- vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[vsnet-grb-info 39655] EP250828a: Liverpool Telescope upper limits
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41590 SUBJECT: EP250828a: Liverpool Telescope upper limits DATE: 25/08/29 09:28:38 GMT FROM: A. Bochenek at Liverpool John Moores University <a.m.bochenek(a)2023.ljmu.ac.uk> A. Bochenek and D. A. Perley (LJMU) report: We observed the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x100s exposures in the SDSS r’ and i’ filters starting at 2025-08-29 22:33:53 UT, approximately 15 hours after the trigger. We do not detect the reported optical counterpart (Taguchi et al., GCN 41582; Yao et al., GCN 41583) or any new objects within the EP/FXT error region of the transient (Liu et al., GCN 41574). The 3-sigma limiting magnitudes on the stacked images are r > 23.7 mag and i > 23.6 mag. This is consistent with the detection of an optical counterpart candidate by NOT at r ~ 24.4 at 15.2 hours post trigger (Fu et al., GCN 41584). View this GCN Circular online at https://gcn.nasa.gov/circulars/41590. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39654] Swift GRB 250828B: Global MASTER-Net observations report
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41589 SUBJECT: Swift GRB 250828B: Global MASTER-Net observations report DATE: 25/08/29 08:58:29 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250828B ( P. A. Evans et al., GCN 41581) errorbox 36970 sec after notice time and 36991 sec after trigger time at 2025-08-29 08:50:09 UT, with upper limit up to 18.0 mag. The observations began at zenith distance = 76 deg. The sun altitude is -27.3 deg. The galactic latitude b = 11 deg., longitude l = 239 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2978941 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 37021 | MASTER-OAFA | C | 60 | 17.9 | 37101 | MASTER-OAFA | C | 60 | 18.0 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41589. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39653] EP250828a: Global MASTER-Net observations report
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41588 SUBJECT: EP250828a: Global MASTER-Net observations report DATE: 25/08/29 08:54:21 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250828a ( EP Team et al., GCN 41574) errorbox 75567 sec after notice time and 1 days 1111 sec after trigger time at 2025-08-29 07:51:04 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 76 deg. The sun altitude is -39.8 deg. The galactic latitude b = -26 deg., longitude l = 76 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2978651 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 87541 | 2025-08-29 07:51:04 | MASTER-OAFA | (21h 51m 57.17s , +20d 28m 10.2s) | C | 60 | 18.8 | 87618 | 2025-08-29 07:52:21 | MASTER-OAFA | (21h 52m 02.14s , +20d 27m 10.6s) | C | 60 | 18.6 | 87694 | 2025-08-29 07:53:37 | MASTER-OAFA | (21h 51m 56.69s , +20d 26m 10.1s) | C | 60 | 18.6 | 87771 | 2025-08-29 07:54:54 | MASTER-OAFA | (21h 52m 01.95s , +20d 26m 11.4s) | C | 60 | 18.6 | 87831 | 2025-08-29 07:54:54 | MASTER-OAFA | (21h 52m 02.01s , +20d 26m 11.0s) | C | 180 | 18.5 | Coadd 87844 | 2025-08-29 07:56:07 | MASTER-OAFA | (21h 50m 57.78s , +20d 27m 39.7s) | C | 60 | 18.5 | 87918 | 2025-08-29 07:57:20 | MASTER-OAFA | (21h 50m 57.71s , +20d 27m 39.6s) | C | 60 | 18.5 | 87991 | 2025-08-29 07:58:34 | MASTER-OAFA | (21h 50m 57.62s , +20d 27m 39.5s) | C | 60 | 18.5 | 88064 | 2025-08-29 07:59:46 | MASTER-OAFA | (21h 50m 57.50s , +20d 27m 39.7s) | C | 60 | 18.4 | 88137 | 2025-08-29 08:00:59 | MASTER-OAFA | (21h 50m 57.38s , +20d 27m 40.1s) | C | 60 | 18.3 | 88723 | 2025-08-29 08:10:46 | MASTER-OAFA | (21h 52m 01.43s , +20d 26m 02.4s) | C | 60 | 17.4 | 88801 | 2025-08-29 08:12:04 | MASTER-OAFA | (21h 51m 59.20s , +20d 27m 52.2s) | C | 60 | 17.3 | 88879 | 2025-08-29 08:13:22 | MASTER-OAFA | (21h 51m 59.14s , +20d 26m 02.3s) | C | 60 | 17.3 | 88939 | 2025-08-29 08:13:22 | MASTER-OAFA | (21h 51m 59.16s , +20d 26m 02.1s) | C | 180 | 17.4 | Coadd 88954 | 2025-08-29 08:14:36 | MASTER-OAFA | (21h 52m 02.29s , +20d 27m 53.3s) | C | 60 | 17.0 | 89030 | 2025-08-29 08:15:53 | MASTER-OAFA | (21h 51m 56.83s , +20d 26m 54.8s) | C | 60 | 16.6 | 89107 | 2025-08-29 08:17:09 | MASTER-OAFA | (21h 51m 55.72s , +20d 27m 52.7s) | C | 60 | 16.0 | 89182 | 2025-08-29 08:18:25 | MASTER-OAFA | (21h 52m 01.38s , +20d 26m 50.8s) | C | 60 | 15.7 | 89256 | 2025-08-29 08:19:38 | MASTER-OAFA | (21h 51m 55.58s , +20d 25m 48.1s) | C | 60 | 15.1 | 89331 | 2025-08-29 08:20:53 | MASTER-OAFA | (21h 52m 01.76s , +20d 26m 13.7s) | C | 60 | 13.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41588. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39652] EP-WXT trigger 01709200800 : KAIT optical observations
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41587 SUBJECT: EP-WXT trigger 01709200800 : KAIT optical observations DATE: 25/08/29 06:20:25 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of EP-WXT trigger 01709200800 (Ni et al., GCN 41561) starting at 11:39, Aug 27 UT, about 17 minutes after the trigger. A set of clear (roughly R) filter images were obtained and lasted for ~70 minutes. We performed the photometry on the suggested flare star LSPM J0220+3320 and saw some variation in the magnitude, which faded from 11.92 +/- 0.03 mag to 11.99 +/- 0.03 mag during our observation. The field was observed again on the following night at ~21.98 hours and lasted for ~2 hours. We saw similar variation in magnitude but in rising phase, suggesting it is a variable star. View this GCN Circular online at https://gcn.nasa.gov/circulars/41587. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39651] EP250828a: KAIT optical upper limit
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41586 SUBJECT: EP250828a: KAIT optical upper limit DATE: 25/08/29 06:16:36 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of EP250828a (Liu et al., GCN 41574) starting at 09:30, Aug 28 UT, about 1.97 hours after the burst. A set of clear (roughly R) filter images were obtained. In our coadd image, we did not detect the reported afterglow (Taguchi et al., GCN 41582; Yao et al., GCN 41583; Fu et al., GCN 41584) down to a limiting mag of 20.5 mag (Vega) at a mid-time of 2.67 hours after the burst. View this GCN Circular online at https://gcn.nasa.gov/circulars/41586. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39650] EP250827a: Swift/UVOT Detection
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41585 SUBJECT: EP250827a: Swift/UVOT Detection DATE: 25/08/29 04:18:21 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of EP250827a 3.2 ks after the EP trigger (Hua et al., GCN Circ. 41553). An uncatalogued source consistent with the XRT position (Evans et al., GCN Circ. 41563) and the optical transient (Levan et al., GCN Circ. 41554; Li et al., GCN Circ. 41555; Lipunov et al., GCN Circ. 41558; Gritsevich et al., GCN Circ. 41559; An et al., GCN Circ. 41560; Ghosh et al., GCN Circ. 41564; O'Neill et al., GCN Circ. 41567) is detected in the UVOT exposures. The preliminary UVOT position is: RA (J2000) = 00:13:52.24 = 3.46767 (deg.) Dec (J2000) = -56:29:52.2 = -56.49784 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 3192 9257 1566 18.29+/-0.04 u 54232 60071 939 19.69+/-0.19 u 121367 127685 754 >20.36 uvw1 53286 59775 1903 19.86+/-0.13 uvw1 120301 127006 2040 >21.21 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.011 in the direction of the burst (Schlegel et al. 1998) View this GCN Circular online at https://gcn.nasa.gov/circulars/41585. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39649] EP250828a: NOT optical counterpart confirmation
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41584 SUBJECT: EP250828a: NOT optical counterpart confirmation DATE: 25/08/29 04:00:42 GMT FROM: syfu(a)nao.cas.cn S.Y. Fu (HUST), J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), A. Henderson (NOT) report: We observed the field of EP250828a (Liu et al., GCN 41574) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). Observations were carried out in the r and z filters. The previously reported counterpart (candidate) within the EP/FXT error circle by Seimei (Taguchi et al., GCN 41582) and SVOM/VT (Yao et al., GCN 41583) is detected in our stacked r-band image and has decayed to r ~ 24.4 (AB) at a median time of 0.634 day post-trigger, calibrated with PanSTARRS-DR2 and not corrected for Galactic extinction. We thus confirm it is the counterpart of EP2508028a. View this GCN Circular online at https://gcn.nasa.gov/circulars/41584. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39648] EP250828a: SVOM/VT optical candidate
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41583 SUBJECT: EP250828a: SVOM/VT optical candidate DATE: 25/08/29 03:36:21 GMT FROM: zhyao(a)bao.ac.cn Z. H. Yao, L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Y. N. Ma, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM/VT performed a Target of Opportunity observation of EP250828a detected by EP/WXT (Liu et al. GCN 41574). SVOM/VT began observing the field at 2025-08-28T11:45:13.5 UTC, 4.21 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. An uncatalogued source compared to Legacy survey is found using VT Xband data, within the error box of EP/FXT (Liu et al. GCN 41574) at R.A., Dec 327.89185, 20.54341 degrees: RA (J2000) = 21:51:34.04 Dec (J2000) = 20:32:36.27 with an uncertainty of 0.5 arcsec. The optical candidate is detected in both VT_B and VT_R band, and the magnitudes are: Mid-Time (T - T0) | exposure time | band | mag (AB) | mag err --------------------|-----------------|------|----------|-------- 4.33 hour | 13*70 sec | VT_B | 23.5 | 0.3 4.33 hour | 12*70 sec | VT_R | 22.27 | 0.08 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41583. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39647] EP250828a: Seimei/TriCCS optical counterpart detection
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41582 SUBJECT: EP250828a: Seimei/TriCCS optical counterpart detection DATE: 25/08/29 02:57:38 GMT FROM: Kenta Taguchi <kentagch(a)kusastro.kyoto-u.ac.jp> Kenta Taguchi (Kyoto U.), Masaomi Tanaka, Seiji Toshikage (Tohoku U.) report on behalf of a larger collaboration: We observed the field of EP250828a detected by the Einstein Probe (Liu et al., GCN 41574, Aryan et al., GCN 41579) using the Tricolor CMOS Camera and Spectrograph (TriCCS) on the 3.8-m Seimei Telescope at the Okayama Observatory of Kyoto University. Our observation started at 13:20:39 UT on 2025-08-28 (MJD = 60915.5560). For each band, we took 37 exposures of 30 seconds (i.e., the total exposure time is 1110 s = 18.5 min). From our preliminary analysis, the 5 sigma limiting magnitudes are 22.3, 22.1, and 22.5 AB mag for g, r, and i-band, respectively. Only in the i-band image, we detected an uncatalogued source of 22.3 +/- 0.2 mag at 21:51:34.02 +20:32:36.3 = 327.8917, 20.5434 (J2000), which is within the FXT error circle (16.5 arcsec from the center). View this GCN Circular online at https://gcn.nasa.gov/circulars/41582. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39646] GRB 250828B: Swift detection of a burst
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41581 SUBJECT: GRB 250828B: Swift detection of a burst DATE: 25/08/28 22:56:15 GMT FROM: David Palmer at LANL <palmer(a)lanl.gov> P. A. Evans (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 22:33:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250828B (trigger=1345836). Swift could not slew to the burst due to an observing constraint. The BAT on-board calculated location is RA, Dec 124.852, -17.388 which is RA(J2000) = 08h 19m 24s Dec(J2000) = -17d 23' 17" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 8 sec. The peak count rate was ~1700 counts/sec (15-350 keV), at ~0 sec after the trigger. Due to a Sun observing constraint, Swift cannot slew to the BAT position until 17:55 UT on 2025 September 07. There will thus be no XRT or UVOT data for this trigger before this time. We note that this GRB is coincident within one minute with a subthreshold Gravitational Wave trigger S250828ef, at 22:32:44 UT. However, this GW trigger is of low confidence, rated as 99% likely to be Terrestrial background noise. Burst Advocate for this burst is P. A. Evans (pae9 AT leicester.ac.uk). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41581. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39645] EP250827a: STEP/T80S Confirmation of fading optical counterpart
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41580 SUBJECT: EP250827a: STEP/T80S Confirmation of fading optical counterpart DATE: 25/08/28 19:34:37 GMT FROM: André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos(a)gmail.com> A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration: We conducted optical follow up with the T80S 0.8-m robotic telescope as part of the S-PLUS Transient Extension Program (Santos et al., 2024, MNRAS, 529, 59) targeting the fast X-ray transient EP250827a by the Einstein Probe instrument (GCN 41533). The T80S observations started on Aug 28 06:34:09 UT (~23 hours after the trigger). We obtained imaging with exposure time 2x120s r-band and 2x150s in i-band with the T80S camera centered at the position of the optical afterglow detection reported in Levan et al., (GCN 41554); Li et al., GCN (41555); Levan et al., (GCN 41557); Lipunov et al., (GCN 41558); Gritsevich et al., (GCN 41559); An et al., (GCN 41560); Ghosh et al., GCN (41564); O’Neill et al., (GCN 41567); Pérez-Fournon et al., (GCN 41568); Passaleva et al., (GCN 41570) to EP250827a Hua et al., (GCN 41553); Ni et al., (GCN 41573) and Eyles-Ferris et al. (GCN 41576). Subtracting DECam template images from the T80S stacked frames using photpipe (Rest et al., 2005, ApJ, 634, 1103), we detect the afterglow at the previously reported position. MEAN-MJD FILTER Magnitude Uncertainty Limit (3sigma) 60915.2823 r 21.32 0.12 22.86 60915.2778 i 20.98 0.16 22.39 The counterpart is detected both in r and i bands. Given the time of imaging at 23.0 hr after the EP trigger and the measured magnitude, we argued the optical counterparting is fading after the rebrightening reported by Ghosh et al. (GCN 41564), confirming the fading reported by Eyles-Ferris (GCN 41576). We acknowledge the T80S technical team with the provided support during the night. View this GCN Circular online at https://gcn.nasa.gov/circulars/41580. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39644] EP250828a: Optical upper limit with Kinder observations
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41579 SUBJECT: EP250828a: Optical upper limit with Kinder observations DATE: 25/08/28 16:27:20 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941(a)gmail.com> A. Aryan, C.-S. Lin (both NCU), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations started at 13:06 UTC on the 28th of August 2025 (MJD 60915.546), 5.56 hr after the EP WXT trigger. We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm. In the stacked as well as in the difference image, we do not find any evidence of a new and uncataloged optical counterpart candidate within the 20 arcseconds error circle of EP-FXT localization. Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured 3-sigma upper limit (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60915.546 | 5.56 | 300 * 12 | >22.3 | 1".69 | 1.08 The absence of any associated optical counterpart down to 22.3 mag with the optical observations starting within the first 6 hours of its X-ray discovery aligns well with the 'Dark' nature of FXTs as proposed by Aryan et al. 2025, arXiv:2504.21096. The presented magnitude is calibrated using the field stars from the PanStarrs1 catalog and is not corrected for the expected Galactic foreground extinction of A_r = 0.28 mag in the direction of the transient (Schlafly & Finkbeiner 2011). View this GCN Circular online at https://gcn.nasa.gov/circulars/41579. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39643] LIGO/Virgo/KAGRA S250827fo: Updated Sky localization
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41578 SUBJECT: LIGO/Virgo/KAGRA S250827fo: Updated Sky localization DATE: 25/08/28 15:42:12 GMT FROM: alexandresebastien.goettel(a)LIGO.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250827fo (GCN Circular 41571). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250827fo For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 470 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1005 +/- 261 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41578. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39642] LIGO/Virgo/KAGRA S250818k: uGMRT 1.3 GHz observations of AT2025ulz
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41577 SUBJECT: LIGO/Virgo/KAGRA S250818k: uGMRT 1.3 GHz observations of AT2025ulz DATE: 25/08/28 14:58:07 GMT FROM: pchandra(a)nrao.edu G. Bruni (INAF-IAPS), P. Chandra (NRAO), S. Chatterjee(Rhodes University) C. Christy (Arizona), R. Kale (NCRA-TIFR), T. Laskar (Utah), S. Mohnani (IIT Indore), S. R. Das (IIST), L. Resmi (IIST), Roberto Ricci (URome) and E. Troja (URome) report on behalf of multiple teams: "We observed AT2025ulz (Stein et al. GCN 41414) with the Giant Metrewave Radio Telescope (GMRT) under programs 48_081 (PI: Mohnani), 48_090 (PI: Troja), 48_096 (PI: Chandra), 48_097 (PI: Bruni), 48_120 (PI: Laskar), and 48_151 (PI: Resmi) beginning on 2025 Aug 26 at 13:30 UT (~8.5 days after the LVK trigger) for 2 hours at Band 5 (1.3 GHz). In preliminary analysis, we do not detect significant radio emission at the position of AT2025ulz or the host. The image rms is ~ 23 microJy/beam, resulting in a 5-sigma upper limit of 115 uJy, at an angular resolution of 2.5x2.0 arcsec. Together with the VLA 3 GHz non-detection at the position of AT2025ulz (<40 uJy, GCN 41542), our GMRT 1.3 GHz upper limit suggests that the radio emission previously detected with MeerKAT at lower angular resolution may include a contribution from the host galaxy (as already noted in the GCN 41500). We thank the GMRT staff for scheduling and executing these observations. GMRT is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. View this GCN Circular online at https://gcn.nasa.gov/circulars/41577. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39641] EP250827a: LCO observations show fading optical counterpart
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41576 SUBJECT: EP250827a: LCO observations show fading optical counterpart DATE: 25/08/28 14:18:51 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris (Leicester), J. A. Quirola-Vasquez (Radboud), A. J. Levan (Radboud and Warwick), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration: We observed the location of the optical counterpart (Levan et al., GCN 41554; Li et al., GCN 41555; Levan et al., GCN 41557; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559; An et al., GCN 41560; Ghosh et al., GCN 41564; O’Neill et al., GCN 41567; Pérez-Fournon et al., GCN 41568; Passaleva et al., GCN 41570) to EP250827a (Hua et al., GCN 41553; Ni et al., GCN 41573) with the Sinistro instruments mounted on 1-m telescopes at the LCO SAAO and Cerro Tololo Inter-American Observatory nodes. 5x300 s exposures were obtained in each of the SDSS u, g and r filters with start times from 2025 Aug 27 at 22:38:04 to 2025 Aug 28 04:36:48 UT. The counterpart is clearly detected in both the g and r filters and is marginally detected in the u filter image. At 15.0 hr after the EP trigger, we measure r = 19.99 +/- 0.05, ~1.5 mags fainter than observed by Ghosh et al. (GCN 41564). Assuming an unbroken power-law decay between the two epochs, we derive a temporal index alpha ~ 1.4 for the decay. View this GCN Circular online at https://gcn.nasa.gov/circulars/41576. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39640] GRB 250828A: Fermi GBM Final Localization
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41575 SUBJECT: GRB 250828A: Fermi GBM Final Localization DATE: 25/08/28 13:52:06 GMT FROM: A. Holzmann Airasca at University of Trento and INFN Bari <a.holzmannairasca(a)unitn.it> The Fermi GBM team reports the detection of a likely LONG GRB "At 12:00:02.52 UT on 28 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250828A (trigger 778075207/250828500). This trigger was initially classified as Unreliable location by the flight software, but is in fact a GRB. The on-ground calculated location, using the Fermi GBM trigger data, is RA = 220.44, Dec = -69.39 (J2000 degrees, equivalent to J2000 14h 41m, -69d 23m'), with a statistical uncertainty of 3.08 degrees. The angle from the Fermi LAT boresight is 14 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250828500/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250828500/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250828500/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/41575. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39639] EP250828a: Einstein Probe detection of an X-ray transient
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41574 SUBJECT: EP250828a: Einstein Probe detection of an X-ray transient DATE: 25/08/28 10:51:02 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Y. Liu (NAO, CAS), K. R. Ni (CCNU) , H. L. Peng (NJNU), Y. J. Song and H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250828a. The transient triggered EP-WXT (ID: 01709200900) at 2025-08-28T07:32:33 (UTC). The WXT position of the source is R.A. = 327.905 deg, DEC = 20.567 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed at 2025-08-28T08:48:40 (UTC) . Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 327.8966 deg, DEC = 20.5435 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41574. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39638] EP250827a: Preliminary analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41573 SUBJECT: EP250827a: Preliminary analysis of the EP-WXT and EP-FXT observations DATE: 25/08/28 08:20:52 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> K. R. Ni (CCNU), Y. L. Hua (PMO,CAS), M. J. Liu and C. C. Jin (NAO,CAS) on behalf of the Einstein Probe (EP) team: The X-ray transient EP250827a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hua et al., GCN 41553). We report on the refined analysis of WXT and FXT observations. The WXT data shows that the event started at T0=2025-08-27T07:39:01 (UTC) and lasted for at least 90 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.19 x 10^20 cm^-2 and a photon index of 0.73 (-0.47/+0.50). The derived average unabsorbed 0.5-4 keV flux is 1.71(-0.34/+0.42) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 102 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 3.46915, DEC = -56.4998 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.19 x 10^20 cm^-2 and a photon index of 2.0(-0.10/+0.10). The derived average unabsorbed 0.5-10 keV flux is 4.38 (-0.38/+0.39) x 10^(-12)erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41573. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39637] GRID detection of GRB 250824A
by GCN Circulars 28 Aug '25

28 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41572 SUBJECT: GRID detection of GRB 250824A DATE: 25/08/28 02:17:34 GMT FROM: GRID Student Team at Tsinghua University <grid(a)tsinghua.edu.cn> Longhao Li, Chenyu Wang and Zirui Yang report on behalf of the GRID Collaboration: GRID-11B reports the detection of the GRB 250824A, which was also detected by Fermi/GBM (GCN Circular 41522). The event was triggered with GRID on 2025-08-24 at 11:24:15 UTC. The measured burst duration (T90) is approximately 78.2 ± 5.6 seconds. The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB250824A/GRID_GRB…. GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4. View this GCN Circular online at https://gcn.nasa.gov/circulars/41572. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39636] LIGO/Virgo/KAGRA S250827fo: Identification of a GW compact binary merger candidate
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41571 SUBJECT: LIGO/Virgo/KAGRA S250827fo: Identification of a GW compact binary merger candidate DATE: 25/08/27 23:51:40 GMT FROM: Jyotirmaya Mohanta at University of Tsukuba <s2430161(a)u.tsukuba.ac.jp> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250827fo during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 22:49:40.311 UTC (GPS time: 1440370198.311). The candidate was found by the cWB BBH [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines. S250827fo is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.3e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250827fo The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%). Noise transients (glitches) were present in LIGO Livingston detector data within a second of the event time, which may affect the parameters or the significance of the candidate. Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 6%. The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 31 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 967 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1064 +/- 295 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41571. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39635] EP250827A: VLT/X-shooter redshift confirmation
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41570 SUBJECT: EP250827A: VLT/X-shooter redshift confirmation DATE: 25/08/27 21:41:57 GMT FROM: N. Passaleva at Sapienza University of Rome <niccolo.passaleva(a)uniroma1.it> Niccolò Passaleva (U Rome), Rosa L. Becerra (U Rome) , Massine El Kabir (U Rome), Yu-Han Yang (U Rome), Eleonora Troja (U Rome) report on behalf of the ERC BHianca team: We observed the optical counterpart of EP250827a (Hua et al., GCN 41553) reported by Levan et al. (GCN 41554) and Wenxiong et al. (GCN 41555) with the X-Shooter spectrograph on the ESO VLT UT3 (Melipal). Observations began at T+1.67 hours and obtained a total of 4x600s spectra at an average airmass of about 1.45. At the position of the optical counterpart we detect a bright continuum. We identify absorption features (e.g. Fe II, Mg I, Mg II, C lV, Si IV, Si II, Al II, Al III) at a common redshift of z ~ 1.61 confirming the result reported by Levan et al. (GCN 41557). We thank the staff at the VLT, for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41570. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39634] Fermi GRB 250827A: Global MASTER-Net observations report
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41569 SUBJECT: Fermi GRB 250827A: Global MASTER-Net observations report DATE: 25/08/27 18:31:07 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250827A ( Fermi GBM team, GCN 41565) errorbox 4990 sec after notice time and 5027 sec after trigger time at 2025-08-27 16:58:40 UT, with upper limit up to 18.5 mag. Observations started at twilight. The observations began at zenith distance = 70 deg. The sun altitude is -9.1 deg. The galactic latitude b = -64 deg., longitude l = 348 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2978193 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 5057 | 2025-08-27 16:58:40 | MASTER-SAAO | (22h 53m 00.45s , -44d 30m 29.7s) | C | 60 | 15.0 | 5149 | 2025-08-27 17:00:12 | MASTER-SAAO | (23h 04m 08.41s , -44d 31m 25.0s) | C | 60 | 15.0 | 5247 | 2025-08-27 17:01:50 | MASTER-SAAO | (22h 51m 38.64s , -42d 35m 50.8s) | C | 60 | 15.4 | 5339 | 2025-08-27 17:03:22 | MASTER-SAAO | (23h 02m 31.98s , -42d 37m 28.4s) | C | 60 | 15.8 | 5430 | 2025-08-27 17:04:53 | MASTER-SAAO | (22h 55m 53.26s , -46d 24m 26.9s) | C | 60 | 16.1 | 5522 | 2025-08-27 17:06:25 | MASTER-SAAO | (23h 07m 29.25s , -46d 23m 23.1s) | C | 60 | 16.1 | 5615 | 2025-08-27 17:07:58 | MASTER-SAAO | (23h 12m 16.06s , -42d 37m 26.7s) | C | 60 | 16.2 | 5707 | 2025-08-27 17:09:30 | MASTER-SAAO | (23h 23m 15.11s , -42d 37m 12.2s) | C | 60 | 16.5 | 5800 | 2025-08-27 17:11:03 | MASTER-SAAO | (22h 52m 56.79s , -44d 29m 35.0s) | C | 60 | 16.9 | 5892 | 2025-08-27 17:12:35 | MASTER-SAAO | (23h 06m 32.59s , -44d 46m 11.8s) | C | 60 | 12.6 | 5892 | 2025-08-27 17:12:35 | MASTER-SAAO | (23h 04m 11.39s , -44d 31m 26.0s) | C | 60 | 16.9 | 5984 | 2025-08-27 17:14:07 | MASTER-SAAO | (22h 54m 00.36s , -42d 52m 10.3s) | C | 60 | 12.9 | 5984 | 2025-08-27 17:14:07 | MASTER-SAAO | (22h 51m 40.13s , -42d 36m 08.1s) | C | 60 | 17.4 | 6076 | 2025-08-27 17:15:38 | MASTER-SAAO | (23h 04m 49.74s , -42d 51m 47.2s) | C | 60 | 12.8 | 6076 | 2025-08-27 17:15:38 | MASTER-SAAO | (23h 02m 27.51s , -42d 36m 37.5s) | C | 60 | 17.5 | 6168 | 2025-08-27 17:17:11 | MASTER-SAAO | (22h 58m 24.36s , -46d 38m 32.3s) | C | 60 | 12.5 | 6168 | 2025-08-27 17:17:11 | MASTER-SAAO | (22h 55m 51.24s , -46d 23m 36.1s) | C | 60 | 17.5 | 6260 | 2025-08-27 17:18:43 | MASTER-SAAO | (23h 10m 04.21s , -46d 40m 47.6s) | C | 60 | 12.7 | 6260 | 2025-08-27 17:18:43 | MASTER-SAAO | (23h 07m 33.75s , -46d 24m 34.4s) | C | 60 | 17.7 | 6352 | 2025-08-27 17:20:15 | MASTER-SAAO | (23h 14m 34.57s , -42d 53m 42.5s) | C | 60 | 12.6 | 6352 | 2025-08-27 17:20:15 | MASTER-SAAO | (23h 12m 15.38s , -42d 37m 38.2s) | C | 60 | 17.8 | 6444 | 2025-08-27 17:21:47 | MASTER-SAAO | (23h 23m 15.76s , -42d 37m 11.0s) | C | 60 | 17.6 | 6537 | 2025-08-27 17:23:20 | MASTER-SAAO | (22h 55m 23.42s , -44d 45m 07.7s) | C | 60 | 12.2 | 6537 | 2025-08-27 17:23:20 | MASTER-SAAO | (22h 52m 55.38s , -44d 30m 04.0s) | C | 60 | 18.0 | 6629 | 2025-08-27 17:24:52 | MASTER-SAAO | (23h 06m 30.14s , -44d 47m 09.0s) | C | 60 | 12.7 | 6629 | 2025-08-27 17:24:52 | MASTER-SAAO | (23h 04m 10.07s , -44d 31m 34.5s) | C | 60 | 18.1 | 6721 | 2025-08-27 17:26:23 | MASTER-SAAO | (22h 54m 01.38s , -42d 52m 37.3s) | C | 60 | 12.6 | 6721 | 2025-08-27 17:26:23 | MASTER-SAAO | (22h 51m 39.24s , -42d 36m 10.8s) | C | 60 | 18.1 | 6812 | 2025-08-27 17:27:55 | MASTER-SAAO | (23h 04m 45.97s , -42d 51m 08.1s) | C | 60 | 12.8 | 6812 | 2025-08-27 17:27:55 | MASTER-SAAO | (23h 02m 26.80s , -42d 36m 48.6s) | C | 60 | 17.8 | 6903 | 2025-08-27 17:29:26 | MASTER-SAAO | (22h 58m 24.48s , -46d 39m 04.3s) | C | 60 | 12.2 | 6903 | 2025-08-27 17:29:26 | MASTER-SAAO | (22h 55m 50.57s , -46d 23m 47.2s) | C | 60 | 18.1 | 6995 | 2025-08-27 17:30:57 | MASTER-SAAO | (23h 10m 03.97s , -46d 40m 48.6s) | C | 60 | 13.3 | 6995 | 2025-08-27 17:30:57 | MASTER-SAAO | (23h 07m 33.26s , -46d 24m 46.2s) | C | 60 | 18.2 | 7087 | 2025-08-27 17:32:30 | MASTER-SAAO | (23h 14m 34.41s , -42d 52m 08.5s) | C | 60 | 12.6 | 7087 | 2025-08-27 17:32:30 | MASTER-SAAO | (23h 12m 15.32s , -42d 37m 50.3s) | C | 60 | 18.2 | 7179 | 2025-08-27 17:34:02 | MASTER-SAAO | (23h 25m 37.26s , -42d 52m 57.4s) | C | 60 | 12.9 | 7179 | 2025-08-27 17:34:02 | MASTER-SAAO | (23h 23m 14.32s , -42d 37m 20.3s) | C | 60 | 18.1 | 7272 | 2025-08-27 17:35:34 | MASTER-SAAO | (22h 55m 22.47s , -44d 45m 30.3s) | C | 60 | 13.1 | 7272 | 2025-08-27 17:35:34 | MASTER-SAAO | (22h 52m 54.03s , -44d 30m 06.0s) | C | 60 | 18.3 | 7363 | 2025-08-27 17:37:06 | MASTER-SAAO | (23h 06m 33.59s , -44d 47m 46.3s) | C | 60 | 12.7 | 7363 | 2025-08-27 17:37:06 | MASTER-SAAO | (23h 04m 08.73s , -44d 31m 42.3s) | C | 60 | 18.2 | 7455 | 2025-08-27 17:38:38 | MASTER-SAAO | (22h 53m 58.35s , -42d 52m 13.3s) | C | 60 | 13.4 | 7455 | 2025-08-27 17:38:38 | MASTER-SAAO | (22h 51m 38.67s , -42d 36m 07.8s) | C | 60 | 18.1 | 7547 | 2025-08-27 17:40:10 | MASTER-SAAO | (23h 04m 46.10s , -42d 51m 19.6s) | C | 60 | 12.6 | 7547 | 2025-08-27 17:40:10 | MASTER-SAAO | (23h 02m 26.49s , -42d 37m 00.5s) | C | 60 | 18.3 | 7639 | 2025-08-27 17:41:41 | MASTER-SAAO | (22h 58m 23.04s , -46d 39m 30.5s) | C | 60 | 13.5 | 7639 | 2025-08-27 17:41:41 | MASTER-SAAO | (22h 55m 49.37s , -46d 23m 58.8s) | C | 60 | 18.2 | 7730 | 2025-08-27 17:43:13 | MASTER-SAAO | (23h 10m 04.09s , -46d 40m 59.5s) | C | 60 | 13.3 | 7730 | 2025-08-27 17:43:13 | MASTER-SAAO | (23h 07m 32.57s , -46d 24m 58.3s) | C | 60 | 18.3 | 7822 | 2025-08-27 17:44:45 | MASTER-SAAO | (23h 14m 34.23s , -42d 54m 05.7s) | C | 60 | 12.9 | 7822 | 2025-08-27 17:44:45 | MASTER-SAAO | (23h 12m 13.06s , -42d 38m 02.3s) | C | 60 | 18.2 | 7914 | 2025-08-27 17:46:17 | MASTER-SAAO | (23h 25m 31.77s , -42d 53m 59.7s) | C | 60 | 12.9 | 7914 | 2025-08-27 17:46:17 | MASTER-SAAO | (23h 23m 11.95s , -42d 37m 54.5s) | C | 60 | 18.3 | 8006 | 2025-08-27 17:47:49 | MASTER-SAAO | (22h 52m 52.65s , -44d 30m 26.9s) | C | 60 | 18.3 | 8098 | 2025-08-27 17:49:21 | MASTER-SAAO | (23h 06m 35.89s , -44d 46m 34.9s) | C | 60 | 12.2 | 8098 | 2025-08-27 17:49:21 | MASTER-SAAO | (23h 04m 07.29s , -44d 31m 42.8s) | C | 60 | 18.2 | 8190 | 2025-08-27 17:50:52 | MASTER-SAAO | (22h 51m 36.87s , -42d 36m 36.2s) | C | 60 | 18.4 | 8281 | 2025-08-27 17:52:24 | MASTER-SAAO | (23h 04m 47.02s , -42d 51m 31.9s) | C | 60 | 12.8 | 8281 | 2025-08-27 17:52:24 | MASTER-SAAO | (23h 02m 24.41s , -42d 37m 11.6s) | C | 60 | 18.1 | 8374 | 2025-08-27 17:53:56 | MASTER-SAAO | (22h 58m 21.84s , -46d 39m 43.1s) | C | 60 | 13.2 | 8374 | 2025-08-27 17:53:56 | MASTER-SAAO | (22h 55m 47.51s , -46d 24m 09.0s) | C | 60 | 18.1 | 8464 | 2025-08-27 17:55:27 | MASTER-SAAO | (23h 10m 03.50s , -46d 41m 10.1s) | C | 60 | 13.5 | 8464 | 2025-08-27 17:55:27 | MASTER-SAAO | (23h 07m 31.49s , -46d 25m 09.3s) | C | 60 | 18.3 | 8556 | 2025-08-27 17:56:59 | MASTER-SAAO | (23h 12m 12.36s , -42d 38m 13.7s) | C | 60 | 18.3 | 8648 | 2025-08-27 17:58:31 | MASTER-SAAO | (23h 25m 31.50s , -42d 54m 10.7s) | C | 60 | 13.3 | 8648 | 2025-08-27 17:58:31 | MASTER-SAAO | (23h 23m 11.42s , -42d 38m 05.6s) | C | 60 | 18.4 | 8741 | 2025-08-27 18:00:04 | MASTER-SAAO | (22h 55m 16.95s , -44d 44m 44.6s) | C | 60 | 12.4 | 8741 | 2025-08-27 18:00:04 | MASTER-SAAO | (22h 52m 51.29s , -44d 30m 30.5s) | C | 60 | 18.5 | 8840 | 2025-08-27 18:01:43 | MASTER-SAAO | (23h 06m 31.53s , -44d 48m 23.1s) | C | 60 | 12.8 | 8840 | 2025-08-27 18:01:43 | MASTER-SAAO | (23h 04m 06.06s , -44d 32m 15.8s) | C | 60 | 18.4 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41569. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39633] EP250827a: confirmation of optical rebrightening with LCO
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41568 SUBJECT: EP250827a: confirmation of optical rebrightening with LCO DATE: 25/08/27 18:04:37 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL) We observed the field of EP250827a, detected by Einstein Probe (Hua et al. GCN 41553) and Swift-XRT (Evans et al. GCN 41563) at a redshift of z = 1.613 (Levan et al. GCN 41557) with one of the three 1-m telescopes of the Las Cumbres Observatory Global Telescope network (LCO) equipped with Sinistro cameras located at the LCO node at the Cerro Tololo Inter-American Observatory (CTIO), Chile. The observation, of 300 seconds in the SDSS-r filter, started at 2025-08-27 10:04:04 UT, about 2.39 hours after the EP trigger. We detect clearly a source within the EP-FXT and Swift-XRT error circles at a position consistent with the detections reported by Levan et al. (GCN 41554) and Li et al. (GCN 41555) and with other optical detections reported by Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559; An et al., GCN 41560, Ghosh et al., GCN 41564; and O'Neill GCN 41567). We measure a magnitude of r = 18.65 +/- 0.04, calibrated with the Legacy Surveys DR10 catalog and not corrected for Galactic extinction. Using a similar r-band filter, we confirm the optical rebrightening detected also by Ghosh et al. (GCN 41564) between the TRT observation (r = 18.93 +/- 0.03, 0.86 hours after the EP trigger) and our LCO observation (r = 18.65 +/- 0.04, 2.39 hours after the EP trigger). This work makes use of observations from the Las Cumbres Observatory global telescope network program IAC2025B-008 (SGLF). View this GCN Circular online at https://gcn.nasa.gov/circulars/41568. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39632] EP250827a: GOTO observation of the optical afterglow
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41567 SUBJECT: EP250827a: GOTO observation of the optical afterglow DATE: 25/08/27 16:51:59 GMT FROM: d.s.oneill(a)bham.ac.uk D. O'Neill, B. P. Gompertz, G. Ramsay, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares Report on behalf of the GOTO collaboration: We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the EP250827a (Hua et al. GCN 41553). Targeted observations were taken at 2025-08-27 15:50:54 UT (+8.17h post trigger). The observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. We detect the optical counterpart (Levan et al., GCNs 41554, 41557; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559, An et al., GCN 41660, Ghosh et al. GCN 41564) at L = 18.63 ± 0.04 AB mag. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2025-08-25 12:46:35 UT down to a 5-sigma limit L>19.42 AB mag. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41567. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39631] GRB 250826A: SVOM/GRM observation
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41566 SUBJECT: GRB 250826A: SVOM/GRM observation DATE: 25/08/27 16:10:33 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Sebastien Guillot (IRAP), Frédéric Piron (CEA) Report on behalf of the SVOM team: SVOM/GRM was triggered on-ground by GRB 250826A at 2025-08-26T07:24:03.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #41545). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 8.0 +4.0/-1.5 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250826A.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 141.8, Dec = 34.7, GCN #41545), is located at about 136 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41566. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39630] GRB 250827A: Fermi GBM Final Real-time Localization
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41565 SUBJECT: GRB 250827A: Fermi GBM Final Real-time Localization DATE: 25/08/27 15:45:24 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 15:34:53 UT on 27 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250827A (trigger 778001698.2092 / 250827649). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 346.8, Dec = -43.8 (J2000 degrees, equivalent to J2000 23h 07m, -43d 47'), with a statistical uncertainty of 7.2 degrees. The angle from the Fermi LAT boresight is 90.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250827649/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250827649/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250827649/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41565. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39629] EP250827A: An optical rebrightening was detected by LCO.
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41564 SUBJECT: EP250827A: An optical rebrightening was detected by LCO. DATE: 25/08/27 15:36:17 GMT FROM: ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994(a)gmail.com> Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Rahul Gupta (NASA GSFC), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES) on behalf of a larger collaboration. We observed the field of the EP 250827A triggered by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hua et al., GCN 41553) in the r, V filter of the 1-meter Sinistro telescope at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Siding Spring is near Coonabarabran, New South Wales, Australia . The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel). Observations began on August 27, 2025, starting 5.135 hours after the GRB trigger. Further observations are currently ongoing. We clearly detect the optical transient (OT) reported by GCNs (Levan et al., GCN 41554; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559, An et al., GCN 41660) in our r band image. A brightening of ≈ 0.5 mag was observed within 1–5 hours after the burst, with An et al,. GCN 41660, reported a magnitude of 18.9 mag at ~1 hour post-burst. |Date| |UTstart| |t-T0 (hours)| |Exp (sec)| |Filter| |Magnitude| -------------------------------------------------------------------------------------------------- 2025-08-27 12:48:48.96 5.135 1 x 900 r r = 18.44 +/- 0.01 The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41564. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39628] EP250827a: Swift-XRT counterpart detection
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41563 SUBJECT: EP250827a: Swift-XRT counterpart detection DATE: 25/08/27 15:35:30 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), R.A.J. Eyles-Ferris (U. Leicester), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250827a, collecting 1.6 ks of Photon Counting (PC) mode data between T0+3.1 ks and T0+9.2 ks after the trigger. A likely counterpart has been found. The details of this source are: Source 1 (SWIFT J001351.7-562952): ================================== RA (J2000.0): 3.4656 = 00 13 51.74 Dec (J2000.0): -56.4979 = -56 29 52.4 Error: 3.7 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.2 arcmin from the Einstein Probe/WXT position. Mean rate: 0.161 +/- 0.011 ct s^-1 Mean flux: (5.48 +/- 0.39)e-12 erg cm^-2 s^-1 Peak rate: 0.253 +/- 0.057 ct s^-1 Peak flux: (8.6 +/- 1.9)e-12 erg cm^-2 s^-1 ECF: 3.41e-11 erg cm^-2 ct^-1 assuming NH=1.45e+20 cm^-2, gamma=1.93 determined from a spectral fit. XMM UL: 2.2e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 3.3-sigma above this 3-sigma upper limit. The source may be fading, at the 1.5-sigma level. This position is consistent with the optical counterpart previously reported (Levan et al. GCN 41554, Li et al. GCN 41555, Lipunov et al. GCN 41558, Gritsevich et al. GCN 41559, An et al. GCN 41560) at a redshift of 1.613 (Levan et al. GCN 41557). All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00062. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41563. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39627] LIGO/Virgo/KAGRA S250827l: Updated Sky localization
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41562 SUBJECT: LIGO/Virgo/KAGRA S250827l: Updated Sky localization DATE: 25/08/27 14:14:22 GMT FROM: Colm Talbot at University of Chicago <talbotcolm(a)gmail.com> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250827l (GCN Circular 41552). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250827l For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 615 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2464 +/- 700 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41562. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39626] GCN Announcement: Super-Kamiokande JSON Notices and Schema v4.5.0
by GCN Announcements 27 Aug '25

27 Aug '25

New Super-Kamiokande JSON Notices The Super-Kamiokande Team and the GCN team are pleased to announce the availability of Super-Kamiokande notice types via the new GCN in JSON format. These notices can be streamed via Kafka. SK_SN notices remain available via GCN Classic and GCN Classic over Kafka (https://gcn.gsfc.nasa.gov/sk_sn.html) The information included in the JSON-format Notices largely overlaps with that in GCN Classic with some additional parameters: the number of detected inverse beta decay (IBD) events, the analysis pipeline used (snwatch or wit), and the type of processed sample (full or partial) used for event detection. Super-Kamiokande Summary: Also known as Super-K or SK, Super-Kamiokande experiment was built and has been operated with funding from the Japanese Ministry of Education, Science, Sports and Culture, and the U.S. Department of Energy. Designed to understand the nature of neutrinos, Super-K is a leading detector in the search for MeV neutrino detections of core-collapse supernovae. https://www-sk.icrr.u-tokyo.ac.jp/en/sk/ Notice topic: new Super-Kamiokande notices provide core-collapse supernova detection and localization as well regular monthly test alerts distributed on the topic gcn.notice.superk.sn_alert. See the SKSN mission page for a more extensive description of the information provided in these new notices. https://gcn.nasa.gov/missions/sksn Schema v4.5.0 GCN Schema v4.5.0 is available with recent changes including the Super-K schema (Release Notes, https://github.com/nasa-gcn/gcn-schema/releases/tag/v4.5.0) For more details on this new feature and an archive of GCN news and announcements, see https://gcn.nasa.gov/news. For questions, issues, or bug reports, please contact us via: - Contact form: https://gcn.nasa.gov/contact - GitHub issue tracker: https://github.com/nasa-gcn/gcn.nasa.gov/issues --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39625] The EP-WXT trigger 01709200800 is likely a flaring star
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41561 SUBJECT: The EP-WXT trigger 01709200800 is likely a flaring star DATE: 25/08/27 12:46:25 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> K. R. Ni (CCNU), Y. L. Hua (PMO,CAS) M. J. Liu and C. C. Jin (NAO,CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709200800 at the time of 2025-08-27T11:22:22, is likely a stellar flare associated with LSPM J0220+3320. The estimated flux of the flare is around 4.87e-11 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 1.61e31 erg/s. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41561. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39624] EP250827a: TRT optical observations
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41560 SUBJECT: EP250827a: TRT optical observations DATE: 25/08/27 10:36:23 GMT FROM: J. An <jiean0813(a)foxmail.com> J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration: We observed the field of EP250827a (Hua et al., GCN 41553), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. (CTO) Observations started at 08:19:45 UTC on 2025-08-27, i.e., ~ 39 min after the EP/WXT trigger and a series of frames in the r band were obtained. We clearly detect the optical counterpart(Levan et al., GCN 41554; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559) in our images. Preliminary photometry shows that the source has r ~ 18.93 +/- 0.03 mag at 51.6 min post-trigger, calibrated with Legacy DR10 catalog and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41560. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39623] EP250827a: BOOTES-7 early optical dectection
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41559 SUBJECT: EP250827a: BOOTES-7 early optical dectection DATE: 25/08/27 10:21:40 GMT FROM: I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg(a)iaa.es> M. Gritsevich (Univ. of Helsinki), I. Perez-Garcia, E. Fernandez-Garcia, S.-Y. Wu, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar (Univ. of Malaga), D.-R. Xiong (YNAO, Kumming), Y.-D. Hu (GXU, Guanxi), B.-B. Zhang (Nanjing Univ.), and A. Maury (Space Obs., San Pedro de Atacama), on behalf of a larger collaboration, report: Following the detection of EP250827a by Einstein Probe (Hua et al., GCNC 41553), the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) automatically observed the X-ray transient location starting on Aug. 27, 07:47:36 UT (i.e. 413 s after trigger). A series of images in clear filter were gathered for 2 h and the optical counterpart reported by Levan et al. (GCNC 41554), Li et al. (GCNC 41555) and Lipunov et al. (GCNC 41558) was detected at early times in our dataset. We measure a magnitude of 18.67 +/- 0.05 on a co-added image (60s x 5 exposures) at 07:53:11 UT mid time. Further data analysis is ongoing. We would like to thank the staff at San Pedro de Atacama Space Observatory for their excellent support. View this GCN Circular online at https://gcn.nasa.gov/circulars/41559. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39622] EP250827a / GRB 250725A: MASTER-OAFA OT observation
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41558 SUBJECT: EP250827a / GRB 250725A: MASTER-OAFA OT observation DATE: 25/08/27 10:09:42 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, A.Kuznetsov, I.Panchenko, G.Antipov, P.Balanutsa, K.Zhirkov (Lomonosov MSU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (OAFA), A.Chasovnikov, E.Gorbovskoy, N.Tiurina, Ya.Kechin, D.Vlasenko, V.Senik, M. Shilova, I.Lipunov, V.Topolev (Lomonosov MSU), O.Gress, N.Budnev(ISU), D.Buckley (SAAO), A.Sosnovskij (Crao RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro AstrophysicsObservatory A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity) MASTER-OAFA robotic telescope (Lipunov et al. GCN 41556) started EP GRB 250827A observation at 2025-08-27 07:46:54 UT and detected optical couterpart of this GRB MASTER OT J001352.27-562952.8 with m_OT (unfiltered) = 18.9m (preliminary) at 2025-08-27 07:48:10 https://master.sai.msu.ru/site/master2/event.php?id=2978010 The light curve demonstrates multipeak structure from 18 to 19 during last hour. This OT coincide with GEMINI-South detection ( GCN 41554 Levan et al.) and Las Cumbres one (GCN 41555 Wenxiong Li et al.) View this GCN Circular online at https://gcn.nasa.gov/circulars/41558. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39621] EP250827a: Gemini-South redshift z = 1.613
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41557 SUBJECT: EP250827a: Gemini-South redshift z = 1.613 DATE: 25/08/27 09:24:15 GMT FROM: Jonathan Quirola at Radboud University <jaquirola1990(a)gmail.com> A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), report on behalf of a larger collaboration: We observed the location of the optical counterpart (Levan et al., GCN 41554) to EP250827a (Hua et al., GCN 41553) with Gemini-South and the GMOS instrument using the B480 grating and a 1 arcsec slit. Four 300 s exposures were obtained. Observations began on 2025 Aug 27 at 08:51 UT, approximately 71 min after the EP trigger time. The data was reduced using DRAGONS. Several metallic absorption lines (due to e.g. Fe II and Mg II) at a redshift of z = 1.613 are present, suggesting the FXT lies at this redshift or beyond. We thank the staff of Gemini, especially K. Silva, for their outstanding work in rapidly obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41557. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39620] EP250827a: Global MASTER-Net observations report
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41556 SUBJECT: EP250827a: Global MASTER-Net observations report DATE: 25/08/27 09:18:21 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250827a ( EP Team et al., GCN 41553) errorbox 371 sec after trigger time at 2025-08-27 07:46:54 UT, with upper limit up to 21.0 mag. The observations began at zenith distance = 29 deg. The sun altitude is -41.2 deg. The galactic latitude b = -60 deg., longitude l = 313 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2978010 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 407 | 2025-08-27 07:46:54 | MASTER-OAFA | (00h 13m 54.06s , -56d 38m 35.7s) | C | 70 | 20.5 | 492 | 2025-08-27 07:48:10 | MASTER-OAFA | (00h 13m 54.34s , -56d 38m 33.9s) | C | 90 | 20.7 | 599 | 2025-08-27 07:49:46 | MASTER-OAFA | (00h 13m 54.64s , -56d 38m 32.6s) | C | 110 | 20.8 | 680 | 2025-08-27 07:51:43 | MASTER-OAFA | (00h 13m 54.87s , -56d 38m 31.2s) | C | 40 | 20.2 | 726 | 2025-08-27 07:52:29 | MASTER-OAFA | (00h 13m 55.02s , -56d 38m 30.5s) | C | 40 | 20.2 | 772 | 2025-08-27 07:53:14 | MASTER-OAFA | (00h 13m 55.19s , -56d 38m 29.9s) | C | 40 | 20.3 | 817 | 2025-08-27 07:54:00 | MASTER-OAFA | (00h 13m 55.31s , -56d 38m 29.1s) | C | 40 | 20.2 | 863 | 2025-08-27 07:54:46 | MASTER-OAFA | (00h 13m 55.44s , -56d 38m 28.5s) | C | 40 | 20.2 | 909 | 2025-08-27 07:55:31 | MASTER-OAFA | (00h 13m 55.59s , -56d 38m 27.9s) | C | 40 | 20.3 | 955 | 2025-08-27 07:56:18 | MASTER-OAFA | (00h 13m 55.72s , -56d 38m 27.0s) | C | 40 | 20.2 | 995 | 2025-08-27 07:56:18 | MASTER-OAFA | (00h 13m 55.72s , -56d 38m 27.0s) | C | 120 | 20.9 | Coadd 1001 | 2025-08-27 07:57:03 | MASTER-OAFA | (00h 13m 55.84s , -56d 38m 26.2s) | C | 40 | 20.2 | 1046 | 2025-08-27 07:57:49 | MASTER-OAFA | (00h 13m 55.97s , -56d 38m 25.7s) | C | 40 | 20.2 | 1093 | 2025-08-27 07:58:35 | MASTER-OAFA | (00h 13m 56.15s , -56d 38m 25.0s) | C | 40 | 20.2 | 1133 | 2025-08-27 07:58:35 | MASTER-OAFA | (00h 13m 56.15s , -56d 38m 25.0s) | C | 120 | 21.0 | Coadd 1138 | 2025-08-27 07:59:21 | MASTER-OAFA | (00h 13m 56.27s , -56d 38m 24.5s) | C | 40 | 20.2 | 1184 | 2025-08-27 08:00:06 | MASTER-OAFA | (00h 13m 56.41s , -56d 38m 23.7s) | C | 40 | 20.3 | 1230 | 2025-08-27 08:00:52 | MASTER-OAFA | (00h 13m 56.58s , -56d 38m 23.2s) | C | 40 | 20.2 | 1270 | 2025-08-27 08:00:52 | MASTER-OAFA | (00h 13m 56.57s , -56d 38m 23.2s) | C | 120 | 21.0 | Coadd 1276 | 2025-08-27 08:01:38 | MASTER-OAFA | (00h 13m 56.69s , -56d 38m 22.7s) | C | 40 | 20.2 | 1321 | 2025-08-27 08:02:24 | MASTER-OAFA | (00h 13m 56.85s , -56d 38m 21.8s) | C | 40 | 20.2 | 1367 | 2025-08-27 08:03:10 | MASTER-OAFA | (00h 13m 56.98s , -56d 38m 21.1s) | C | 40 | 20.2 | 1413 | 2025-08-27 08:03:55 | MASTER-OAFA | (00h 13m 57.14s , -56d 38m 20.5s) | C | 40 | 20.2 | 1458 | 2025-08-27 08:04:40 | MASTER-OAFA | (00h 13m 57.27s , -56d 38m 19.9s) | C | 40 | 20.2 | 1504 | 2025-08-27 08:05:26 | MASTER-OAFA | (00h 13m 57.42s , -56d 38m 19.4s) | C | 40 | 20.2 | 1544 | 2025-08-27 08:05:26 | MASTER-OAFA | (00h 13m 57.42s , -56d 38m 19.4s) | C | 120 | 21.0 | Coadd 1549 | 2025-08-27 08:06:12 | MASTER-OAFA | (00h 13m 57.57s , -56d 38m 18.6s) | C | 40 | 20.2 | 1595 | 2025-08-27 08:06:58 | MASTER-OAFA | (00h 13m 57.70s , -56d 38m 18.1s) | C | 40 | 20.2 | 1642 | 2025-08-27 08:07:44 | MASTER-OAFA | (00h 13m 57.88s , -56d 38m 17.5s) | C | 40 | 20.2 | 1687 | 2025-08-27 08:08:30 | MASTER-OAFA | (00h 13m 58.02s , -56d 38m 17.0s) | C | 40 | 20.1 | 1732 | 2025-08-27 08:09:15 | MASTER-OAFA | (00h 13m 58.16s , -56d 38m 16.4s) | C | 40 | 20.1 | 3986 | 2025-08-27 08:46:38 | MASTER-OAFA | (00h 14m 05.29s , -56d 37m 56.2s) | C | 60 | 19.4 | 4046 | 2025-08-27 08:46:38 | MASTER-OAFA | (00h 14m 05.29s , -56d 37m 56.2s) | C | 180 | 20.0 | Coadd 4061 | 2025-08-27 08:47:53 | MASTER-OAFA | (00h 14m 08.12s , -56d 36m 11.6s) | C | 60 | 19.3 | 4137 | 2025-08-27 08:49:10 | MASTER-OAFA | (00h 14m 03.64s , -56d 37m 03.5s) | C | 60 | 19.3 | 4215 | 2025-08-27 08:50:28 | MASTER-OAFA | (00h 14m 02.43s , -56d 36m 02.5s) | C | 60 | 19.3 | 4275 | 2025-08-27 08:50:28 | MASTER-OAFA | (00h 14m 02.43s , -56d 36m 02.5s) | C | 180 | 20.0 | Coadd 4291 | 2025-08-27 08:51:43 | MASTER-OAFA | (00h 14m 10.29s , -56d 37m 02.2s) | C | 60 | 19.4 | 4369 | 2025-08-27 08:53:01 | MASTER-OAFA | (00h 14m 02.79s , -56d 38m 01.6s) | C | 60 | 19.5 | 4447 | 2025-08-27 08:54:19 | MASTER-OAFA | (00h 14m 09.55s , -56d 37m 48.8s) | C | 60 | 19.4 | 4525 | 2025-08-27 08:55:37 | MASTER-OAFA | (00h 14m 07.17s , -56d 36m 15.0s) | C | 60 | 19.3 | 4599 | 2025-08-27 08:56:51 | MASTER-OAFA | (00h 14m 07.39s , -56d 37m 46.8s) | C | 60 | 19.3 | 4674 | 2025-08-27 08:58:07 | MASTER-OAFA | (00h 14m 10.80s , -56d 36m 08.9s) | C | 60 | 19.4 | 4750 | 2025-08-27 08:59:23 | MASTER-OAFA | (00h 14m 04.33s , -56d 36m 58.4s) | C | 60 | 19.4 | 4827 | 2025-08-27 09:00:39 | MASTER-OAFA | (00h 14m 05.93s , -56d 35m 57.5s) | C | 60 | 19.4 | 4900 | 2025-08-27 09:02:02 | MASTER-OAFA | (00h 14m 10.40s , -56d 36m 57.1s) | C | 40 | 20.2 | 4945 | 2025-08-27 09:02:48 | MASTER-OAFA | (00h 14m 10.58s , -56d 36m 56.9s) | C | 40 | 20.2 | 4991 | 2025-08-27 09:03:33 | MASTER-OAFA | (00h 14m 10.70s , -56d 36m 56.6s) | C | 40 | 20.1 | 5047 | 2025-08-27 09:04:30 | MASTER-OAFA | (00h 14m 04.87s , -56d 37m 56.3s) | C | 40 | 20.1 | 5093 | 2025-08-27 09:05:16 | MASTER-OAFA | (00h 14m 05.01s , -56d 37m 55.8s) | C | 40 | 20.1 | 5138 | 2025-08-27 09:06:01 | MASTER-OAFA | (00h 14m 05.17s , -56d 37m 55.6s) | C | 40 | 20.1 | 5193 | 2025-08-27 09:06:55 | MASTER-OAFA | (00h 14m 11.73s , -56d 37m 28.0s) | C | 40 | 20.1 | 5238 | 2025-08-27 09:07:41 | MASTER-OAFA | (00h 14m 11.89s , -56d 37m 27.8s) | C | 40 | 20.2 | 5284 | 2025-08-27 09:08:26 | MASTER-OAFA | (00h 14m 12.04s , -56d 37m 27.5s) | C | 40 | 20.2 | 5342 | 2025-08-27 09:09:25 | MASTER-OAFA | (00h 14m 09.80s , -56d 36m 14.9s) | C | 40 | 20.2 | 5388 | 2025-08-27 09:10:10 | MASTER-OAFA | (00h 14m 09.96s , -56d 36m 14.4s) | C | 40 | 20.1 | 5433 | 2025-08-27 09:10:56 | MASTER-OAFA | (00h 14m 10.12s , -56d 36m 14.4s) | C | 40 | 20.1 | 5489 | 2025-08-27 09:11:52 | MASTER-OAFA | (00h 14m 10.28s , -56d 37m 47.0s) | C | 40 | 20.1 | 5535 | 2025-08-27 09:12:37 | MASTER-OAFA | (00h 14m 10.41s , -56d 37m 46.7s) | C | 40 | 20.1 | 5580 | 2025-08-27 09:13:23 | MASTER-OAFA | (00h 14m 10.58s , -56d 37m 46.4s) | C | 40 | 20.2 | 5637 | 2025-08-27 09:14:19 | MASTER-OAFA | (00h 14m 12.90s , -56d 35m 55.2s) | C | 40 | 20.1 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39619] EP250827a: Las Cumbres independent detection of the optical counterpart
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41555 SUBJECT: EP250827a: Las Cumbres independent detection of the optical counterpart DATE: 25/08/27 09:02:44 GMT FROM: Wenxiong Li at NAOC <liwenxiong1992(a)gmail.com> Wenxiong Li (NAOC), Ido Keinan (TAU), Iair Arcavi (TAU), David Sand (U of Arizona) We observed the position of EP250827a (Hua et al. GCN 41553) with a Las Cumbres 1m telescope at CTIO starting on 2025-08-27 07:51:31 UT (less than 5 minutes after the alert and less than 11 minutes after the event). We obtained 2x300s exposures in the broad optical w band. We find a source at RA=3.4679 Dec=56.4979, not present in archival Legacy Survey (https://www.legacysurvey.org/viewer) images and measure a magnitude of 18.71 +- 0.06, calibrated to the r band (consistent with the Gemini detection; Levan et al. GCN 41554). Addition followup is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41555. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39618] EP250827a: Gemini-South optical counterpart detection
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41554 SUBJECT: EP250827a: Gemini-South optical counterpart detection DATE: 25/08/27 08:44:22 GMT FROM: Andrew Levan at Radboud University <a.levan(a)astro.ru.nl> A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), report on behalf of a larger collaboration: We observed the location of EP250827a (Hua et al. GCN 41553) with Gemini-South and the GMOS instrument. Observations began at 08:14 UT approximately 34 minutes after the EP trigger time. Within the WXT and FXT error circles we locate a new source in our first r-band exposure with a location of RA(J2000): 00:13:52.32 DEC(J2000) -56:29:52.9 The error is about 0.3" in each coordinate. The source is not visible in the Legacy Survey and has a magnitude of r = 18.70 +- 0.02 (calibrated against nearby stars from the SkyMapper catalog and not corrected for Galactic extinction). This is very likely the optical counterpart of EP250827a. We thank the staff of Gemini for their outstanding work in rapidly obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41554. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39617] EP250827a: Einstein Probe detection of an X-ray transient
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41553 SUBJECT: EP250827a: Einstein Probe detection of an X-ray transient DATE: 25/08/27 08:32:27 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. L. Hua (PMO,CAS), K. R. Ni (CCNU) , M. J. Liu and C. C. Jin (NAO,CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250827a. The transient triggered EP-WXT (ID: 01709200787) at 2025-08-27T07:40:43 (UTC). The WXT position of the source is R.A. = 3.488 deg, DEC = -56.463 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 3.46915 deg, DEC = -56.4998 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41553. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39616] LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41552 SUBJECT: LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate DATE: 25/08/27 02:45:38 GMT FROM: Jyotirmaya Mohanta at University of Tsukuba <s2430161(a)u.tsukuba.ac.jp> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250827l during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 01:27:57.538 UTC (GPS time: 1440293295.538). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines. S250827l is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250827l The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time. * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1145 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2132 +/- 580 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [5] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41552. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39615] Fermi GRB 250826A: Global MASTER-Net observations report
by GCN Circulars 27 Aug '25

27 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41551 SUBJECT: Fermi GRB 250826A: Global MASTER-Net observations report DATE: 25/08/27 00:30:41 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 250826A ( Fermi GBM team, GCN 41545) errorbox 61289 sec after notice time and 61324 sec after trigger time at 2025-08-27 00:26:06 UT, with upper limit up to 16.5 mag. The observations began at zenith distance = 80 deg. The sun altitude is -20.9 deg. The galactic latitude b = 47 deg., longitude l = 190 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2977441 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 61355 | 2025-08-27 00:26:06 | MASTER-Kislovodsk | (09h 12m 54.84s , +37d 15m 47.2s) | C | 60 | 16.2 | 61355 | 2025-08-27 00:26:06 | MASTER-Kislovodsk | (09h 17m 46.54s , +36d 51m 26.0s) | C | 60 | 16.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41551. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39614] GRB 250806A: HiPERCAM/GTC limits on a possible supernova
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41550 SUBJECT: GRB 250806A: HiPERCAM/GTC limits on a possible supernova DATE: 25/08/26 16:15:04 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), and D. Gonzalez Gonzalez (GTC) report: We observed the field of GRB 250806A (Xie et al., GCN 41243) with the 5-band HiPERCAM imager mounted on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted of 20 x 60 s exposures in the u, g, r, i, and z bands, with mean epoch 2025-08-23 01:51:07 UT (16.75 days after the burst). Our images have 3-sigma detection limits of u > 26.1, g > 26.4, r > 25.9, i > 25.6, and z > 24.7 mag as compared to SDSS field stars (all the magnitudes in this GCN are in the AB system). Within the refined XRT error box (Campana et al., GCN 41249) we clearly detect the two galaxies mentioned by Malesani et al. (GCN 41279), the brightest of which has been proposed as the host of GRB 250806A and has a measured redshift of 0.367 (Sevilla et al., GCN 41499). Under the assumption that the GRB is indeed associated with this galaxy, its low redshift prompted us to perform a search for a possible emerging supernova (SN). The time of our observation would correspond to 14.7 days after the burst in the rest frame, close to the time of expected maximum light. No other significant new source is detected within the XRT error box beyond the mentioned galaxies. Image subtraction with respect to the Legacy Survey (LS) does not reveal any significant residuals, although we note that the reference images are significantly shallower than our data. A deeper search for the SN component will require obtaining deeper templates once the possible SN has faded away. Assuming as limits for the subtraction images the depth of the Legacy Survey, we would not be detecting a SN component down to r > 25.0 mag and i > 24.5 mag. At a redshift of 0.367, this is equivalent to absolute magnitudes of M_4500AA (g-band) > -16.1 and M_5500AA (V-band) > -16.6, or approximately 2.7 magnitudes fainter than SN 1998bw in each of these bands. This could indicate a SN intrinsically fainter than SN 1998bw by a comparable amount, or might indicate line-of-sight extinction of A_V > 2.7 mag. The latter option would be consistent with the lack of optical/NIR afterglow detection (Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Xin et al., GCN 41250; Zheng et al., GCN 41251; Schneider et al., GCN 41253; Malesani et al., GCN 41279; Pankov et al., GCN 41285). As an alternative, the galaxy at z = 0.367 might be of course a chance association, and the GRB might be simply further away. View this GCN Circular online at https://gcn.nasa.gov/circulars/41550. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39613] EP250821a: Gemini GMOS Redshift Confirmation
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41549 SUBJECT: EP250821a: Gemini GMOS Redshift Confirmation DATE: 25/08/26 15:49:43 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2(a)andrew.cmu.edu> Brendan O'Connor (CMU), Xander Hall (CMU), Antonella Palmese (CMU) report on behalf of a larger collaboration: We observed the location of EP250821a (Hu et al. GCN 41459; Liang et al., GCN 41467) with the Gemini GMOS-S spectrograph starting on 2025-08-26 at 01:53:43 UT. Our spectra cover wavelengths 575 to 1060 nm and consist of 4x1200 s exposures using the R400 grating. In a preliminary reduction, at the location of the optical counterpart (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485), we clearly detect a bright trace. We detect multiple emission lines from the underlying host galaxy, including [OII], [OIII], Hbeta, [NII], and Halpha, at a consistent redshift of z=0.577. This confirms the previous spectroscopic redshift obtained by the VLT (An et al., GCN 41490). We thank the staff of the Gemini Observatory, in particular Karleyne Silva, Cinthya Rodriguez, Aleksandar Cikota, Emanuele Farina, for assistance with scheduling and obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41549. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39612] LIGO/Virgo/KAGRA S250818k: SAO RAS BTA z-band observations of AT2025ulz
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41548 SUBJECT: LIGO/Virgo/KAGRA S250818k: SAO RAS BTA z-band observations of AT2025ulz DATE: 25/08/26 11:49:10 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen(a)gmail.com> N. Pankov (HSE, IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), S. Kotov (SAO RAS), A. Volnova (IKI) report on behalf of the IKI-GRB-FuN collaboration: We observed the field of an optical transient AT2025ulz detected by ZTF (Stein et. al, GCN 41414) in the localization region of a candidate binary neutron star merger S250818k (LVK, GCN 41437). The observations with BTA 6-meter telescope of SAO RAS started on 2025-08-22 at 17:18:46 UT, i.e. ~4.67 days since LVK trigger. We obtained 2.4 ks exposure in the z'-band using the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev, 2011, BaltA, 20, 363) installed in the prime focus of the telescope. We applied image subtraction using LS DR10 z-band image as reference. We clearly detect the optical source in the difference image at the position given by ZTF (Stein et. al, GCN 41414). The preliminary photometry of the source is presented below: Date UTstart Exptime,s T-T0,days Filter OT Err UL(3sigma) FWHM," ---------- -------- ------- ------- -- ----- ---- ---- --- 2025-08-22 17:18:46 2400.13 4.67963 z' 24.09 0.24 24.3 1.8 The photometry was calibrated using comparison stars from SDSS-DR12 and not corrected for the Galactic extinction. The OT was also followed by (Busmann et. al, GCN 41421; Hall et. al, GCN 41433; Karambelkar et. al, GCN 41436; Nicholl et. al, GCN 41439; Srivastav et. al, GCN 41451; O’Connor et. al, GCN 41452; Gillanders et. al, GCN 41454; Mo et. al, GCN 41456; Liu et. al, GCN 41461; Ackley et. al, GCN 41468; Klose et. al, GCN 41474; Banerjee et. al, GCN 41476; Perley et. al, GCN 41480; D'Avanzo et. al, GCN 41489; Malesani et. al, GCN 41492; Smartt et. al, GCN 41493; Santos et. al, GCN 41501; Becerra et. al, GCN 41502; An et. al, GCN 41503; Passaleva et. al, GCN 41504; Taguchi et. al, GCN 41505; Troja et. al, GCN 41506; Freeburn et. al, GCN 41507; Angulo et. al, GCN 41518; Antier et. al, GCN 41519; Banerjee et. al, GCN 41532; Lipunov et. al, GCN 41534; Busmann et. al, GCN 41535; Kasliwal et. al, GCN 41538; Gillanders et. al, GCN 41540; Becerra et. al, GCN 41544). We note that our measurements may be affected during image subtraction and should be considered as a lower limit on the source brightness. View this GCN Circular online at https://gcn.nasa.gov/circulars/41548. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39611] Fermi trigger No 777883624: Global MASTER-Net observations report
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41547 SUBJECT: Fermi trigger No 777883624: Global MASTER-Net observations report DATE: 25/08/26 08:30:49 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB250826.28 (trigger No 777883624,23h 40m 02.40s , -64d 12m 00.0s, R=4.94) errorbox 90 sec after notice time and 132 sec after trigger time at 2025-08-26 06:49:11 UT, with upper limit up to 20.3 mag. The observations began at zenith distance = 34 deg. The sun altitude is -53.0 deg. The galactic latitude b = -52 deg., longitude l = 315 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2977396 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 147 | 2025-08-26 06:49:11 | MASTER-OAFA | (23h 39m 27.05s , -64d 23m 49.4s) | C | 30 | 19.9 | 183 | 2025-08-26 06:49:47 | MASTER-OAFA | (23h 39m 27.12s , -64d 23m 50.0s) | C | 30 | 19.9 | 224 | 2025-08-26 06:50:23 | MASTER-OAFA | (23h 39m 27.23s , -64d 23m 50.4s) | C | 40 | 19.9 | 275 | 2025-08-26 06:51:09 | MASTER-OAFA | (23h 39m 27.37s , -64d 23m 50.7s) | C | 50 | 19.9 | 336 | 2025-08-26 06:52:05 | MASTER-OAFA | (23h 39m 27.59s , -64d 23m 50.9s) | C | 60 | 20.3 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41547. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39610] Fermi Gamma-ray Burst Monitor trigger 777883624/250826283 is not a GRB
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41546 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 777883624/250826283 is not a GRB DATE: 25/08/26 08:16:30 GMT FROM: Rushikesh Sonawane at IISER, TVM <rushikesh23(a)iisertvm.ac.in> R. Sonawane (IISER, TVM) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 777883624/250826283 at 06:46:59.09 UT on 26 August 2025, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to local particles." View this GCN Circular online at https://gcn.nasa.gov/circulars/41546. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39609] GRB 250826A: Fermi GBM Final Real-time Localization
by GCN Circulars 26 Aug '25

26 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41545 SUBJECT: GRB 250826A: Fermi GBM Final Real-time Localization DATE: 25/08/26 07:34:29 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 07:24:01 UT on 26 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250826A (trigger 777885846.969916 / 250826308). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 141.8, Dec = 34.7 (J2000 degrees, equivalent to J2000 09h 27m, 34d 42'), with a statistical uncertainty of 3.6 degrees. The angle from the Fermi LAT boresight is 40.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250826308/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250826308/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250826308/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41545. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39608] LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Confirmation of Rebrightening of AT2025ulz
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41544 SUBJECT: LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Confirmation of Rebrightening of AT2025ulz DATE: 25/08/25 19:06:17 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Rosa L. Becerra (U Rome), Yuhan Yang (U Rome), Alan M. Watson (UNAM), Eleonora Troja (U Rome) and William H. Lee (UNAM) report on behalf of a larger team: We continued monitoring AT2025ulz/ZTF25abjmnps, initially reported by Stein et al. (GCN 41414) within the localization region of the candidate gravitational wave event S250818k (LVK Collaboration, GCN 41437, 41440), using the OSIRIS instrument mounted on the GTC telescope. Our observations commenced at 2025-08-24 21:18 UTC (6.8 days after the trigger) and were carried out in the g, r, i and z filters at an average airmass of ~1.3. By comparison with our first epoch at T+3.9 days (Becerra et al., GCN 41502), we confirm a significant rebrightening in both the r and z bands, measuring: delta_r = 0.9 +/- 0.3 delta_z = 0.6 +/- 0.1 Our observations are in agreement with the rebrightening observed in i-band (Freeburn et al. GCN 41507; Angulo et al. GCN 41518; Busmann et al. GCN 41535), a behavior that is not expected for kilonova emission. Further observations are planned. We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41544. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39607] Fermi GRB 250824B: Global MASTER-Net observations report
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41543 SUBJECT: Fermi GRB 250824B: Global MASTER-Net observations report DATE: 25/08/25 17:16:08 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 250824B ( Fermi GBM team, GCN 41527) errorbox 33953 sec after notice time and 33960 sec after trigger time at 2025-08-24 23:04:13 UT, with upper limit up to 23.0 mag. Observations started at twilight. The observations began at zenith distance = 4 deg. The sun altitude is -11.4 deg. MASTER-SAAO robotic telescope located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250824B errorbox 1 days 11812 sec after notice time and 1 days 11818 sec after trigger time at 2025-08-25 16:55:12 UT, with upper limit up to 17.7 mag. Observations started at twilight. The observations began at zenith distance = 3 deg. The sun altitude is -8.7 deg. The galactic latitude b = 8 deg., longitude l = 346 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2976525 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 33990 | 2025-08-24 23:04:13 | MASTER-OAFA | (16h 35m 33.85s , -34d 39m 23.0s) | C | 60 | 17.6 | 34050 | 2025-08-24 23:04:13 | MASTER-OAFA | (16h 35m 33.85s , -34d 39m 23.0s) | C | 180 | 18.0 | Coadd 34067 | 2025-08-24 23:05:30 | MASTER-OAFA | (16h 45m 12.85s , -34d 40m 49.9s) | C | 60 | 17.6 | 34127 | 2025-08-24 23:05:30 | MASTER-OAFA | (16h 45m 12.85s , -34d 40m 50.0s) | C | 180 | 18.3 | Coadd 34143 | 2025-08-24 23:06:46 | MASTER-OAFA | (16h 35m 35.17s , -34d 40m 03.8s) | C | 60 | 17.7 | 34218 | 2025-08-24 23:08:02 | MASTER-OAFA | (16h 45m 15.27s , -34d 38m 56.7s) | C | 60 | 17.8 | 34294 | 2025-08-24 23:09:18 | MASTER-OAFA | (16h 35m 31.47s , -34d 40m 14.9s) | C | 60 | 17.7 | 34372 | 2025-08-24 23:10:35 | MASTER-OAFA | (16h 21m 40.27s , -36d 32m 28.1s) | C | 60 | 18.2 | 34448 | 2025-08-24 23:11:52 | MASTER-OAFA | (16h 31m 38.29s , -36d 31m 55.2s) | C | 60 | 18.0 | 34525 | 2025-08-24 23:13:08 | MASTER-OAFA | (16h 21m 47.37s , -36d 32m 26.9s) | C | 60 | 18.3 | 34601 | 2025-08-24 23:14:25 | MASTER-OAFA | (16h 31m 38.55s , -36d 33m 53.4s) | C | 60 | 18.0 | 34677 | 2025-08-24 23:15:40 | MASTER-OAFA | (16h 21m 47.32s , -36d 32m 56.9s) | C | 60 | 18.2 | 34752 | 2025-08-24 23:16:55 | MASTER-OAFA | (16h 17m 03.71s , -34d 39m 09.2s) | C | 60 | 18.4 | 34828 | 2025-08-24 23:18:11 | MASTER-OAFA | (16h 26m 50.74s , -34d 38m 10.8s) | C | 60 | 18.2 | 34903 | 2025-08-24 23:19:27 | MASTER-OAFA | (16h 17m 01.48s , -34d 38m 26.9s) | C | 60 | 18.1 | 34980 | 2025-08-24 23:20:43 | MASTER-OAFA | (16h 26m 45.27s , -34d 37m 50.9s) | C | 60 | 18.1 | 35056 | 2025-08-24 23:21:59 | MASTER-OAFA | (16h 17m 06.81s , -34d 38m 26.1s) | C | 60 | 18.4 | 35132 | 2025-08-24 23:23:15 | MASTER-OAFA | (16h 31m 46.56s , -32d 46m 14.8s) | C | 60 | 18.3 | 35192 | 2025-08-24 23:23:15 | MASTER-OAFA | (16h 31m 46.56s , -32d 46m 14.8s) | C | 180 | 18.4 | Coadd 35207 | 2025-08-24 23:24:31 | MASTER-OAFA | (16h 41m 14.46s , -32d 44m 51.6s) | C | 60 | 17.9 | 35267 | 2025-08-24 23:24:31 | MASTER-OAFA | (16h 41m 14.46s , -32d 44m 51.7s) | C | 180 | 18.0 | Coadd 35284 | 2025-08-24 23:25:47 | MASTER-OAFA | (16h 31m 43.58s , -32d 46m 05.1s) | C | 60 | 18.2 | 35359 | 2025-08-24 23:27:03 | MASTER-OAFA | (16h 41m 17.58s , -32d 44m 52.4s) | C | 60 | 17.9 | 35436 | 2025-08-24 23:28:19 | MASTER-OAFA | (16h 31m 40.60s , -32d 45m 16.7s) | C | 60 | 18.2 | 35513 | 2025-08-24 23:29:37 | MASTER-OAFA | (16h 13m 41.38s , -32d 44m 35.6s) | C | 60 | 18.4 | 35591 | 2025-08-24 23:30:54 | MASTER-OAFA | (16h 23m 06.58s , -32d 45m 55.9s) | C | 60 | 18.3 | 35667 | 2025-08-24 23:32:10 | MASTER-OAFA | (16h 13m 42.72s , -32d 45m 24.3s) | C | 60 | 18.2 | 35743 | 2025-08-24 23:33:26 | MASTER-OAFA | (16h 23m 10.45s , -32d 44m 24.2s) | C | 60 | 18.3 | 35819 | 2025-08-24 23:34:42 | MASTER-OAFA | (16h 13m 40.11s , -32d 45m 05.5s) | C | 60 | 18.1 | 35896 | 2025-08-24 23:35:59 | MASTER-OAFA | (16h 29m 04.22s , -30d 51m 29.6s) | C | 60 | 18.2 | 35956 | 2025-08-24 23:35:59 | MASTER-OAFA | (16h 29m 04.21s , -30d 51m 29.6s) | C | 180 | 18.8 | Coadd 35971 | 2025-08-24 23:37:14 | MASTER-OAFA | (16h 38m 25.02s , -30d 50m 49.4s) | C | 60 | 17.8 | 36046 | 2025-08-24 23:38:30 | MASTER-OAFA | (16h 29m 11.62s , -30d 51m 29.4s) | C | 60 | 18.2 | 36121 | 2025-08-24 23:39:45 | MASTER-OAFA | (16h 38m 23.42s , -30d 52m 48.9s) | C | 60 | 17.6 | 36197 | 2025-08-24 23:41:01 | MASTER-OAFA | (16h 29m 11.50s , -30d 52m 17.2s) | C | 60 | 18.1 | 36273 | 2025-08-24 23:42:16 | MASTER-OAFA | (16h 45m 18.86s , -34d 38m 42.1s) | C | 60 | 17.6 | 36348 | 2025-08-24 23:43:31 | MASTER-OAFA | (16h 45m 19.00s , -34d 40m 22.5s) | C | 60 | 17.6 | 36425 | 2025-08-24 23:44:48 | MASTER-OAFA | (16h 45m 21.36s , -34d 38m 28.6s) | C | 60 | 17.6 | 36501 | 2025-08-24 23:46:04 | MASTER-OAFA | (16h 35m 33.55s , -34d 39m 01.5s) | C | 60 | 17.5 | 36575 | 2025-08-24 23:47:19 | MASTER-OAFA | (16h 35m 33.38s , -34d 38m 01.0s) | C | 60 | 17.5 | 36649 | 2025-08-24 23:48:32 | MASTER-OAFA | (16h 35m 38.91s , -34d 39m 00.9s) | C | 60 | 17.5 | 36727 | 2025-08-24 23:49:50 | MASTER-OAFA | (16h 31m 43.42s , -36d 33m 30.7s) | C | 60 | 17.6 | 36801 | 2025-08-24 23:51:04 | MASTER-OAFA | (16h 31m 50.46s , -36d 33m 30.5s) | C | 60 | 17.7 | 36861 | 2025-08-24 23:51:04 | MASTER-OAFA | (16h 31m 50.46s , -36d 33m 30.6s) | C | 180 | 18.3 | Coadd 36877 | 2025-08-24 23:52:21 | MASTER-OAFA | (16h 31m 47.08s , -36d 31m 35.2s) | C | 60 | 17.7 | 36952 | 2025-08-24 23:53:36 | MASTER-OAFA | (16h 21m 49.89s , -36d 32m 43.5s) | C | 60 | 18.2 | 37027 | 2025-08-24 23:54:50 | MASTER-OAFA | (16h 21m 52.23s , -36d 31m 13.0s) | C | 60 | 18.0 | 37103 | 2025-08-24 23:56:07 | MASTER-OAFA | (16h 21m 46.32s , -36d 32m 04.0s) | C | 60 | 18.0 | 37179 | 2025-08-24 23:57:22 | MASTER-OAFA | (16h 26m 49.33s , -34d 37m 37.5s) | C | 60 | 17.8 | 37253 | 2025-08-24 23:58:36 | MASTER-OAFA | (16h 26m 54.89s , -34d 38m 37.4s) | C | 60 | 17.8 | 37330 | 2025-08-24 23:59:54 | MASTER-OAFA | (16h 26m 49.70s , -34d 39m 36.7s) | C | 60 | 17.9 | 37406 | 2025-08-25 00:01:09 | MASTER-OAFA | (16h 17m 11.71s , -34d 39m 08.6s) | C | 60 | 18.0 | 37483 | 2025-08-25 00:02:26 | MASTER-OAFA | (16h 17m 09.75s , -34d 37m 39.5s) | C | 60 | 18.1 | 37560 | 2025-08-25 00:03:44 | MASTER-OAFA | (16h 17m 09.94s , -34d 39m 03.3s) | C | 60 | 18.2 | 37638 | 2025-08-25 00:05:01 | MASTER-OAFA | (16h 41m 20.88s , -32d 44m 38.6s) | C | 60 | 17.7 | 37716 | 2025-08-25 00:06:19 | MASTER-OAFA | (16h 41m 15.67s , -32d 45m 31.0s) | C | 60 | 17.7 | 37776 | 2025-08-25 00:06:19 | MASTER-OAFA | (16h 41m 15.62s , -32d 45m 30.6s) | C | 180 | 17.9 | Coadd 37794 | 2025-08-25 00:07:37 | MASTER-OAFA | (16h 41m 15.13s , -32d 44m 30.9s) | C | 60 | 17.8 | 37870 | 2025-08-25 00:08:53 | MASTER-OAFA | (16h 31m 52.02s , -32d 45m 11.1s) | C | 60 | 18.2 | 37945 | 2025-08-25 00:10:08 | MASTER-OAFA | (16h 31m 45.37s , -32d 46m 10.7s) | C | 60 | 18.3 | 38021 | 2025-08-25 00:11:24 | MASTER-OAFA | (16h 31m 52.37s , -32d 45m 55.4s) | C | 60 | 18.2 | 38097 | 2025-08-25 00:12:41 | MASTER-OAFA | (16h 23m 15.22s , -32d 44m 03.7s) | C | 60 | 18.4 | 38172 | 2025-08-25 00:13:55 | MASTER-OAFA | (16h 23m 15.38s , -32d 45m 37.0s) | C | 60 | 18.4 | 38246 | 2025-08-25 00:15:09 | MASTER-OAFA | (16h 23m 18.45s , -32d 44m 02.9s) | C | 60 | 18.4 | 38322 | 2025-08-25 00:16:26 | MASTER-OAFA | (16h 13m 42.34s , -32d 44m 36.1s) | C | 60 | 18.5 | 38382 | 2025-08-25 00:16:26 | MASTER-OAFA | (16h 13m 42.34s , -32d 44m 36.2s) | C | 180 | 19.3 | Coadd 38397 | 2025-08-25 00:17:40 | MASTER-OAFA | (16h 13m 42.50s , -32d 43m 36.5s) | C | 60 | 18.5 | 38472 | 2025-08-25 00:18:55 | MASTER-OAFA | (16h 13m 48.41s , -32d 44m 36.7s) | C | 60 | 18.6 | 38551 | 2025-08-25 00:20:14 | MASTER-OAFA | (16h 38m 28.04s , -30d 52m 51.3s) | C | 60 | 18.1 | 38629 | 2025-08-25 00:21:32 | MASTER-OAFA | (16h 38m 35.38s , -30d 52m 26.8s) | C | 60 | 18.1 | 38707 | 2025-08-25 00:22:50 | MASTER-OAFA | (16h 38m 31.94s , -30d 51m 07.4s) | C | 60 | 18.1 | 38782 | 2025-08-25 00:24:05 | MASTER-OAFA | (16h 29m 12.64s , -30d 52m 29.6s) | C | 60 | 19.0 | 38860 | 2025-08-25 00:25:24 | MASTER-OAFA | (16h 29m 15.31s , -30d 50m 59.5s) | C | 60 | 19.1 | 38920 | 2025-08-25 00:25:24 | MASTER-OAFA | (16h 29m 15.31s , -30d 50m 59.5s) | C | 180 | 18.7 | Coadd 38936 | 2025-08-25 00:26:39 | MASTER-OAFA | (16h 29m 10.42s , -30d 51m 37.5s) | C | 60 | 19.2 | 39014 | 2025-08-25 00:27:57 | MASTER-OAFA | (16h 45m 23.00s , -34d 38m 11.4s) | C | 60 | 18.4 | 39087 | 2025-08-25 00:29:10 | MASTER-OAFA | (16h 45m 28.91s , -34d 39m 11.4s) | C | 60 | 18.4 | 39147 | 2025-08-25 00:29:10 | MASTER-OAFA | (16h 45m 28.91s , -34d 39m 11.4s) | C | 180 | 18.0 | Coadd 39165 | 2025-08-25 00:30:28 | MASTER-OAFA | (16h 45m 22.63s , -34d 40m 11.1s) | C | 60 | 18.4 | 39240 | 2025-08-25 00:31:43 | MASTER-OAFA | (16h 35m 44.77s , -34d 39m 41.5s) | C | 60 | 18.5 | 39318 | 2025-08-25 00:33:01 | MASTER-OAFA | (16h 35m 42.48s , -34d 37m 51.6s) | C | 60 | 18.5 | 39393 | 2025-08-25 00:34:17 | MASTER-OAFA | (16h 35m 42.66s , -34d 39m 40.6s) | C | 60 | 18.5 | 39468 | 2025-08-25 00:35:31 | MASTER-OAFA | (16h 31m 57.69s , -36d 31m 21.2s) | C | 60 | 18.6 | 39543 | 2025-08-25 00:36:47 | MASTER-OAFA | (16h 31m 51.08s , -36d 32m 13.9s) | C | 60 | 18.6 | 39621 | 2025-08-25 00:38:04 | MASTER-OAFA | (16h 31m 51.67s , -36d 31m 13.9s) | C | 60 | 18.6 | 39741 | 2025-08-25 00:40:04 | MASTER-OAFA | (16h 22m 00.21s , -36d 43m 40.2s) | C | 60 | 18.7 | 39818 | 2025-08-25 00:41:22 | MASTER-OAFA | (16h 21m 54.49s , -36d 44m 39.5s) | C | 60 | 18.7 | 39895 | 2025-08-25 00:42:38 | MASTER-OAFA | (16h 22m 00.31s , -36d 44m 11.6s) | C | 60 | 18.7 | 39971 | 2025-08-25 00:43:54 | MASTER-OAFA | (16h 27m 00.83s , -34d 49m 15.4s) | C | 60 | 23.0 | 40046 | 2025-08-25 00:45:09 | MASTER-OAFA | (16h 27m 00.88s , -34d 50m 21.8s) | C | 60 | 18.6 | 40122 | 2025-08-25 00:46:25 | MASTER-OAFA | (16h 27m 03.85s , -34d 49m 14.4s) | C | 60 | 23.0 | 40197 | 2025-08-25 00:47:41 | MASTER-OAFA | (16h 17m 14.37s , -34d 49m 23.6s) | C | 60 | 18.7 | 40275 | 2025-08-25 00:48:58 | MASTER-OAFA | (16h 17m 14.82s , -34d 48m 22.8s) | C | 60 | 18.7 | 40350 | 2025-08-25 00:50:13 | MASTER-OAFA | (16h 17m 19.54s , -34d 49m 22.7s) | C | 60 | 18.7 | 40428 | 2025-08-25 00:51:32 | MASTER-OAFA | (16h 41m 24.66s , -32d 57m 24.9s) | C | 60 | 18.5 | 40503 | 2025-08-25 00:52:46 | MASTER-OAFA | (16h 41m 30.48s , -32d 56m 55.6s) | C | 60 | 18.4 | 40582 | 2025-08-25 00:54:06 | MASTER-OAFA | (16h 41m 28.42s , -32d 55m 43.1s) | C | 60 | 18.5 | 40659 | 2025-08-25 00:55:23 | MASTER-OAFA | (16h 31m 57.16s , -32d 56m 30.8s) | C | 60 | 19.0 | 40719 | 2025-08-25 00:55:23 | MASTER-OAFA | (16h 31m 57.16s , -32d 56m 30.8s) | C | 180 | 19.5 | Coadd 40737 | 2025-08-25 00:56:40 | MASTER-OAFA | (16h 31m 59.77s , -32d 55m 15.2s) | C | 60 | 19.2 | 40813 | 2025-08-25 00:57:56 | MASTER-OAFA | (16h 31m 54.45s , -32d 55m 55.8s) | C | 60 | 19.0 | 40889 | 2025-08-25 00:59:12 | MASTER-OAFA | (16h 23m 20.33s , -32d 54m 34.1s) | C | 60 | 19.5 | 40964 | 2025-08-25 01:00:27 | MASTER-OAFA | (16h 23m 27.22s , -32d 55m 34.0s) | C | 60 | 19.5 | 41024 | 2025-08-25 01:00:27 | MASTER-OAFA | (16h 23m 27.22s , -32d 55m 34.1s) | C | 180 | 19.8 | Coadd 41049 | 2025-08-25 01:01:52 | MASTER-OAFA | (16h 23m 20.82s , -32d 56m 33.3s) | C | 60 | 19.5 | 41124 | 2025-08-25 01:03:07 | MASTER-OAFA | (16h 13m 54.25s , -32d 56m 10.7s) | C | 60 | 19.4 | 41202 | 2025-08-25 01:04:25 | MASTER-OAFA | (16h 13m 52.54s , -32d 54m 14.5s) | C | 60 | 19.4 | 41277 | 2025-08-25 01:05:40 | MASTER-OAFA | (16h 13m 52.58s , -32d 56m 07.8s) | C | 60 | 19.3 | 41354 | 2025-08-25 01:06:57 | MASTER-OAFA | (16h 38m 43.17s , -31d 01m 29.4s) | C | 60 | 18.5 | 41430 | 2025-08-25 01:08:13 | MASTER-OAFA | (16h 38m 38.55s , -31d 02m 09.8s) | C | 60 | 18.5 | 41507 | 2025-08-25 01:09:30 | MASTER-OAFA | (16h 38m 38.27s , -31d 01m 09.0s) | C | 60 | 18.5 | 41582 | 2025-08-25 01:10:45 | MASTER-OAFA | (16h 29m 24.82s , -31d 01m 46.2s) | C | 60 | 19.5 | 41661 | 2025-08-25 01:12:04 | MASTER-OAFA | (16h 29m 19.49s , -31d 02m 45.6s) | C | 60 | 19.6 | 41740 | 2025-08-25 01:13:23 | MASTER-OAFA | (16h 29m 24.61s , -31d 02m 32.0s) | C | 60 | 19.5 | 41820 | 2025-08-25 01:14:43 | MASTER-OAFA | (16h 45m 32.41s , -34d 49m 49.3s) | C | 60 | 18.8 | 41895 | 2025-08-25 01:15:59 | MASTER-OAFA | (16h 45m 32.47s , -34d 51m 02.0s) | C | 60 | 18.8 | 41972 | 2025-08-25 01:17:15 | MASTER-OAFA | (16h 45m 35.46s , -34d 49m 48.7s) | C | 60 | 18.8 | 42048 | 2025-08-25 01:18:32 | MASTER-OAFA | (16h 31m 54.17s , -36d 43m 48.6s) | C | 60 | 18.6 | 42124 | 2025-08-25 01:19:48 | MASTER-OAFA | (16h 31m 54.28s , -36d 42m 48.2s) | C | 60 | 18.6 | 42200 | 2025-08-25 01:21:04 | MASTER-OAFA | (16h 32m 02.27s , -36d 43m 48.4s) | C | 60 | 18.6 | 42277 | 2025-08-25 01:22:21 | MASTER-OAFA | (16h 27m 00.81s , -34d 50m 36.1s) | C | 60 | 23.0 | 42352 | 2025-08-25 01:23:35 | MASTER-OAFA | (16h 27m 05.57s , -34d 50m 26.4s) | C | 60 | 23.0 | 42429 | 2025-08-25 01:24:52 | MASTER-OAFA | (16h 27m 03.53s , -34d 48m 58.9s) | C | 60 | 23.0 | 42505 | 2025-08-25 01:26:08 | MASTER-OAFA | (16h 41m 30.59s , -32d 57m 02.3s) | C | 60 | 18.4 | 42580 | 2025-08-25 01:27:24 | MASTER-OAFA | (16h 41m 33.91s , -32d 55m 37.4s) | C | 60 | 18.5 | 42659 | 2025-08-25 01:28:42 | MASTER-OAFA | (16h 41m 26.77s , -32d 56m 08.2s) | C | 60 | 18.5 | 42738 | 2025-08-25 01:30:02 | MASTER-OAFA | (16h 11m 42.80s , -31d 01m 05.8s) | C | 60 | 19.6 | 42814 | 2025-08-25 01:31:17 | MASTER-OAFA | (16h 20m 56.77s , -31d 02m 24.3s) | C | 60 | 19.6 | 42890 | 2025-08-25 01:32:33 | MASTER-OAFA | (16h 11m 42.58s , -31d 01m 52.9s) | C | 60 | 19.6 | 42966 | 2025-08-25 01:33:49 | MASTER-OAFA | (16h 20m 59.81s , -31d 00m 51.5s) | C | 60 | 19.6 | 43042 | 2025-08-25 01:35:05 | MASTER-OAFA | (16h 11m 39.84s , -31d 01m 54.5s) | C | 60 | 19.6 | 43121 | 2025-08-25 01:36:24 | MASTER-OAFA | (16h 23m 28.18s , -32d 54m 32.8s) | C | 60 | 19.5 | 43196 | 2025-08-25 01:37:39 | MASTER-OAFA | (16h 23m 22.49s , -32d 55m 28.0s) | C | 60 | 19.5 | 43284 | 2025-08-25 01:39:07 | MASTER-OAFA | (16h 23m 22.23s , -32d 54m 27.9s) | C | 60 | 19.5 | 43362 | 2025-08-25 01:40:25 | MASTER-OAFA | (16h 38m 45.90s , -31d 02m 01.6s) | C | 60 | 18.3 | 43435 | 2025-08-25 01:41:39 | MASTER-OAFA | (16h 38m 40.56s , -31d 03m 01.5s) | C | 60 | 18.4 | 43513 | 2025-08-25 01:42:56 | MASTER-OAFA | (16h 38m 46.40s , -31d 02m 33.2s) | C | 60 | 18.4 | 44833 | 2025-08-25 02:04:56 | MASTER-OAFA | (16h 21m 04.33s , -31d 00m 58.7s) | C | 60 | 19.5 | 44910 | 2025-08-25 02:06:13 | MASTER-OAFA | (16h 20m 58.26s , -31d 01m 38.5s) | C | 60 | 19.4 | 44970 | 2025-08-25 02:06:13 | MASTER-OAFA | (16h 20m 58.26s , -31d 01m 38.6s) | C | 180 | 19.6 | Coadd 44986 | 2025-08-25 02:07:29 | MASTER-OAFA | (16h 20m 58.30s , -31d 00m 38.4s) | C | 60 | 19.5 | 45063 | 2025-08-25 02:08:47 | MASTER-OAFA | (16h 11m 44.82s , -31d 01m 23.1s) | C | 60 | 19.5 | 45138 | 2025-08-25 02:10:02 | MASTER-OAFA | (16h 11m 38.38s , -31d 02m 23.4s) | C | 60 | 19.4 | 45214 | 2025-08-25 02:11:17 | MASTER-OAFA | (16h 11m 44.92s , -31d 02m 18.3s) | C | 60 | 19.5 | 47426 | 2025-08-25 02:48:09 | MASTER-OAFA | (16h 21m 03.31s , -31d 02m 28.1s) | C | 60 | 19.3 | 47503 | 2025-08-25 02:49:26 | MASTER-OAFA | (16h 21m 06.15s , -31d 01m 10.4s) | C | 60 | 19.5 | 47579 | 2025-08-25 02:50:42 | MASTER-OAFA | (16h 11m 39.75s , -31d 01m 45.0s) | C | 60 | 19.4 | 47653 | 2025-08-25 02:51:57 | MASTER-OAFA | (16h 11m 41.24s , -31d 00m 45.8s) | C | 60 | 19.4 | 47729 | 2025-08-25 02:53:13 | MASTER-OAFA | (16h 11m 47.36s , -31d 01m 46.3s) | C | 60 | 19.4 | 49692 | 2025-08-25 03:25:56 | MASTER-OAFA | (16h 21m 01.37s , -31d 03m 21.4s) | C | 60 | 18.6 | 49768 | 2025-08-25 03:27:11 | MASTER-OAFA | (16h 21m 07.46s , -31d 03m 05.3s) | C | 60 | 18.5 | 49844 | 2025-08-25 03:28:27 | MASTER-OAFA | (16h 21m 05.27s , -31d 01m 47.1s) | C | 60 | 18.6 | 98249 | 2025-08-25 16:55:12 | MASTER-SAAO | (16h 34m 21.01s , -35d 04m 06.8s) | C | 60 | 16.5 | 98339 | 2025-08-25 16:56:43 | MASTER-SAAO | (16h 43m 57.78s , -35d 05m 36.2s) | C | 60 | 16.7 | 98431 | 2025-08-25 16:58:14 | MASTER-SAAO | (16h 20m 34.54s , -36d 56m 18.5s) | C | 60 | 17.2 | 98521 | 2025-08-25 16:59:44 | MASTER-SAAO | (16h 30m 30.26s , -36d 58m 30.6s) | C | 60 | 17.2 | 98618 | 2025-08-25 17:01:21 | MASTER-SAAO | (16h 15m 54.89s , -35d 02m 54.0s) | C | 60 | 17.4 | 98710 | 2025-08-25 17:02:53 | MASTER-SAAO | (16h 25m 35.36s , -35d 02m 24.3s) | C | 60 | 17.5 | 98800 | 2025-08-25 17:04:24 | MASTER-SAAO | (16h 30m 32.11s , -33d 10m 42.4s) | C | 60 | 17.5 | 98891 | 2025-08-25 17:05:54 | MASTER-SAAO | (16h 40m 07.88s , -33d 10m 54.7s) | C | 60 | 17.5 | 98980 | 2025-08-25 17:07:24 | MASTER-SAAO | (16h 34m 23.67s , -35d 02m 57.0s) | C | 60 | 17.7 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41543. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39606] LIGO/Virgo/KAGRA S250818k: VLA upper limits on AT2025ulz
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41542 SUBJECT: LIGO/Virgo/KAGRA S250818k: VLA upper limits on AT2025ulz DATE: 25/08/25 14:13:32 GMT FROM: muskan.yadav(a)students.uniroma2.eu R. Ricci, M. Yadav, E. Troja (U Rome) report on behalf of the ERC BHianca team: We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) within the localization of the candidate gravitational wave event S250818k (LVKC GCN 41437; GCN 41440) with the Very Large Array in S-band and C-band at the centre frequency of 3 GHz and 6 GHz respectively with a bandwidth of 2 GHz and 4 GHz on Aug 24st 2025, 5.9 days after the GW trigger. A possible radio counterpart with flux 77 +/- 16 microJy/beam at 3 GHz was claimed by Bruni et al. (GCN 41500). After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the transient position down to a 3-sigma flux density upper limit of about 40 microJy/beam and 25 microJy/beam for 3 GHz and 6 GHz respectively. We thank the VLA staff for promptly executing the observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41542. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39605] LIGO/Virgo/KAGRA S250818k: nature of the AT2025ulz/ZTF25abjmnnps OT
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41541 SUBJECT: LIGO/Virgo/KAGRA S250818k: nature of the AT2025ulz/ZTF25abjmnnps OT DATE: 25/08/25 13:17:47 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.M. Lipunov, (Lomonosov MSU), In several recent telegrams (GCN 41532, 41538) the nature of the AT2025ulz/ZTF25abjmnps source is discussed. An alternative between the KN and SN II models has emerged. The unusual observable properties of the optical flare are emphasized. This alternative seems strange. For example, where can SN II appear in an elliptical galaxy? In this regard, I propose to include a third possible model, namely the merger of two heavy white dwarfs (O-Ne-Mg dwarfs) turning into a neutron star - a dumbbell with powerful radiation of gravitational waves [1]. As was shown earlier, such a phenomenon can generate an unusual form of gravitational wave impulse, which can be taken for a phenomenon caused by a terrestrial artifact. Is this not related to the fact that the initial probability of a terrestrial nature turned out to be more than 70%? [1] Lipunov V.M., Double O-Ne-Mg white dwarfs merging as the source of the powerful gravitational waves for LIGO/VIRGO type interferometers., New Astronomy, Volume 56, p. 84-85, 2017 https://doi.org/10.1016/j.newast.2017.04.013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41541. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39604] LIGO/Virgo/KAGRA S250818k: Pan-STARRS imaging confirms re-brightening of SN2025ulz
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41540 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS imaging confirms re-brightening of SN2025ulz DATE: 25/08/25 13:07:11 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro(a)gmail.com> J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard). We have been observing the optical transient SN2025ulz (Banerjee et al., GCN 41532), formerly AT2025ulz (Stein et al., GCN 41414), the candidate optical counterpart associated with the sub-threshold gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560); see also Gillanders et al. (GCN 41454) and Smartt et al. (GCN 41493). Our observations span grizy-bands, but here we report on the optical re-brightening evident in our i-band observations. On each of MJDs 60909.26, 60910.26, 60911.26 and 60912.26, we observed SN2025ulz for a total exposure time of 1800s. The images were processed with the Pan-STARRS pipeline, where, after astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). For this process, we utilise proprietary data to compile significantly deeper reference stacks (~3400s) than those that are publicly available through the STScI website (~300s) and our stacked target images (1800s). From these difference images, we measure the following preliminary AB magnitudes: MJD t-t0 (days) Filter AB mag 60909.26 4.2 i 22.4 +/- 0.3 60910.26 5.2 i 22.5 +/- 0.3 60911.26 6.2 i 22.1 +/- 0.2 60912.26 7.2 i 21.6 +/- 0.1 Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437). SN2025ulz has risen in our i-band observations by ~0.5 mag (~0.9 mag) in the last one day (two days). While there are barriers to accurately measuring the photometry of this source (e.g., it is relatively faint; it lies close to the bright nucleus and body of the host), our photometry values, and the inferred re-brightening, are robust, given our access to deep reference stacks. We note that these are vital to accurately infer the true brightness of the transient, as performing difference imaging with a shallower reference image can lead to systematic errors (as much as ~0.7 AB mag, in the specific case of SN2025ulz utilising the public reference stack available through the STScI website). Re-brightening of SN2025ulz has been previously noted by Liu et al. (GCN 41461), Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518). Our observed re-brightening supports the interpretation of SN2025ulz being a young SN, based on the VLT spectral observations reported by ENGRAVE (Banerjee et al., GCN 41532). Our consistent i-band rise is likely to suggest it is now on a radioactively powered rise following an initial shock-cooling phase, and thus indicates that SN2025ulz is likely not the optical counterpart to S250818k. Further Pan-STARRS observations are continuing to determine if the colour and lightcurve shape remains consistent with a type II or IIb supernova, to compare with the conclusions of Kasliwal et al. (GCN 41538). Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys. View this GCN Circular online at https://gcn.nasa.gov/circulars/41540. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39603] GRB 250823A: Swift-BAT refined analysis
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41539 SUBJECT: GRB 250823A: Swift-BAT refined analysis DATE: 25/08/25 12:44:24 GMT FROM: Rahul Gupta at NASA GSFC <rahul.gupta(a)nasa.gov> C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU), M. H. Siegel (PSU) (i.e. the Swift-BAT team): Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250823A (trigger #1344586) (Siegel, et al., GCN Circ. 41511). The BAT ground-calculated position is RA, Dec = 297.465, -24.986 deg which is RA(J2000) = 19h 49m 51.7s Dec(J2000) = -24d 59' 09.0" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%. The mask-weighted BAT light curve shows multi-peaked emission starting from ~T-25 to ~T+180 sec. T90 (15-350 keV) is 193.60 +- 10.19 sec (estimated error including systematics). The time-averaged spectrum from T-23.84 to T+180.82 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.76 +- 0.16. The fluence in the 15-150 keV band is 1.8 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T+4.24 sec in the 15-150 keV band is 1.1 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1344586 View this GCN Circular online at https://gcn.nasa.gov/circulars/41539. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39602] LIGO/Virgo/KAGRA S250818k: Continued Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41538 SUBJECT: LIGO/Virgo/KAGRA S250818k: Continued Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz) DATE: 25/08/25 10:04:54 GMT FROM: Mansi Kasliwal at Caltech <mansikasliwal(a)gmail.com> Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Christoffer Fremling (Caltech), Tomas Ahumada (Caltech), Xander J. Hall (CMU), Daniel A. Perley (LJMU), Shreya Anand (Stanford), Chang Liu (Northwestern), Kaustav Das (Caltech), Varun Bhalerao (IIT-B), Vishwajeet Swain (IIT-B), Aditya Saikia (IIT-B) report on behalf of a larger ZTF and GROWTH collaboration We continue to observe ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) with the LRIS spectrograph on Keck I. Our latest epoch starts at UTC 2025-08-25 06:50. We reduce the data with lpipe (Perley et al. 2019) and subtract a spectrum at an earlier epoch to understand the evolution (Karambelkar et al. GCN 41436). As reported by Banerjee et al. GCN 41532, we also see a feature with an absorption minimum at 6720A. If this is Halpha, this would imply a velocity of 17000 km/s at z=0.0848 (we measure a higher velocity than what was reported in Banerjee et al. GCN 41532). However, we posit the classification of this transient as a supernova or a kilonova currently remains ambiguous. If it is a supernova, it is an unusual supernova for at least four reasons: (i) There is no robust (rlap>5) spectral match in the SNID library, (ii) No supernova light curve in the ZTF supernova library matches especially the blue-to-red color evolution at this early phase (e.g. Busmann et al. GCN 41535), (iii) The feature at 6720A is not smooth in shape and has additional structure, (iv) There appear to be other weak, broad undulations in the spectra. If it is a kilonova, all photometric data in-hand continue to be consistent with this hypothesis, however, additional theoretical modeling may be needed to fully explain the spectra. Looking ahead, we encourage continued follow-up of the panchromatic light curve, including the radio and X-ray bands, to unambiguously classify this source. View this GCN Circular online at https://gcn.nasa.gov/circulars/41538. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39601] GRB 250821B: AstroSat CZTI detection
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41537 SUBJECT: GRB 250821B: AstroSat CZTI detection DATE: 25/08/25 09:32:36 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar(a)iitb.ac.in> M. Tembhurnikar (IUCAA), A. Arya (IITB), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of GRB 250821B which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41473 and 41477), SVOM/GRM (SVOM team, GCN Circ. 41486), Konus-Wind (Frederiks et al., GCN Circ. 41508), and Insight-HXMT (Wang et al., GCN Circ. 41533). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-08-21 16:07:47.95 UTC. The measured peak count rate associated with the burst is 343 (+153, -46) counts/s above the background in the combined data of all quadrants, with a total of 218 (+60, -62) counts. The local mean background count rate was 252 (+10, -12) counts/s. Using cumulative rates, we measure a T90 of 1.2 (+0.2, -0.5) s. The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-08-21 16:07:47.16 UTC. The measured peak count rate associated with the burst is 1724 (+84, -94) counts/s above the background in the combined data of all quadrants, with a total of 2373 (+175, -194) counts. The local mean background count rate was 1175 (+9, -11) counts/s. We measure a T90 of 1.9 (+1.0, -0.2) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41537. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39600] GRB 250819A: AstroSat CZTI detection
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41536 SUBJECT: GRB 250819A: AstroSat CZTI detection DATE: 25/08/25 09:11:22 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar(a)iitb.ac.in> M. Tembhurnikar (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of GRB 250819A which was also detected by Insight-HXMT (Wang et. al., GCN Circ. 41457), and SVOM/GRM (SVOM team, GCN Circ. 41458). The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-08-19 17:49:46.59 UTC. The measured peak count rate associated with the burst is 396 (+68, -65) counts/s above the background in the combined data of all quadrants, with a total of 1066 (+262, -291) counts. The local mean background count rate was 1300 (+10, -12) counts/s. We measure a T90 of 3.3 (+1.8, -1.0) s from the cumulative Veto light curve. The source was also faintly detected in the CZT detectors in the 20-200 keV energy range. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41536. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39599] LIGO/Virgo/KAGRA S250818k: FTW Observations Show Continued Reddening of AT 2025ulz
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41535 SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Observations Show Continued Reddening of AT 2025ulz DATE: 25/08/25 09:09:19 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann(a)physik.lmu.de> Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), and Mansi Kasliwal (Caltech) report: We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, i, and J bands simultaneously starting at 2025-08-24T19:33:25 for 20 x 180 s with a few interruptions due to clouds. The difference imaging with templates from the Legacy Survey for the g-band and PS1 for the i-band, we report that AT 2025ulz has g - i ~ 1.7 AB mag. We note our differenced i-band brightness is consistent with observations reported by Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518). The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. Further analysis is underway. We thank Christoph Ries from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41535. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39598] LIGO/Virgo/KAGRA S250818k: MASTER predicovery limits of the AT2025ulz/ZTF25abjmnps KN candidate
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41534 SUBJECT: LIGO/Virgo/KAGRA S250818k: MASTER predicovery limits of the AT2025ulz/ZTF25abjmnps KN candidate DATE: 25/08/25 08:21:58 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, N.Tiurina, D.Vlasenko, P.Balanutsa, I.Panchenko, A.Sankovich, A.Chasovnikov K.Zhirkov, G.Antipov, I.Gorbunov, A.Sosnovskij, Yu.Tselik, Ya.Kechin, V.Senik, V.Topolev (Lomonosov MSU), O.Gress, N.Budnev, O.Ershova (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), D.Buckley (SAAO), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) Global MASTER robotic net [1] observed the field with AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) before and after trigger time. At the location of AT2025ulz the twin MASTER600-Tunka telescope (Cmos QHY 6060, FOW = 18 square degress) did not detect a source with next 5-sigma limits: Date UT Tstart-Ttrig Exp Filter tube lim (middle) (day) (s) 250815 14:55:18 -3.12 60 Clear East 19.8 250822 16:52:05 3.95 8x60 Clear West 20.9 250823 15:37:06 4.90 38x60 Clear West 21.8 The first observation was a lucky coincidence. Since there was no automatic LVC alert, subsequent observations had to be carried out in the Internet telescope mode, taking into account the specific quality of the weather. [1] V.M. Lipunov, V.G. Kornilov, E.Gorbovskoy, N. Tiurina & A.Kuznetsov, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http://www.pereplet.ru/lipunov/625.html#625 View this GCN Circular online at https://gcn.nasa.gov/circulars/41534. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39597] GRB 250821B: Insight-HXMT detection
by GCN Circulars 25 Aug '25

25 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41533 SUBJECT: GRB 250821B: Insight-HXMT detection DATE: 25/08/25 03:30:20 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Wen-Jun Tan, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team: At 2025-08-21T16:07:47.500 (T0), Insight-HXMT/HE detected the bright intermedium duration burst GRB 250821B, which is also detected by Fermi/GBM (Fermi GBM team., GCN #41473), SVOM/GRM (Tan et al., GCN #41486) and Konus-Wind (D. Frederiks et al., GCN #41508). The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 2.05 +1.70/-0.45 s. The 1s peak rate, measured from T0-0.075 s, is 11927 cnts/sec. The total counts from this burst is 15817 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb250821B.png All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org. View this GCN Circular online at https://gcn.nasa.gov/circulars/41533. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39596] LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of SN 2025ulz as a type II supernova
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41532 SUBJECT: LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of SN 2025ulz as a type II supernova DATE: 25/08/24 20:46:51 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo(a)ucd.ie> Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Valerio D’Elia (ASI-SSDC), Massimiliano De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Matt Nicholl (QUB), Sam Oates (Lancaster U.), Silvia Piranomonte (INAF-OAR), Luigi Piro (INAF-IAPS), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Avinash Singh (Stockholm University), Stephen J. Smartt (Oxford), Albert Sneppen (DAWN/NBI), Jesper Sollerman (Stockholm), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS), report for the ENGRAVE collaboration: We report preliminary analysis from follow-up ESO VLT observations obtained by the ENGRAVE collaboration of SN 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization region of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440). Starting on 2025-08-24 00:03:40 UT, we observed the field of SN 2025ulz using the ESO VLT UT4 (Yepun) equipped with the MUSE integral-field spectrograph. A series of 4x700 s exposures were taken for this observation. From a preliminary reduction, extraction of a spectrum at the position of SN 2025ulz reveals a continuum well detected over the full wavelength range of 5000 to 9000 Å. The spectrum shows a prominent broad feature which we interpret as a P-Cygni profile of the H-alpha line, similar to that observed in young type II or IIb supernovae. The peak of the emission and the absorption trough is consistent with the host galaxy redshift of z = 0.0848 (ENGRAVE GCN 41476, Karambelkar et al., GCN 41436) and an expansion velocity of around 12,000 km/s. There is weaker evidence for an H-beta P-Cygni profile. Plausible matches to young type II and IIb SNe are found with SNID-SAGE (Stoppa et al. in prep, a new enhanced, python version of SNID), although the transient object continuum is contaminated by the host and the absorption line strengths are therefore diluted. We suggest that SN 2025ulz is a type II supernova (of unconfirmed subtype at this point) and is therefore unrelated to S250818k. We acknowledge the excellent support from the ESO staff in Paranal, in particular Camila de Sa Freitas, Israel Blanchard and Sam Kim. View this GCN Circular online at https://gcn.nasa.gov/circulars/41532. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39595] GRB 250823A: Swift-XRT refined Analysis
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41531 SUBJECT: GRB 250823A: Swift-XRT refined Analysis DATE: 25/08/24 20:31:05 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> S. Dichiara (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 4.8 ks of XRT data for GRB 250823A, from 111 s to 44.7 ks after the trigger. The data comprise 313 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The light curve can be modelled with a series of power-law decays. The light curve initially rises, with an index alpha=-1.5 (+0.0, -0.6). At T+120 s it breaks to an alpha of 3.33 (+/-0.09). The light curve breaks again at T+349 s to a decay with alpha=2.31 (+0.23, -0.21), before a final break at T+1667 s s after which the decay index is 0.78 (+0.13, -0.12). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 (+/-0.04). The best-fitting absorption column is 1.99 (+0.16, -0.15) x 10^21 cm^-2, in excess of the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.92 (+0.28, -0.26) and a best-fitting absorption column of 1.9 (+1.0, -0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.9 (+1.0, -0.8) x 10^21 cm^-2 Galactic foreground: 1.1 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.92 (+0.28, -0.26) If the light curve continues to decay with a power-law decay index of 0.78, the count rate at T+24 hours will be 9.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-13 (4.8 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01344586. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41531. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39594] EP250821a: MeerKAT radio counterpart
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41530 SUBJECT: EP250821a: MeerKAT radio counterpart DATE: 25/08/24 16:50:44 GMT FROM: Francesco Carotenuto at INAF/OAR <francesco.carotenuto(a)inaf.it> Authors: F. Carotenuto (INAF-OAR), J. Bright (Oxford), P. G. Jonker (Radboud), on behalf of a larger collaboration We observed the field of EP250821a detected by the Einstein Probe (GCN 41459, 41470, 41490) with the MeerKAT radio telescope from 2025-08-23 21:44 UTC to 2025-08-23 22:31 UTC (with a total time on source of 44 minutes) using the S-band receivers at a central frequency of 3.06 GHz. Analysing the SARAO Science Data Processor output continuum image, we detect a source at the position of EP250821a with a flux density of around 40 uJy/beam, with a typical image noise of around 6 uJy/beam. Pending verification on the host galaxy emission, we tentatively ascribe it to EP250821a. Further observations are planned. We thank the SARAO staff for rapidly scheduling these observations. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. This work has made use of the "MPIfR S-band receiver system" designed, constructed and maintained by funding of the MPI für Radioastronomie and the Max-Planck-Society. View this GCN Circular online at https://gcn.nasa.gov/circulars/41530. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39593] EP triggers 01709200706 and 01709200707: GOTO confirmation of flaring stars
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41529 SUBJECT: EP triggers 01709200706 and 01709200707: GOTO confirmation of flaring stars DATE: 25/08/24 16:50:29 GMT FROM: d.s.oneill(a)bham.ac.uk D. O'Neill, A. Kumar, G. Ramsay, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the EP/WXT alerts 01709200706 and 01709200707. Observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Targeted observations of EP-WXT 01709200706 were taken at 2025-08-24 12:46:36 UT (+3.13h post trigger) and 2025-24-08 14:10:18 UT (+4.53h post trigger). We detect a single new source in difference imaging, consistent with the flaring star UCAC4 277-216984, as identified by Li et al. (GCN 41524). The star is saturated in our unsubtracted frames, meaning we cannot robustly determine the magnitude other than place lower limits of L>11.94 AB mag and L>12.11 AB mag at +3.13h and +4.53h post trigger, respectively. Targeted observations of EP-WXT 01709200707 were taken at 2025-08-24 12:54:10 UT (+0.22h post trigger) and 2025-08-24 14:02:40 UT (+1.36h post trigger). We identify a new source in differencing imaging consistent with UCAC4 268-197742 identified by Li et al. (GCN 41526). The star reaches a peak magnitude in our unsubtracted frames of L=14.31±0.01 and L=15.16±0.01 AB mag at +0.22h and +1.36h post trigger, respectively. Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41529. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39592] LIGO/Virgo/KAGRA S250818k: Swift Observations of AT 2025ulz - Second Epoch
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41528 SUBJECT: LIGO/Virgo/KAGRA S250818k: Swift Observations of AT 2025ulz - Second Epoch DATE: 25/08/24 16:00:28 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Rosa L. Becerra (U Rome), Eleonora Troja (U Rome) and Simone Dichiara (PSU) report: We requested a second epoch of ToO observations of AT2025ulz/ZTF25abjmnps (Stein et al.; GCN 41414) discovered within the localization of the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440). The observations started on 2025-08-22 at 21:56 UT, approximately 4.9 days after the trigger, with a total exposure of 3.2 ks. At the transient position, we detect no X-ray source down to a 3-sigma upper limit of 0.004 cts/s (0.3-10 keV), consistent with the result reported by Hall et al. (GCN 41453). Using a power-law with nH=2.2E+20 and a photon index=2, we convert the count rate into an unabsorbed flux of 1.5e-13 erg/cm2/s, which corresponds to a luminosity of 3e42 erg/s at z=0.0848 (Karambelkar et al. GCN 41436). In simultaneous UVOT observations performed with the u filter, we identify some faint diffuse emission likely related to the host galaxy and place 3-sigma upper limit u > 21 AB at the transient’s position, in agreement with the faint detection reported by Troja et al. (GCN 41506). We thank the Swift team and the PI for promptly scheduling and making these observations possible. View this GCN Circular online at https://gcn.nasa.gov/circulars/41528. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39591] GRB 250824B: Fermi GBM Final Real-time Localization
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41527 SUBJECT: GRB 250824B: Fermi GBM Final Real-time Localization DATE: 25/08/24 13:48:54 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely SHORT GRB At 13:38:13 UT on 24 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250824B (trigger 777735498.8624 / 250824568). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 248.3, Dec = -35.6 (J2000 degrees, equivalent to J2000 16h 33m, -35d 36'), with a statistical uncertainty of 7.7 degrees. The angle from the Fermi LAT boresight is 27.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824568/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824568/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824568/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41527. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39590] The EP-WXT trigger 01709200707 is likely a flaring star
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41526 SUBJECT: The EP-WXT trigger 01709200707 is likely a flaring star DATE: 25/08/24 13:31:38 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> A. Li (BNU), C. Y. Wang（THU）, X. Y. Zhou (PRIC), W. D. Zhang (NAOC) report on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709200707 at the time of 2025-08-24T12:40:53, is likely a stellar flare associated with UCAC4 268-197742. The estimated flux of the flare is around 6e-11 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 1.1e31 erg/s. A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an X-ray source was detected at R.A. = 328.5143 deg, DEC = -36.5634 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic), 10 arcsec from UCAC4 268-197742. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41526. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39589] GRB 250823A: COLIBRÍ optical observations
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41525 SUBJECT: GRB 250823A: COLIBRÍ optical observations DATE: 25/08/24 12:59:39 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Alan M. Watson (UNAM), Damien Dornic (CPPM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of GRB 250823A, detected by Swift/BAT (Siegel et al., GCN Circ. 41511) and Fermi/GBM (Fermi GBM team et al., GCN Circ. 41510) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-24 05:54 to 06:33 UTC (from 9.73 to 10.38 hours after the trigger) and obtained 32 minutes of exposure in the i filter. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detected the optical counterpart first reported by Gompertz et al. (GCN Circ. 41512) and subsequently by Fernandez-Garcia et al. (GCN Circ. 41513), Strausbaugh & Cucchiara (GCN Circ. 41514), Malesani et al. (GCN Circ. 41520), Kuin et al. (GCN Circ. 41521), and Mal et al. (GCN Circ. 41523), at a preliminary magnitude of: i = 22.74 +/- 0.21 This measurement is contemporaneous and consistent with the value reported by SVOM/VT (Mal et al., GCN Circ. 41523). We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/41525. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39588] The EP-WXT trigger 01709200706 is likely a flaring star
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41524 SUBJECT: The EP-WXT trigger 01709200706 is likely a flaring star DATE: 25/08/24 12:37:36 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> A. Li (BNU), C. Y. Wang（THU）, X. Y. Zhou (PRIC), W. D. Zhang (NAOC) report on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709200706 at the time of 2025-08-24T09:38:33, is likely a stellar flare associated with UCAC4 277-216984. The estimated flux of the flare is around 8e-11 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 1.4e31 erg/s. A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an X-ray source was detected at R.A. = 355.8633 deg, DEC = -34.7836 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic), 6 arcsec from UCAC4 277-216984. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41524. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39587] GRB 250823A: SVOM/VT optical observation
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41523 SUBJECT: GRB 250823A: SVOM/VT optical observation DATE: 25/08/24 12:20:24 GMT FROM: Yinuo Ma <mayn(a)bao.ac.cn> Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed a Target of Opportunity observation of GRB 250823A detected by Swift/BAT (Siegel et al., GCN 41511) and Fermi/GBM (Fermi GBM Team, GCN 41510). SVOM/VT began observing the field at 2025-08-23T20:38:57 UTC, 0.48 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X-band data availible, the optical counterpart (Gompertz & Dimple, GCN 41512; Fernandez-Garcia et al., GCN 41513; Strausbaugh & Cucchiara, GCN 41514; Malesani et al., GCN 41520; Kuin & Siegel, GCN 41521) at the position consistent with the locations of Swift/XRT (Siegel et al., GCN 41511; Evans et al., GCN 41515), was clearly detected in both VT_B and VT_R. It decayed by about 2 mag in 8 hours. The magnitudes are: mid time (h) | exposure time (s) | band | mag (AB) | mag err -------------|-------------------|------|----------|-------- 1.934 | 37*70 | VT_B | 21.80 | 0.08 1.934 | 37*70 | VT_R | 20.92 | 0.06 9.913 | 28*70 | VT_R | 22.8 | 0.3 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41523. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39586] GRB 250824A: Fermi GBM Final Real-time Localization
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41522 SUBJECT: GRB 250824A: Fermi GBM Final Real-time Localization DATE: 25/08/24 11:34:48 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 11:24:18 UT on 24 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250824A (trigger 777727463.376698 / 250824475). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 312.0, Dec = 49.5 (J2000 degrees, equivalent to J2000 20h 48m, 49d 30'), with a statistical uncertainty of 2.0 degrees. The angle from the Fermi LAT boresight is 61.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824475/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824475/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250824475/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41522. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39585] GRB 250823A: Swift/UVOT Detection
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41521 SUBJECT: GRB 250823A: Swift/UVOT Detection DATE: 25/08/24 09:33:50 GMT FROM: Paul Kuin at MSSL <npkuin(a)gmail.com> Paul Kuin (UCL/MSSL) and M. H. Siegel (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250823A 126 s after the BAT trigger (Siegel et al., GCN Circ. 41511). A source consistent with the XRT position is detected in the initial UVOT exposures. Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 126 1020 334 20.42 +/- 0.17 v 668 1244 78 18.43 +/- 0.28 b 594 1169 58 19.25 +/- 0.28 u 338 1317 304 >19.5 w1 718 1293 58 >19.1 m2 692 1268 58 >18.8 w2 644 1391 97 >19.3 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.111 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41521. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39584] GRB 250823A: NOT optical observations
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41520 SUBJECT: GRB 250823A: NOT optical observations DATE: 25/08/24 08:54:25 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), A de Ugarte Postigo (LAM), A. Henderson de la Fuente (NOT) report on behalf of a larger collaboration: We observed the optical counterpart (Gompertz & Dimple, GCN 41512; Fernandez-Garcia et al., GCN 41513; Strausbaugh & Cucchiara, GCN 41514) of the Swift and Fermi GRB 250823A (Siegel et al., GCN 41511; Smith et al., GCN 41517) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). Observations were carried out in the r and z filters. Calibrated against nearby Pan-STARRS pointlike objects, we report the following magnitudes: | Filter | Mean epoch (UT) | Time since trigger (hr) | Exp time (s) | Magnitude (AB) | | ------ | --------------- | ----------------------- | ------------ | -------------- | | r | 2025 Aug 24.022 | 4.36 | 3x300 | 21.89 +- 0.05 | | z | 2025 Aug 24.036 | 4.69 | 5x250 | 21.15 +- 0.12 | These values are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41520. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39583] LIGO/Virgo/KAGRA S250818k: CFH/MegaCam follow-up observation for AT2025ulz
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41519 SUBJECT: LIGO/Virgo/KAGRA S250818k: CFH/MegaCam follow-up observation for AT2025ulz DATE: 25/08/24 07:14:08 GMT FROM: Sarah Antier at OCA <sarah.antier(a)oca.eu> S. Antier (OCA/IJCLAB), M. Pillas (Uliege), D. Akl (NYUAD), S. Karpov (FZU), M. Coughlin (UMN), P. Hello (IJCLAB), A. Jacquesson (IJCLAB) report on behalf on ZTF collaboration: We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414) over several nights, which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, Angulo et al., GCN 41518 as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440). The observations were carried out using MegaCam on the CFHT. We report here our preliminary measurement in r-band taken around 2025-08-21 06:50 UTC by co-adding several images and analyzed with STDPipe (Karpov 2025) with photometric calibration against Pan-STARRS. The quality of the sky during that night was not ideal. r = 22.85 +/- 0.15 (AB) Differential image analysis against archival CFH data has been performed. Further observations are planned. We would like to thank the CFHT staff enabling these timely observations, in particular Nadine Manset and Heather Flewelling. View this GCN Circular online at https://gcn.nasa.gov/circulars/41519. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39582] LIGO/Virgo/KAGRA S250818k: COLIBRÍ confirmation of rebrightening of AT2025ulz
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41518 SUBJECT: LIGO/Virgo/KAGRA S250818k: COLIBRÍ confirmation of rebrightening of AT2025ulz DATE: 25/08/24 06:21:08 GMT FROM: Alan Watson at UNAM <alan(a)astro.unam.mx> Camila Angulo (UNAM), Alan M. Watson (UNAM), Damien Dornic (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd .Sánchez Álvarez (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the AT2025ulz (Stein et al., GCN Circ. 41414) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed at two epochs, from 2025-08-22 03:19 to 05:29 UTC (4.13 days after the trigger) and from 2025-08-24 03:29 to 05:41 UTC (6.14 days after the trigger). At both epochs we obtained 96 minutes of exposure in the i filter. The data were reduced and coadded with custom software and then analyzed in STDWeb (Karpov 2021). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. After subtracting a Pan-STARRS DR2 template image from our first epoch image (4.13 days after the trigger), we detect a source at the reported position of AT2025ulz with i = 23.1 +/- 0.25 After subtracting a Pan-STARRS DR2 template image from our second epoch image (6.14 days after the trigger), we detect a source at the reported position of AT2025ulz with i = 22.0 +/- 0.1 Subtracting the first epoch image from the second epoch image, we find a brightening that corresponds to Delta i = 22.7 +/- 0.2 We caution that these analyses are preliminary, especially the photometry after subtracting the shallower PS template images. Nevertheless, our results would appear to confirm the rebrightening reported by Freeburn et al. (GCN Circ. 41507). Further observations are planned. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/41518. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39581] GRB 250823A: Fermi GBM Observation
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41517 SUBJECT: GRB 250823A: Fermi GBM Observation DATE: 25/08/24 02:09:14 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> Jacob Smith (UAH), B. Mailyan (Florida Tech) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: At 20:10:10.67 UT on 23 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250823A (trigger 777672615/250823840), which was also detected by Swift BAT (M. H. Siegel et al. 2025, GCN 41511), and optical follow-up was first reported by Liverpool Telescope (B. P. Gompertz et al. 2025, GCN 41512). The Fermi GBM on-ground location is consistent with the Swift BAT position. The angle from the Fermi LAT boresight is 72 degrees. The GBM light curve consists of a single emission episode with a duration (T90) of about 13 s (50-300 keV). The time-averaged spectrum from T0-6.1 to T0+10.2 s is best fit by a simple power law function with index -1.67 +/- 0.05. A power law function with an exponential high-energy cutoff fits equally well with power law index = -1.44 +/- 0.17 and Epeak = 206.5 +/- 95.8. The event fluence (10-1000 keV) in this time interval is (1.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.1 s in the 10-1000 keV band is 1.8 +/- 0.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/ View this GCN Circular online at https://gcn.nasa.gov/circulars/41517. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39580] GRB 250818B: MeerKAT Detection
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41516 SUBJECT: GRB 250818B: MeerKAT Detection DATE: 25/08/24 01:11:07 GMT FROM: Genevieve Schroeder at Cornell University <genevieveschroeder(a)u.northwestern.edu> G. Schroeder (Cornell), W. Fong (Northwestern), T. Laskar (Utah), L. Rhodes (TSI/McGill) report: We observed the location of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407) with the MeerKAT radio telescope under Director’s Discretionary program DDT-20250822-GS-01 (PI Schroeder) at 1.3 GHz and 3.1 GHz on 2025 August 23 (~5 days post burst). In preliminary analysis, we detect the radio afterglow (Ricci et al., GCN 41455) in the 3.1 GHz image, but not in 1.3 GHz (rms of 7 microJy/beam). We measure a flux density of ~30 microJy at 3.1 GHz. Further observations are planned. We thank Fernando Camilo for the rapid approval of the DDT proposal, and the staff at the South African Radio Astronomy Observatory for scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41516. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39579] GRB 250823A: Enhanced Swift-XRT position
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41515 SUBJECT: GRB 250823A: Enhanced Swift-XRT position DATE: 25/08/24 01:08:20 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) report on behalf of the Swift-XRT team. Using 950 s of XRT Photon Counting mode data and 1 UVOT images for GRB 250823A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 297.47814, -24.99557 which is equivalent to: RA (J2000): 19h 49m 54.75s Dec (J2000): -24d 59' 44.1" with an uncertainty of 2.2 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41515. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39578] GRB 250823A: LCOGT Optical Detection
by GCN Circulars 24 Aug '25

24 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41514 SUBJECT: GRB 250823A: LCOGT Optical Detection DATE: 25/08/24 00:24:31 GMT FROM: Robert Strausbaugh at Eastern Illinois University <rstrausbaugh(a)eiu.edu> R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration: We observed the Swift GRB 250823A field (Siegel et al., GCN 41511) with the LCOGT 1-meter Sinistro instrument at the Cerro Tololo Interamerican Observatory, Chile site, on August 23, from 23:17 to 23:49 UT (corresponding to 3.12 to 3.65 hours after the GRB trigger time) with the SDSS r and i filters. We performed a series of 3x300s exposures in i-band and r-band. We detect an uncatalogued source coincident with the XRT error region, and other optical detections (Gompertz et al., GCN 41512; Fernandez-Garcia et al., GCN 41513) in both bands. The following magnitudes are calculated using the PanSTARRS catalog as reference: r = 21.2 +/- 0.15 i = 22.5 +/- 0.55 These magnitudes are not corrected for galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41514. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39577] GRB 250823A: BOOTES6/DPRT early optical detection
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41513 SUBJECT: GRB 250823A: BOOTES6/DPRT early optical detection DATE: 25/08/23 23:11:13 GMT FROM: Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct(a)iaa.es> E. Fernandez-Garcia, I. Perez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, Bloemfontein), A. Martin-Carrillo and L. Hanlon (UCD, Dublin), C. J. Perez del Pulgar (UMA, Malaga), Y.-D. Hu (GXI, Guangxi) and M. Gritsevich (Univ. of Helsinki), on behalf of a larger collaboration, report: Following the detection of GRB250823A by Fermi (Fermi GBM Team, GCN Circ. 41510) and Swift/BAT (Siegel et al., GCN Circ. 41511), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) automatically observed the Swift/XRT X-ray afterglow location starting on Aug. 23, 20:12:44 UT (i.e. 157 s after trigger). A series of images in clear filter were gathered and the optical counterpart reported by the LT (Gompertz et al., GCNC 41512) was detected at early times in our dataset. We measure a preliminary magnitude of 19.30 +- 0.15 on the co-added 10 s x 45 image (at 20:28:26 UT mid time). Further data analysis is ongoing. We thank the staff at Boyden Observatory for their excellent support. View this GCN Circular online at https://gcn.nasa.gov/circulars/41513. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39576] GRB 250823A: Liverpool Telescope optical detection
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41512 SUBJECT: GRB 250823A: Liverpool Telescope optical detection DATE: 25/08/23 22:19:16 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz(a)bham.ac.uk> B. P. Gompertz (U. Birmingham) and Dimple (U. Birmingham) report on behalf of a larger collaboration: We conducted follow-up observations of GRB 250823A (Fermi GBM Team, GCN Circ. 41510, Siegel et al., GCN Circ. 41511) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 21:26:55 UT on 2025-08-23, 1.28 hr after the Swift trigger, and consisted of 5 x 120 s exposures in each of the SDSS g, r, i, and z filters. We detect a new optical source with an AB magnitude of r = 21.25 +/- 0.09 (mid-time t0+1.45 hr) at a position of RA(J2000) 19:49:54.7, Dec -24:59:44.9 (0.2” uncertainty). The source is not present in archival Pan-STARRs imaging and is spatially consistent with the localisation by Swift/XRT (Siegel et al., GCN Circ. 41511). We therefore conclude it is the optical counterpart of GRB 250823A. Magnitudes are calibrated against nearby Pan-STARRS stars and are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41512. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39575] GRB 250823A: Swift detection of a burst
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41511 SUBJECT: GRB 250823A: Swift detection of a burst DATE: 25/08/23 20:31:35 GMT FROM: K.L. Page at U Leicester <klp5(a)leicester.ac.uk> M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), S. Lanava (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:10:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250823A (trigger=1344586). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 297.521, -24.976 which is RA(J2000) = 19h 50m 05s Dec(J2000) = -24d 58' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a complex structure with a duration of 200 sec. The peak count rate was ~1600 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 20:17:28.0 UT, 440.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 297.47788, -24.99582 which is equivalent to: RA(J2000) = 19h 49m 54.69s Dec(J2000) = -24d 59' 45.0" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 157 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.12 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.9 (+3.01/-2.59) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 338 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.111. Some data, such as earlier XRT observations, are not available due to a telemetry gap, and will be produced on the full ground download. Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu) Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41511. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39574] GRB 250823A: Fermi GBM Final Real-time Localization
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41510 SUBJECT: GRB 250823A: Fermi GBM Final Real-time Localization DATE: 25/08/23 20:20:56 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 20:10:10 UT on 23 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250823A (trigger 777672615.66839 / 250823840). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 315.5, Dec = -23.6 (J2000 degrees, equivalent to J2000 21h 02m, -23d 36'), with a statistical uncertainty of 8.7 degrees. The angle from the Fermi LAT boresight is 70.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41510. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39573] Swift GRB250823.84: Global MASTER-Net observations report
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41509 SUBJECT: Swift GRB250823.84: Global MASTER-Net observations report DATE: 25/08/23 20:20:37 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB250823.84 (trigger No 1344586,19h 50m 05.04s , -24d 58m 33.6s, R=0.05) errorbox 247 sec after notice time and 298 sec after trigger time at 2025-08-23 20:15:06 UT, with upper limit up to 12.6 mag. The observations began at zenith distance = 70 deg. The sun altitude is -34.1 deg. The galactic latitude b = -24 deg., longitude l = 16 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2976085 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 329 | MASTER-Kislovodsk | P- | 60 | 12.6 | The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41509. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39572] Konus-Wind detection of GRB 250821B
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41508 SUBJECT: Konus-Wind detection of GRB 250821B DATE: 25/08/23 16:06:36 GMT FROM: Dmitry Frederiks at Ioffe Institute <fred(a)mail.ioffe.ru> D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The bright, long-duration GRB 250821B (Ferm-GBM detection: The Fermi GBM team, GCN 41473; Smith and Meegan, GCN 41477; SVOM-GRM detection: Tan et al., GCN 41486; IPN localization Kozyrev et al., GCN 41498) triggered Konus-Wind (KW) at T0=58069.135 s UT (16:07:49.135). The burst light curve shows a single emission pulse with a total duration of ~2.5 s. The emission is seen up to ~5 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250821_T58069/ As observed by Konus-Wind, the burst had a fluence of (1.34 ± 0.13)x10^-5 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 0.512 s, of (1.77 ± 0.17)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 15 MeV range by a GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.60 (-0.14,+0.15), the high energy photon index beta = -2.24 (-0.13,+0.10), the peak energy Ep = 193(-18,+21) keV, chi2 = 70/96 dof. All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/41508. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39571] LIGO/Virgo/KAGRA S250818k: Rebrightening detected with Gemini/GMOS
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41507 SUBJECT: LIGO/Virgo/KAGRA S250818k: Rebrightening detected with Gemini/GMOS DATE: 25/08/23 15:18:41 GMT FROM: James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54(a)gmail.com> J. Freeburn (UNC), B. O’Connor (CMU), X. J. Hall (CMU), M. Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: Using Gemini/GMOS-N, starting at 2025-08-23 05:30:48 UTC, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k) which is a candidate binary neutron star merger. We took images in g and i bands. We subtracted Gemini/GMOS-N observations taken on the previous night (starting at 2025-08-22 05:43:02) from these observations. In the g-band, no source is detected in the difference image. The i-band difference image reveals a positive source at the location of AT2025ulz/ZTF25abjmnps. This positive detection corresponds to an increase in brightness of 0.63+/-0.09 microJanksy, which indicates rising emission in i-band at this phase. Further observations are planned and additional follow-up is encouraged to further monitor the transient’s evolution. We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41507. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39570] LIGO/Virgo/KAGRA S250818k: HST nIR detection of AT 2025ulz
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41506 SUBJECT: LIGO/Virgo/KAGRA S250818k: HST nIR detection of AT 2025ulz DATE: 25/08/23 14:34:22 GMT FROM: Eleonora <nora.gsfc(a)gmail.com> Eleonora Troja (U Rome), Brendan O’Connor (Carnegie Mellon U.), and Rosa L. Becerra (U Rome) report on behalf of a larger team: We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) with the Hubble Space Telescope (PI: Troja) starting from 2025-08-22 20:04 UTC (~T_0+4.8 days). The observations were carried out with the Wide-Field Camera 3 (WFC3) using the UVIS channel with the F336W filter and the IR channel with the F110W and F160W filters. Due to orbital constraints, we could acquire only short exposures of ~120 s per filter. At the location of AT2025ulz we clearly detect a source with preliminary AB magnitudes of: F110W = 22.91 +/- 0.12 F160W = 22.8 +/- 0.3 (partial result based on 60 s exposure) F336W = 23.9 +/ -0.4 The above values were derived from aperture photometry after subtracting a 2D GALFIT (Peng et al. 2004) model for the host galaxy, and are not corrected for Galactic extinction. The source is resolved in our nIR images and offset from the galactic center by ~0.9 arcsec, corresponding to a projected physical offset of ~1.5 kpc. The measured nIR brightness matches well the kilonova AT2017gfo placed at the same distance of 388 Mpc. However, the blue color implied by the F336W observations is not consistent with a kilonova spectrum. At present we cannot determine whether the F336W flux is mostly dominated by the transient’s light, e.g. a rising supernova, or by host emission, e.g. underlying star-formation or a young star cluster. While the bulk of the galaxy is not detected in F336W, a star forming clump is still possible, and a clump is seen in the IR filters at the southern edge of the disk. Continued monitoring of the source is encouraged to assess its variability. We thank the HST staff, in particular Julia Roman-Duval, Patricia Royle, and William Januszweski for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41506. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39569] LIGO/Virgo/KAGRA S250818k: J-GEM follow-up observation for AT2025ulz
by GCN Circulars 23 Aug '25

23 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41505 SUBJECT: LIGO/Virgo/KAGRA S250818k: J-GEM follow-up observation for AT2025ulz DATE: 25/08/23 05:37:03 GMT FROM: Mahito Sasada at Tokyo Institute of Technology <sasada(a)hp.phys.titech.ac.jp> K. Taguchi (Kyoto U.), Y. Utsumi (NAOJ), M. Sasada (Science Tokyo), R. Itoh (Bisei Astronomical Observatory), T. Morokuma (Chiba Tech), K. Ohta (Kyoto U.), I. Takahashi (Science Tokyo), M. Tanaka (Tohoku), N. Tominaga (NAOJ), M. Yoshida (NAOJ), on behalf of J-GEM collaboration We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414), which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440), using TriCCS, a three-color simultaneous CMOS imager, on the 3.8m Seimei Telescope in Okayama, Japan. We began the observation at 10:47:59 on Aug 20, 2025 (UT), 2.39 days after the merger. Images in g, r, i, and z were integrated for 3899, 3899, 1919, and 1979 secs, respectively, resulting in coadded images with limiting magnitudes of 22.12, 21.87, 21.85, and 21.11 (AB, 5 sigma) Differential image analysis against the PanSTARRS archival data (Chambers et al. 2016) using ZOGY (Zackay et al. 2016) has been attempted, but no obvious source has been revealed. A Sersic model of n=0.5 subtracted the host galaxy reasonably well, which revealed a possible remnant in g, r, i at the source location reported in GCN 41414; however, the forced aperture photometry at the reported location was consistent with the background sky fluctuation. Further analysis is underway. View this GCN Circular online at https://gcn.nasa.gov/circulars/41505. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39568] LIGO/Virgo/KAGRA S250818k: PRIME nIR observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41504 SUBJECT: LIGO/Virgo/KAGRA S250818k: PRIME nIR observations of AT2025ulz DATE: 25/08/22 23:39:31 GMT FROM: N. Passaleva at Sapienza University of Rome <niccolo.passaleva(a)uniroma1.it> N. Passaleva (U Rome), J. Durbak (UMD), O. Guiffreda (UMD), E. Troja (U Rome), R.Hamada (Osaka U), A. S. Kutyrev (NASA/GSFC), D. Suzuki (Osaka U), T. Sumi (Osaka U), A. Idei (Osaka U), S. B. Cenko (NASA/GSFC) We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O'Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) in H filter and in J filter with PRIME ~87.62 hours and ~111.62 hours after the GW trigger. Using nearby stars from the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) for calibration we estimate a 3-sigma limiting magnitude of H>20 AB for the image. Within the transient's host galaxy we see no obvious point source, although template subtraction is required to place a meaningful upper limit to its brightness. PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024). We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41504. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39567] LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41503 SUBJECT: LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz DATE: 25/08/22 18:13:18 GMT FROM: J. An <jiean0813(a)foxmail.com> J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Our first epoch observation started on 2025 Aug 20.61 UT, i.e., 2.55 days after the GW trigger time, in the Sloan g-, r-, and i- filters with 1 hr exposure for each filter. By image subtraction with the Legacy Survey template, we obtained a provisional magnitude of r = 22.8 +/- 0.3 (AB) at 2025 August 20.63 UT for AT2025ulz. We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41503. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39566] LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41502 SUBJECT: LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz DATE: 25/08/22 17:59:55 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Rosa L. Becerra (U Rome), Alan M. Watson (UNAM), and Eleonora Troja (U Rome) report on behalf of a larger team: We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440), with the OSIRIS instrument mounted on the GTC telescope. Our observations began at 2025-08-21 21:40 UTC (3.9 days after the trigger) and were carried out in the r and z filters with an average airmass of ~1.3 and excellent seeing. After image subtraction against Legacy Survey templates (Dey et al. 2019), we detect the source with a preliminary magnitude of r~23.4 AB, consistent with the near-contemporaneous observations of Malesani et al. (GCN 41492). We also confirm a very red color, in agreement with earlier observations (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480). Further observations are planned. We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41502. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39565] LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41501 SUBJECT: LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz DATE: 25/08/22 17:12:58 GMT FROM: André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos(a)gmail.com> A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; P. D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493), possibly associated with the GW superevent S250818k (LVK Collaboration, GCN 41437, GCN 41440), using the T80N 0.8-m robotic telescope using the 1.4 x 1.4 FoV T80N-Cam imager. The observations started on Aug 21, 21:46 UT (~3.85 days after trigger). We obtained 600s (2x300s)and 1200s (4x300s) stacked exposures in r and i bands respectively, with the T80-Cam centered at the position of the reported transient. Subtracting Pan-SSTARRS templates available at MAST archive using photpipe (Rest et al. 2005), we do not detect any sources in our difference images and derive 3-sigma limiting magnitudes of r > 22.9 and i > 22.8 mag for the transient. We thank the OAJ Data Processing and Archiving Department (DPAD) for reducing and calibrating the OAJ data used in this work, as well as the distribution of the data products through a dedicated web portal. View this GCN Circular online at https://gcn.nasa.gov/circulars/41501. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39564] LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41500 SUBJECT: LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz DATE: 25/08/22 16:13:06 GMT FROM: Aishwarya L Thakur at INAF-IAPS, Rome <aishth(a)outlook.com> G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report: We observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 21, 16:30 UT (at 3.6 days post-trigger) for a total of 2 hours (1.5 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 6.4 uJy/beam. We detect radio emission at the position of the transient, and we measure an integrated flux density of 77 +/- 16 uJy (corresponding to a ~ 10-sigma detection). However, given the northern declination of the field, the angular resolution is insufficient to disentangle any potential contamination from the host with this epoch of observation alone. Further MeerKAT observations are planned. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. —------------------------ [1] INAF-IAPS View this GCN Circular online at https://gcn.nasa.gov/circulars/41500. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39563] GRB 250806A: Gemini observations of host galaxy candidate
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41499 SUBJECT: GRB 250806A: Gemini observations of host galaxy candidate DATE: 25/08/22 16:12:48 GMT FROM: gsriniv2(a)umd.edu C. Sevilla (Cornell), G. Srinivasaragavan (UMD), D. A. Perley (LJMU), A. Y. Q. Ho (Cornell), A. Bochenek (LJMU) report: We observed the candidate host galaxy of GRB 250806A (Xie et al., GCN 41243, Malesani et al. GCN 41279) beginning UTC 2025-08-13 13:14:40 with the Gemini Multi-Object Spectrograph on Gemini North under program GN-2025B-Q-125 (PI: Srinivasaragavan). We use a 1 arcsecond slit, the R400 grating, and the GG455 filter. We obtained four exposures of 450 seconds each, two at a central wavelength of 520 nm and two at 525 nm. We obtain a redshift of 0.367 from galactic Hβ, [O II], and [O III] emission lines. At this redshift, using the 4 - 120 keV flux from Bouchet et al. (GCN 41287) and observed burst time of 20 seconds, we estimate the burst has an isotropic equivalent energy of ~ 8e49 erg. We thank the Gemini Contact Scientists and the Gemini Helpdesk for their assistance in observing and data reduction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41499. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39562] IPN triangulation of GRB 250821B (long/bright)
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41498 SUBJECT: IPN triangulation of GRB 250821B (long/bright) DATE: 25/08/22 15:46:04 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team, D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team, and W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report: The bright, long-duration GRB 250821B (Ferm-GBM detection: The Fermi GBM team, GCN 41473; Smith and Meegan, GCN 41477; BALROG localization: Preis and Greiner, GCN 41475; SVOM-GRM detection: Tan et al., GCN 41486) was detected by Fermi (GBM trigger 777485272), SVOM (GRM), Konus-Wind, and Mars-Odyssey (HEND), at about 58067 s UT (16:07:47). We have triangulated it to a preliminary, 3 sigma error box whose coordinates are: --------------------------------------------- RA(2000), deg Dec(2000), deg --------------------------------------------- Center: 29.373 (01h 57m 30s) -82.081 (-82d 04' 51") Corners: 26.171 (01h 44m 41s) -82.255 (-82d 15' 19") 32.420 (02h 09m 41s) -81.925 (-81d 55' 30") 32.427 (02h 09m 42s) -81.883 (-81d 52' 58") 26.212 (01h 44m 51s) -82.213 (-82d 12' 48") --------------------------------------------- The error box area is 127 sq. arcmin, and its maximum dimension is 56 arcmin (the minimum one is 2 arcmin). The Sun distance was 106 deg. This localization may be improved. The IPN localization is consistent with, but reduces the area of, the Fermi-GBM (GCN 41473) and BALROG (GCN 41475) localizations. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250821_T58069/IPN The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/41498. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39561] GRB 250821A: Kilonova-Catcher optical upper limits
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41497 SUBJECT: GRB 250821A: Kilonova-Catcher optical upper limits DATE: 25/08/22 15:19:10 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration: We observed the field of GRB 250821A detected by Swift/BAT (Dichiara et al., GCN 41462) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T32 telescope located at Siding Spring Observatory and operated by M. Freeberg. Our observations started at T0+17 min. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the Swift/XRT enhanced position (Beardmore et al., GCN 41472). We report our follow-up results in the table below: +---------------+-----------+-----------+----------------------+-------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+===========+======================+=============+ | 0.86 | 20 x 180s | Ic (Vega) | 19.4 (U.L., 5 sigma) | T32 | +---------------+-----------+-----------+----------------------+-------------+ All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog. We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41497. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39560] EP250821a: Swift/XRT counterpart fading
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41496 SUBJECT: EP250821a: Swift/XRT counterpart fading DATE: 25/08/22 14:41:37 GMT FROM: Simone Dichiara at Pennsylvania State University <sbd5667(a)psu.edu> S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU), K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team: Swift-XRT has performed further follow-up observations of the EP-WXT transient EP250821a (Hu et al., GCN 41459), collecting a total of 3.0 ks of Photon Counting (PC) mode data between T0+61.0 ks and T0+72.9 ks. The source detected in XRT in the first 1.9 ks of data previously reported as “Source 1” (Dichiara et al., GCN 41469), consistent with the optical counterpart (Li et al., GCN 41470, Yao et al., GCN 41485, An et al., GCN 41490), has faded more than 3 sigma in the latest X-ray observation. The count rate derived from the follow-up observation is 2.2 (+/-0.3) x 10^-2 cts/s. The light curve can be modelled with a power-law decay with an index of alpha=1.03 +/- 0.07. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 +/- 0.2. The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2 , in excess of the Galactic value of 7.26 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.51 x 10^-11 (4.48 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from the follow-up observations from T0+61.0 ks to T0+72.9 ks is thus 7.7 x 10^-13 (9.9 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00059/ This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41496. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39559] GRB 250812A: SVOM/ECLAIRs refined analysis
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41495 SUBJECT: GRB 250812A: SVOM/ECLAIRs refined analysis DATE: 25/08/22 14:39:39 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> L. Bouchet, J.-L. Atteia , O. Godet, M. Brunet (IRAP), L. P. Xin(NAOC), Z.M. Wang(BNU) report on behalf of the SVOM/ECLAIRs team: Using the ECLAIRs event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250812A (SVOM burst-id sb25081201), triggered by ECLAIRs onboard SVOM (Xin et al., GCN 41322). The burst is faint and the 4-120 keV lightcurve is featureless. The time-averaged spectrum from T0-51 s to T0+3 s (T0 = 2025-08-12T02:46:03 UTC) in the energy range 4-120 keV is relatively well fitted by a powerlaw model with index -1.19+/-0.27. With this model, the fluence in 4-120 keV is about 7.5 (-1.9,+0.6) e-7 erg/cm^2. All quoted errors are at the 68% confidence level. The spectral index suggests that the Epeak value is above 120 keV. The source redshift is found to be z=2.571 (Van Dalen et al., GCN 41347; de Ugarte Postigo et al., GCN 41348). At this redshift, GRB 250812A lies in the region of type II GRBs in the Amati plot for values of Epeak ≤ 200 keV. We note that the calibration of SVOM/ECLAIRs is ongoing thus these results are preliminary. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this GCN circular is: Laurent Bouchet (laurent.bouchet(a)irap.omp.eu) View this GCN Circular online at https://gcn.nasa.gov/circulars/41495. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39558] GRB 250821A: Swift-BAT refined analysis
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41494 SUBJECT: GRB 250821A: Swift-BAT refined analysis DATE: 25/08/22 14:31:30 GMT FROM: Rahul Gupta at NASA GSFC <rahul.gupta(a)nasa.gov> A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC), S. Dichiara (PSU), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-240 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250821A (trigger #1344131) (Dichiara, et al., GCN Circ. 41462). The BAT ground-calculated position is RA, Dec = 358.427, -28.631 deg which is RA(J2000) = 23h 53m 42.6s Dec(J2000) = -28d 37' 53.3" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%. The mask-weighted BAT light curve exhibits a short-duration main emission starting from ~T-0.5 to ~T+1.2 sec, followed by a weaker, extended emission which last up to ~ T+18 sec. T90 (15-350 keV) is 1.52 +- 0.37 sec (estimated error including systematics). The time-averaged spectrum from T-0.46 to T+1.20 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.83 +- 0.41. The fluence in the 15-150 keV band is 1.3 +- 0.3 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band is 0.9 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1344131 View this GCN Circular online at https://gcn.nasa.gov/circulars/41494. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39557] LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41493 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts DATE: 25/08/22 14:09:35 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro(a)gmail.com> S. J. Smartt, J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard). We surveyed the skymap of the gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Following on from the reported optical candidate AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480, D’Avanzo et al. GCN 41489), we have been taking targeted grizy imaging of this transient (Gillanders et al., GCN 41454). At this specific region we have proprietary data to make significantly deeper reference stacks than those that are available through the STScI public website, and monitoring with Pan-STARRS is ongoing. For the rest of the northern portion of the GW skymap, we surveyed the region in the i-band over three nights: 2025 Aug 20, 21, 22. We covered ~288 square degrees and ~33% of the bilby.fits skymap 90% area each night, with multiple 45s (Aug 20) and 120s (Aug 21, 22) exposures. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artefacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g., Smith et al., 2020, PASP, 132, 1; Smartt et al., 2024, MNRAS 528, 2299). The depth of our images are typically i ~ 21 - 21.5 per exposure, with the first night having better seeing and transparency conditions. We detected 68 candidate extragalactic transients, of which 30 were already registered on the IAU Transient Name Server (TNS) as having been discovered and detected before S250818k merger time. We have registered the other 38 sources on the TNS. To down-select for optical counterpart candidates of S250818k, we applied the following filters: 1. Spatially associated with a host galaxy that has a photometric or spectroscopic redshift within the 2-sigma limit from LVK of 0.025 < z < 0.08, 2. Has some indication of lightcurve evolution over the three nights of observation, 3. We rejected all faint transients spatially coincident with the nucleus of galaxies as being due to either AGN variability or possible subtraction artefacts. We found no plausible candidate for the optical counterpart of S250818k other than AT2025ulz. Six sources with plausible kilonova-like absolute magnitudes and in hosts with consistent redshifts (within errors) with S250818k were found (AT2025uuw, AT2025uso, AT2025uuf, AT2025utr, AT205usy, AT2025uxs), but their lightcurves are either rising or are flat and show no evolution over 48hrs, and they do not appear to be viable candidates at this point. Discovery details and magnitudes of the ATs can be found on the TNS. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys. View this GCN Circular online at https://gcn.nasa.gov/circulars/41493. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39556] LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41492 SUBJECT: LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz DATE: 25/08/22 11:30:51 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), A. Boye (DTU Space), L. Izzo (INAF/OAC and DARK/NBI), G. Leloudas (DTU Space), J. An (NAOC), X. Liu (NAOC), D. Xu (NAOC), M. Fraser (UCD), S. J. Brennan (MPE), J. Broe Bendsten (SDU), I. A. Koch (SDU), S. Lund Wagner (SDU), J. Magaard Knudsen (SDU), J. Hein Pedersen (SDU), J. P. U. Fynbo (DAWN/NBI), R. Holmberg Rasmussen (NOT and Aarhus), K. Valeckas (NOT and NBI), M. De Pasquale (U. Messina) report on behalf of a larger collaboration: We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were started on 2025 Aug 20.875 UT (2.82 days after the GW trigger time). We adopted the filters r, i, and z (exposure times of 1200, 1200, and 1500 s, respectively). Image subtraction using the Legacy sky survey as reference reveals a transient at coordinates (J2000): RA = 15:51:54.19 Dec = +30:54:08.3 with an estimated error of < 0.5". These are consistent with the ZTF position (Stein et al., GCN 41414). The transient is located 1" southwest of the nucleus of its host galaxy, at a projected distance of 1.6 kpc (z = 0.0848; Karambelkar et al., GCN 41436; Banerjee et al., GCN 41476). Using nearby calibrators from the Legacy Survey, we obtained a provisional magnitude measurement of r = 23.2 ± 0.2 (AB), at a mean time of 2025 August 20.883 UT. We caution that we expect to refine our results after securing a deep template with the same instrument and filter. View this GCN Circular online at https://gcn.nasa.gov/circulars/41492. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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