August 2025 - vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[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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