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

[vsnet-grb-info 39486] GRB 250818B: EP-FXT counterpart detection
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41424 SUBJECT: GRB 250818B: EP-FXT counterpart detection DATE: 25/08/19 02:25:18 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> R.-Z. Li (YNAO, CAS), Y.-C. Fu (BNU), W.-D. Zhang and Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250818B (SVOM/sb25081801, Wang et al. GCN 41405) at 2025-08-18 06:00:27 (UTC), about 2.52 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~2 ks. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Kumar et al. GCN 41406, Ferro et al. GCN 41407, Yao et al. GCN 41409, Zheng et al. GCN 41417, Fong et al. GCN 41419). Preliminary analysis on this source are conducted, with the details listed as follows. Source 1: EPF_J030413.4-030730 RA (J2000): 46.056 Dec (J2000): -3.1247 Flux: 7.5432e-12 erg/s/cm2 (observed, 0.5-10 kev) Flux_err: 6.1924e-13 erg/s/cm2 (90% C. L.) The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic). 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/41424. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39485] GRB 250818A: Swift-BAT refined analysis
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41423 SUBJECT: GRB 250818A: Swift-BAT refined analysis DATE: 25/08/19 01:42:00 GMT FROM: Amy <yarleen(a)gmail.com> S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa), 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-721 to T+482 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250818A (trigger #1343270) (Cenko et al., GCN Circ. 41403). The BAT ground-calculated position is RA, Dec = 213.816, -58.010 deg which is RA(J2000) = 14h 15m 15.8s Dec(J2000) = -58d 00' 36.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 44%. The mask-weighted light curve shows a multi-peaked structure that starts at ~T-15 s and ends at ~T+140 s. The main peak occurs at ~T+78 s. T90 (15-350 keV) is 106.86 +- 3.79 sec (estimated error including systematics). The time-averaged spectrum from T-15.92 to T+141.43 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 +- 0.04. The fluence in the 15-150 keV band is 1.2 +- 0.03 x 10^-05 erg/cm2. The 1-sec peak photon flux measured from T+77.63 sec in the 15-150 keV band is 5.5 +- 0.3 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/1343270/ View this GCN Circular online at https://gcn.nasa.gov/circulars/41423. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39484] Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41422 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB DATE: 25/08/19 01:41:04 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> Jacob Smith (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 777251788/250818970 at 23:16:23.51 UT on 18 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/41422. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39483] LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41421 SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz DATE: 25/08/19 01:00:05 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.), and Antonella Palmese (Carnegie Mellon U.) report: We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-18T19:35:11 for 40 x 180 s. We took 30 observations in the giJ configuration and 10 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at g = (21.25 +/- 0.03) AB mag, r = (21.43 +/- 0.06) AB mag. The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41421. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39482] GRB 250818C: Fermi GBM Final Real-time Localization
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41420 SUBJECT: GRB 250818C: Fermi GBM Final Real-time Localization DATE: 25/08/18 23:26:54 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 LONG GRB At 23:16:23 UT on 18 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818C (trigger 777251788.512804 / 250818970). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 203.8, Dec = 74.7 (J2000 degrees, equivalent to J2000 13h 35m, 74d 42'), with a statistical uncertainty of 1.0 degrees. The angle from the Fermi LAT boresight is 0.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41420. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39481] GRB 250818B: Keck redshift of the optical afterglow
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41419 SUBJECT: GRB 250818B: Keck redshift of the optical afterglow DATE: 25/08/18 22:13:52 GMT FROM: Wen-fai Fong at Northwestern University <wfong(a)northwestern.edu> W. Fong, A. C. Gordon (Northwestern), A. J. Levan (Radboud), N. R. Tanvir (Leicester), Y. Dong, A. Suresh, C. Liu (Northwestern) report: We observed the position of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407). We obtained spectroscopy of the optical afterglow candidate detected by GOTO (Kumar et al., GCN 41406) and SVOM/VT (Yao et al., GCN 41409) using the Low-Resolution Imaging Spectrograph (LRIS) mounted on the Keck I telescope atop Mauna Kea in Hawaii (PI: Chang Liu; Program O397). We obtained 3x300-sec exposures using the 400/3400 grism on the blue side and the 400/8500 grating on the red side at an airmass of 1.2 in 0.9” seeing and clear conditions, starting on 2025 Aug 18 at 14:10 UT (approximately 10.68 hr post-burst). The spectrum spans approximately 3500 to 9500 Ang. The continuum is well-detected along with several prominent absorption features. In particular, we identify absorption features of FeII (2344, 2374, 2382), MgII (2796, 2803), MgI (2852), CaII H&K, and possibly AlII (1670), at a common redshift of z=1.216 which we consider to be the redshift of the GRB. This implies an optical luminosity of ~5e45 erg/s at 0.67 hr after the burst (using Kumar et al., GCN 41406), which is on the bright side compared to the handful of short GRBs observed at these epochs. We thank Keck Observatory staff Rita Morris and Josh Walawender for their assistance with our observing night. View this GCN Circular online at https://gcn.nasa.gov/circulars/41419. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39480] Fermi trigger No 776849714: Global MASTER-Net observations report
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41418 SUBJECT: Fermi trigger No 776849714: Global MASTER-Net observations report DATE: 25/08/18 22:09:57 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-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB250814.32 (trigger No 776849714,19h 52m 09.60s , +47d 27m 36.0s, R=8.61) errorbox 28727 sec after notice time and 28760 sec after trigger time at 2025-08-14 15:34:30 UT, with upper limit up to 19.5 mag. The observations began at zenith distance = 6 deg. The sun altitude is -21.4 deg. The galactic latitude b = 10 deg., longitude l = 82 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2968191 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 28851 | 2025-08-14 15:34:30 | MASTER-Tunka | (20h 33m 22.90s , +48d 36m 16.9s) | C | 180 | 16.4 | 28851 | 2025-08-14 15:34:30 | MASTER- | (20h 17m 19.59s , +48d 42m 56.2s) | C | 180 | 16.3 | 30056 | 2025-08-14 15:54:35 | MASTER-Tunka | (20h 33m 27.75s , +48d 35m 30.4s) | C | 180 | 16.3 | 30056 | 2025-08-14 15:54:35 | MASTER- | (20h 17m 24.16s , +48d 42m 05.5s) | C | 180 | 16.1 | 31332 | 2025-08-14 16:15:51 | MASTER-Tunka | (20h 33m 21.59s , +48d 34m 42.2s) | C | 180 | 16.3 | 31332 | 2025-08-14 16:15:51 | MASTER- | (20h 17m 18.27s , +48d 41m 13.3s) | C | 180 | 16.1 | 32228 | 2025-08-14 16:30:47 | MASTER-Tunka | (20h 33m 26.53s , +48d 35m 48.2s) | C | 180 | 16.4 | 32228 | 2025-08-14 16:30:47 | MASTER- | (20h 17m 22.76s , +48d 42m 16.4s) | C | 180 | 16.1 | 32361 | 2025-08-14 16:33:59 | MASTER-Tunka | (20h 33m 26.48s , +48d 34m 27.3s) | C | 60 | 19.5 | 32361 | 2025-08-14 16:33:59 | MASTER- | (20h 17m 23.23s , +48d 40m 55.6s) | C | 60 | 19.3 | 32433 | 2025-08-14 16:35:11 | MASTER-Tunka | (20h 33m 30.41s , +48d 35m 46.3s) | C | 60 | 19.5 | 32433 | 2025-08-14 16:35:11 | MASTER- | (20h 17m 26.76s , +48d 42m 14.4s) | C | 60 | 19.3 | 32505 | 2025-08-14 16:36:23 | MASTER-Tunka | (20h 33m 24.26s , +48d 35m 02.8s) | C | 60 | 19.4 | 32505 | 2025-08-14 16:36:23 | MASTER- | (20h 17m 20.90s , +48d 41m 30.9s) | C | 60 | 19.3 | 32577 | 2025-08-14 16:37:36 | MASTER-Tunka | (20h 33m 24.14s , +48d 36m 00.8s) | C | 60 | 19.4 | 32577 | 2025-08-14 16:37:36 | MASTER- | (20h 17m 20.53s , +48d 42m 28.1s) | C | 60 | 19.3 | 32650 | 2025-08-14 16:38:49 | MASTER-Tunka | (20h 33m 30.50s , +48d 34m 58.3s) | C | 60 | 19.4 | 32650 | 2025-08-14 16:38:49 | MASTER- | (20h 17m 27.29s , +48d 41m 25.8s) | C | 60 | 19.3 | 32723 | 2025-08-14 16:40:02 | MASTER-Tunka | (20h 33m 22.90s , +48d 33m 55.9s) | C | 60 | 19.4 | 32723 | 2025-08-14 16:40:02 | MASTER- | (20h 17m 20.07s , +48d 40m 23.1s) | C | 60 | 19.3 | 32797 | 2025-08-14 16:41:16 | MASTER-Tunka | (20h 33m 30.19s , +48d 34m 09.2s) | C | 60 | 19.4 | 32797 | 2025-08-14 16:41:16 | MASTER- | (20h 17m 27.34s , +48d 40m 36.2s) | C | 60 | 19.3 | 32870 | 2025-08-14 16:42:29 | MASTER-Tunka | (20h 33m 27.31s , +48d 35m 42.0s) | C | 60 | 19.4 | 32870 | 2025-08-14 16:42:29 | MASTER- | (20h 17m 24.00s , +48d 42m 08.7s) | C | 60 | 19.3 | 32942 | 2025-08-14 16:43:41 | MASTER-Tunka | (20h 33m 27.15s , +48d 34m 10.2s) | C | 60 | 19.4 | 32942 | 2025-08-14 16:43:41 | MASTER- | (20h 17m 24.35s , +48d 40m 36.5s) | C | 60 | 19.3 | 33018 | 2025-08-14 16:44:57 | MASTER-Tunka | (20h 33m 31.39s , +48d 35m 22.4s) | C | 60 | 19.4 | 33018 | 2025-08-14 16:44:57 | MASTER- | (20h 17m 28.24s , +48d 41m 48.4s) | C | 60 | 19.3 | 33094 | 2025-08-14 16:46:12 | MASTER-Tunka | (20h 33m 24.22s , +48d 34m 44.4s) | C | 60 | 19.4 | 33094 | 2025-08-14 16:46:12 | MASTER- | (20h 17m 21.30s , +48d 41m 10.3s) | C | 60 | 19.3 | 33169 | 2025-08-14 16:47:28 | MASTER-Tunka | (20h 33m 25.21s , +48d 35m 42.1s) | C | 60 | 19.4 | 33169 | 2025-08-14 16:47:28 | MASTER- | (20h 17m 21.99s , +48d 42m 07.7s) | C | 60 | 19.3 | 33241 | 2025-08-14 16:48:40 | MASTER-Tunka | (20h 33m 30.18s , +48d 34m 39.6s) | C | 60 | 19.5 | 33241 | 2025-08-14 16:48:40 | MASTER- | (20h 17m 27.28s , +48d 41m 05.0s) | C | 60 | 19.3 | 33315 | 2025-08-14 16:49:54 | MASTER-Tunka | (20h 33m 24.74s , +48d 33m 37.1s) | C | 60 | 19.4 | 33315 | 2025-08-14 16:49:54 | MASTER- | (20h 17m 22.16s , +48d 40m 02.3s) | C | 60 | 19.3 | 33389 | 2025-08-14 16:51:07 | MASTER-Tunka | (20h 33m 30.29s , +48d 33m 58.2s) | C | 60 | 19.4 | 33389 | 2025-08-14 16:51:08 | MASTER- | (20h 17m 27.60s , +48d 40m 23.0s) | C | 60 | 19.2 | 33463 | 2025-08-14 16:52:22 | MASTER-Tunka | (20h 33m 28.09s , +48d 35m 05.7s) | C | 60 | 19.4 | 33463 | 2025-08-14 16:52:22 | MASTER- | (20h 17m 25.02s , +48d 41m 30.4s) | C | 60 | 19.2 | 33536 | 2025-08-14 16:53:35 | MASTER-Tunka | (20h 33m 28.01s , +48d 33m 59.1s) | C | 60 | 19.3 | 33536 | 2025-08-14 16:53:35 | MASTER- | (20h 17m 25.29s , +48d 40m 23.5s) | C | 60 | 19.1 | 33608 | 2025-08-14 16:54:46 | MASTER-Tunka | (20h 33m 31.60s , +48d 35m 24.4s) | C | 60 | 19.3 | 33608 | 2025-08-14 16:54:46 | MASTER- | (20h 17m 28.43s , +48d 41m 48.6s) | C | 60 | 19.1 | 33680 | 2025-08-14 16:55:59 | MASTER-Tunka | (20h 33m 24.74s , +48d 34m 26.1s) | C | 60 | 19.4 | 33680 | 2025-08-14 16:55:59 | MASTER- | (20h 17m 21.87s , +48d 40m 50.0s) | C | 60 | 19.2 | 33753 | 2025-08-14 16:57:12 | MASTER-Tunka | (20h 33m 25.74s , +48d 35m 24.3s) | C | 60 | 19.4 | 33753 | 2025-08-14 16:57:12 | MASTER- | (20h 17m 22.56s , +48d 41m 47.6s) | C | 60 | 19.2 | 33828 | 2025-08-14 16:58:27 | MASTER-Tunka | (20h 33m 31.07s , +48d 34m 21.8s) | C | 60 | 19.4 | 33828 | 2025-08-14 16:58:27 | MASTER- | (20h 17m 28.21s , +48d 40m 45.1s) | C | 60 | 19.3 | 33902 | 2025-08-14 16:59:41 | MASTER-Tunka | (20h 33m 26.34s , +48d 33m 19.5s) | C | 60 | 19.4 | 33902 | 2025-08-14 16:59:41 | MASTER- | (20h 17m 23.82s , +48d 39m 42.3s) | C | 60 | 19.3 | 33974 | 2025-08-14 17:00:53 | MASTER-Tunka | (20h 33m 30.83s , +48d 33m 42.9s) | C | 60 | 19.4 | 33974 | 2025-08-14 17:00:53 | MASTER- | (20h 17m 28.18s , +48d 40m 05.7s) | C | 60 | 19.3 | 34052 | 2025-08-14 17:02:11 | MASTER-Tunka | (20h 33m 28.99s , +48d 34m 50.2s) | C | 60 | 19.5 | 34052 | 2025-08-14 17:02:11 | MASTER- | (20h 17m 25.96s , +48d 41m 12.6s) | C | 60 | 19.3 | 34125 | 2025-08-14 17:03:24 | MASTER-Tunka | (20h 33m 28.87s , +48d 33m 29.3s) | C | 60 | 19.4 | 34125 | 2025-08-14 17:03:24 | MASTER- | (20h 17m 26.28s , +48d 39m 51.3s) | C | 60 | 19.3 | 34199 | 2025-08-14 17:04:37 | MASTER-Tunka | (20h 33m 32.08s , +48d 34m 44.8s) | C | 60 | 19.4 | 34199 | 2025-08-14 17:04:37 | MASTER- | (20h 17m 29.07s , +48d 41m 06.9s) | C | 60 | 19.3 | 34275 | 2025-08-14 17:05:54 | MASTER-Tunka | (20h 33m 26.69s , +48d 34m 08.9s) | C | 60 | 19.4 | 34275 | 2025-08-14 17:05:54 | MASTER- | (20h 17m 23.88s , +48d 40m 30.2s) | C | 60 | 19.3 | 34349 | 2025-08-14 17:07:08 | MASTER-Tunka | (20h 33m 26.26s , +48d 35m 06.8s) | C | 60 | 19.4 | 34349 | 2025-08-14 17:07:08 | MASTER- | (20h 17m 23.14s , +48d 41m 27.8s) | C | 60 | 19.2 | 34422 | 2025-08-14 17:08:21 | MASTER-Tunka | (20h 33m 31.92s , +48d 34m 04.4s) | C | 60 | 19.4 | 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/41418. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39479] GRB 250818B: KAIT optical observations
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41417 SUBJECT: GRB 250818B: KAIT optical observations DATE: 25/08/18 21:38:07 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 GRB 250818B (Wang et al., GCN 41405) with a set of 90x60s images in the clear (roughly R) filters, at a mid time of 8.0 hours after the trigger. We clearly detected the optical afterglow (Kumar et al., GCN 41406; Yao et al, GCN 41409) in the coadd image with a brightness of 19.9 +/- 0.2 mag (Vega). View this GCN Circular online at https://gcn.nasa.gov/circulars/41417. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39478] GRB 250818A: Swift/UVOT Upper Limits
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41416 SUBJECT: GRB 250818A: Swift/UVOT Upper Limits DATE: 25/08/18 17:50:17 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250818A 162 s after the BAT trigger (Cenko et al., GCN Circ. 41403). No optical afterglow consistent with the XRT position (Osborn et al., GCN Circ. 41403) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u (fc) 162 412 246 >20.3 b 418 438 19 >18.6 v 32212 32641 429. >20.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.602 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41416. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39477] LIGO/Virgo/KAGRA S250818t: Updated Sky localization
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41415 SUBJECT: LIGO/Virgo/KAGRA S250818t: Updated Sky localization DATE: 25/08/18 17:16:28 GMT FROM: lucy.thomas(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 S250818t (GCN Circular 41404). 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/S250818t For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 319 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1209 +/- 290 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/41415. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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