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@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.eyJlbWFpbCI6InZzbmV0LW...