TITLE: GCN CIRCULAR NUMBER: 40694 SUBJECT: GRB 250612A: Swift detection of a burst or new Galactic transient Swift J1643.6-3854 DATE: 25/06/12 01:27:51 GMT FROM: James DeLaunay at PSU <jjd330@psu.edu> M. H. Siegel (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:02:05.82 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250612A (trigger=1323295). The BAT on-board calculated location is RA, Dec 250.903, -38.902 which is RA(J2000) = 16h 43m 37s Dec(J2000) = -38d 54’ 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 5 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~1 sec after the trigger. Due to a Moon observing constraint, Swift cannot slew to the BAT position until 11:59 UT on 2025 June 12. There will thus be no XRT or UVOT data for this trigger before this time. Given the location of the BAT detection at a Galactic latitude of 4.6 deg and no confirmation of a fading afterglow due to observing constraints, we cannot rule out a Galactic origin. If this is a new Galactic transient we would name it Swift J1643.6-3854. 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/40694. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...