TITLE: GCN CIRCULAR NUMBER: 41959 SUBJECT: GRB 250924A: Swift detection of a burst with an optical counterpart DATE: 25/09/24 08:35:33 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> M. H. Siegel (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), S. Lanava (PSU), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 08:18:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250924A (trigger=1351790). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 51.865, +74.652 which is RA(J2000) = 03h 27m 28s Dec(J2000) = +74d 39' 07" 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 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~17 sec after the trigger. The XRT began observing the field at 08:21:32.8 UT, 183.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 51.87141, 74.65894 which is equivalent to: RA(J2000) = 03h 27m 29.14s Dec(J2000) = +74d 39' 32.2" with an uncertainty of 2.7 arcseconds (radius, 90% containment). This location is 25 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 consistent with the Galactic value of 2.77 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 1.03e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 192 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:27:29.62 = 51.87340 DEC(J2000) = +74:39:34.0 = 74.65945 with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.74 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.316. 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/41959. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...