TITLE: GCN CIRCULAR NUMBER: 40224 SUBJECT: GRB 250424A: Swift detection of a burst with an optical counterpart DATE: 25/04/24 07:12:30 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> S. B. Cenko (GSFC), V. D'Elia (SSDC & INAF-OAR), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), A. Melandri (INAF-OAR), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 06:52:28.51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250424A (trigger=1306404). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 217.513, -35.022 which is RA(J2000) = 14h 30m 03s Dec(J2000) = -35d 01' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 30 sec. The peak count rate was ~25000 counts/sec (15-350 keV), at ~15 sec before the trigger. The XRT began observing the field at 06:56:39.3 UT, 250.9 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 217.50010, -35.02493 which is equivalent to: RA(J2000) = 14h 30m 00.03s Dec(J2000) = -35d 01' 29.7" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 39 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 does not constrain the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 254 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 14:29:60.00 = 217.49998 DEC(J2000) = -35:01:30.6 = -35.02518 with a 90%-confidence error radius of about 0.63 arc sec. This position is 1 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.80 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.064. Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 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/40224. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...