TITLE: GCN CIRCULAR NUMBER: 38796 SUBJECT: GRB 250103B: Swift detection of a burst DATE: 25/01/03 15:58:32 GMT FROM: P.A. Evans at U. Leicester <pae9@leicester.ac.uk> R. A. J. Eyles-Ferris (U Leicester), S. Dichiara (PSU), P. A. Evans (U Leicester), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 15:44:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250103B (trigger=1278865). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 54.651, -33.772 which is RA(J2000) = 03h 38m 36s Dec(J2000) = -33d 46' 18" 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 ~600 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 15:46:48.8 UT, 151.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 54.66403, -33.74965 which is equivalent to: RA(J2000) = 03h 38m 39.37s Dec(J2000) = -33d 44' 58.7" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 89 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 in excess of the Galactic value (9.99 x 10^19 cm^-2, Willingale et al. 2013), with an excess column of 4.1 (+3.96/-3.24) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 155 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.008. Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 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/38796. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...