TITLE: GCN CIRCULAR NUMBER: 38752 SUBJECT: GRB 250101A: Swift detection of a burst with an optical counterpart DATE: 25/01/01 13:39:49 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> K. L. Page (U Leicester), R. Gupta (NASA GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 13:22:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250101A (trigger=1278305). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 37.061, +19.154 which is RA(J2000) = 02h 28m 15s Dec(J2000) = +19d 09' 14" 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 25 sec. The peak count rate was ~400 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 13:24:57.4 UT, 127.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 37.06963, 19.19521 which is equivalent to: RA(J2000) = 02h 28m 16.71s Dec(J2000) = +19d 11' 42.8" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 151 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.44 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 132 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 02:28:16.50 = 37.06873 DEC(J2000) = +19:11:39.9 = 19.19442 with a 90%-confidence error radius of about 1.10 arc sec. This position is 3.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.30. No correction has been made for the expected extinction corresponding to E(B-V) of 0.164. Burst Advocate for this burst is K. L. Page (klp5 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/38752. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...