TITLE: GCN CIRCULAR NUMBER: 42924 SUBJECT: GRB 251201B: Swift detection of a burst with an optical counterpart DATE: 25/12/01 16:38:08 GMT FROM: Tyler Parsotan at NASA GSFC <tyler.parsotan@nasa.gov> M. J. Moss (GSFC), S. Dichiara (PSU), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester) and T. M. Parsotan (GSFC) report on behalf of the Neil Gehrels Swift Observatory Team: At 16:26:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251201B (trigger=1418729). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 6.409, -44.237 which is RA(J2000) = 00h 25m 38s Dec(J2000) = -44d 14' 12" 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 40 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 16:28:03.7 UT, 93.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 6.3925, -44.2312 which is equivalent to: RA(J2000) = 00h 25m 34.20s Dec(J2000) = -44d 13' 52.3" with an uncertainty of 7.0 arcseconds (radius, 90% containment). This location is 47 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 0.1 s image was 8.63e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 101 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:25:34.59 = 6.39413 DEC(J2000) = -44:14:01.1 = -44.23364 with a 90%-confidence error radius of about 0.75 arc sec. This position is 9.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.28 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.008. Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 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/42924. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...