TITLE: GCN CIRCULAR NUMBER: 13993 SUBJECT: GRB 121125A: Swift detection of a burst with optical counterpart DATE: 12/11/25 08:43:43 GMT FROM: Scott Barthelmy at NASA/GSFC B. N. Barlow (PSU), S. D. Barthelmy (GSFC), D. Grupe (PSU), E. A. Helder (PSU) and C. J. Mountford (U Leicester) report on behalf of the Swift Team: At 08:32:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 121125A (trigger=539563). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 228.520, +55.322 which is RA(J2000) = 15h 14m 05s Dec(J2000) = +55d 19' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multiple-peaked structure with a duration of about 70 sec. The peak count rate was ~4600 counts/sec (15-350 keV), at ~34 sec after the trigger. The XRT began observing the field at 08:33:34.1 UT, 66.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 228.52496, 55.31330 which is equivalent to: RA(J2000) = 15h 14m 05.99s Dec(J2000) = +55d 18' 47.9" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 32 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 http://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 in excess of the Galactic value (1.27 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.7 (+2.20/-1.89) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 4.27e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 75 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 15:14:06.54 = 228.52724 DEC(J2000) = +55:18:48.0 = 55.31332 with a 90%-confidence error radius of about 0.74 arc sec. This position is 1.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.19 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is B. N. Barlow (bnb2 AT 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/too.html.)