TITLE: GCN CIRCULAR NUMBER: 12720 SUBJECT: GRB 111225A: Swift detection of a burst with an optical counterpart DATE: 11/12/25 04:12:26 GMT FROM: David Palmer at LANL M. H. Siegel (PSU), W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), D. Grupe (PSU), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), C. A. Swenson (PSU), T. N. Ukwatta (MSU) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 03:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 111225A (trigger=510341). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 13.102, +51.589 which is RA(J2000) = 00h 52m 24s Dec(J2000) = +51d 35' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The immediately available lightcurve was truncated for contact with the ground station. The BAT lightcurve showed a rise at around T-10 seconds with flux continuing at least until T+8 seconds, the limit of of the immediately available data. The peak count rate was ~1300 counts/sec (15-350 keV), at ~-2 sec after the trigger. The XRT began observing the field at 03:52:05.9 UT, 88.1 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 13.1548, 51.5741 which is equivalent to: RA(J2000) = +00h 52m 37.15s Dec(J2000) = +51d 34' 26.8" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 129 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 2.5 s image was 2.96e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 95 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:52:37.21 = 13.15503 DEC(J2000) = +51:34:19.5 = 51.57207 with a 90%-confidence error radius of about 0.75 arc sec. This position is 7.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.42 with a 1-sigma error of about 0.20. No correction has been made for the expected extinction corresponding to E(B-V) of 0.26. Burst Advocate for this burst is M. H. Siegel (siegel AT astro.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.)