TITLE: GCN CIRCULAR NUMBER: 12215 SUBJECT: GRB 110731A: Swift detection of a burst with an optical counterpart DATE: 11/07/31 11:23:28 GMT FROM: David Palmer at LANL S. R. Oates (UCL-MSSL), A. P. Beardmore (U Leicester), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), A. Melandri (INAF-OAB), C. J. Mountford (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), C. J. Saxton (UCL-MSSL) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 11:09:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110731A (trigger=458448). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 280.521, -28.546 which is RA(J2000) = 18h 42m 05s Dec(J2000) = -28d 32' 44" 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 20 sec. The peak count rate was ~20000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 11:10:36.9 UT, 66.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 280.5042, -28.5363 which is equivalent to: RA(J2000) = 18h 42m 01.00s Dec(J2000) = -28d 32' 10.6" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 63 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. 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) = 18:42:01.01 = 280.50419 DEC(J2000) = -28:32:13.7 = -28.53713 with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.80 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.17. Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.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/too.html.)