TITLE: GCN CIRCULAR NUMBER: 11991 SUBJECT: GRB 110503A: Swift detection of a burst with a bright optical counterpart DATE: 11/05/03 17:59:20 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester), V. Mangano (INAF-IASFPA), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), P. T. O\'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), M. H. Siegel (PSU), R. L. C. Starling (U Leicester), C. A. Swenson (PSU), E. Troja (NASA/GSFC/ORAU) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 17:35:45 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110503A (trigger=452685). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 132.800, +52.216 which is RA(J2000) = 08h 51m 12s Dec(J2000) = +52d 12' 56" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single structure with a duration of about 10 sec. The peak count rate was ~17,000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 17:37:19.2 UT, 93.6 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 132.7778, +52.2067 which is equivalent to: RA(J2000) = 08h 51m 06.67s Dec(J2000) = +52d 12' 24.1" with an uncertainty of 5.1 arcseconds (radius, 90% containment). This location is 58 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 nominal 150.000 seconds with the White filter starting 212 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 08:51:06.24 = 132.77598 DEC(J2000) = +52:12:27.5 = 52.20764 with a 90%-confidence error radius of about 1.10 arc sec. This position is 5.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.75. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 AT nasa.gov). 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.)