TITLE: GCN CIRCULAR NUMBER: 12256 SUBJECT: GRB 110808A: Swift detection of a burst with optical counterpart DATE: 11/08/08 06:36:32 GMT FROM: Giulia Stratta at ASDC G. Stratta (ASDC), W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), A. Melandri (INAF-OAB), K. L. Page (U Leicester) and C. J. Saxton (UCL-MSSL) report on behalf of the Swift Team: At 06:18:54 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110808A (trigger=458918). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 57.264, -44.179 which is RA(J2000) = 03h 49m 03s Dec(J2000) = -44d 10' 42" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 10 sec. The peak count rate was ~400 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 06:20:45.1 UT, 110.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 57.2666, -44.1944 which is equivalent to: RA(J2000) = 03h 49m 3.97s Dec(J2000) = -44d 11' 40.0" with an uncertainty of 2.6 arcseconds (radius, 90% containment). This location is 55 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.04 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 3 (+3.41/-2.82) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 5.42e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 120 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:49:04.28 = 57.26782 DEC(J2000) = -44:11:40.3 = -44.19452 with a 90%-confidence error radius of about 0.83 arc sec. This position is 5.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.93 with a 1-sigma error of about 0.19. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is G. Stratta (giulia.stratta AT asdc.asi.it). 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.)