TITLE: GCN CIRCULAR NUMBER: 13566 SUBJECT: GRB 120803B: Swift detection of a burst DATE: 12/08/03 11:21:49 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT J. L. Racusin (NASA/GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P. A. Evans (U Leicester), C. Guidorzi (U Ferrara), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), O. M. Littlejohns (U Leicester) and K. L. Page (U Leicester) report on behalf of the Swift Team: At 11:06:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120803B (trigger=529606). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 314.224, +53.282 which is RA(J2000) = 20h 56m 54s Dec(J2000) = +53d 16' 57" 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 30 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 11:07:22.4 UT, 75.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 314.23606, 53.30380 which is equivalent to: RA(J2000) = 20h 56m 56.65s Dec(J2000) = +53d 18' 13.7" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 82 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 consistent with the Galactic value of 5.67 x 10^21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 149 seconds with the White filter starting 78 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 3.5% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 1.71. Burst Advocate for this burst is J. L. Racusin (judith.racusin 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.)