TITLE: GCN CIRCULAR NUMBER: 13565 SUBJECT: GRB 120803A: Swift detection of a possible burst DATE: 12/08/03 07:55:44 GMT FROM: David Palmer at LANL J. L. Racusin (NASA/GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), C. Guidorzi (U Ferrara), J. A. Kennea (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. Sakamoto (NASA/UMBC) report on behalf of the Swift Team: At 07:22:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120803A (trigger=529582). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 269.565, -6.715 which is RA(J2000) = 17h 58m 16s Dec(J2000) = -06d 42' 53" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 07:24:43.1 UT, 147.0 seconds after the BAT trigger. No source was detected in 1.0 ks of promptly downlinked data, which covered 91% of the BAT error circle. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 149 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 25% of the BAT 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 BAT 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 0.94. This event is of low significance (7.3 sigma) in the BAT trigger image, and no XRT source has been found in the 91% of the BAT error circle covered by the immediately-available data. It is unusual for a real long GRB to lack an XRT counterpart. Therefore, we cannot confirm the reality of this event until we receive the complete downlinked data. 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.)