GCN Circular 13565

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.)