GCN Circular 4363

A. Parsons (GSFC), S. Barthelmy (GSFC), D. Burrows (PSU), M. Capalbi (ASDC),
J. Cummings (GSFC/NRC), N. Gehrels (GSFC), C. Gronwall (PSU),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC),
K. Page (U Leicester), D. Palmer (LANL),
on behalf of the Swift team:

At 01:51:16 UT, Swift-BAT triggered and located GRB 051221 (trigger=173780).
The spacecraft slewed immediately.  The BAT on-board calculated location
is RA,Dec 328.711d,+16.896d {21h 54m 51s,+16d 53' 45"} with an uncertainty
of 3 arcmin (radius, 90% containment, stat+sys).  The BAT light curve shows
a single bright short peak at T0 with a duration less than 128 msec
followed by a 2nd smaller and much softer peak at T+1 sec with an exponential
decay lasting ~3 sec.  The peak count rate was ~70,000 counts/sec (15-350 keV),
at ~0 seconds after the trigger.

The XRT began observing the target at 01:52:44 UT, 88 sec after the burst.
There was no source bright enough for an on-board centroid determination,
but the TDRSS spectrum and lightcurve suggest the presence of an X-ray source
in the field of view.  Further analysis will require the full data dump
through the Malindi ground station.

UVOT took a finding chart exposure of 199.9 seconds with the V filter
starting 86.1 seconds after the BAT trigger.  No 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 is about 18.7 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 18.0 mag.  No correction has been made for the expected
visual extinction of about 0.23 magnitudes.