GRB 051221: Swift detection of a bright short burst

TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 4363
SUBJECT: GRB 051221: Swift detection of a bright short burst
DATE: 05/12/21 02:24:36 GMT
FROM: Scott Barthelmy at NASA/GSFC Scott@lheamail.gsfc.nasa.gov

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.