TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4365
SUBJECT: GRB 051221:  Refined analysis of the Swift-BAT short hard burst
DATE:    05/12/21 05:07:18 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott@lheamail.gsfc.nasa.gov>

J. Cummings (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cannizzo (GSFC-UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (GSFC/UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), 
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), M. Tashiro (Saitama U.), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

Using the data from the recent telemetry downlink, we report
further analysis of Swift-BAT GRB 051221 (trigger #173780)
(Parsons, et al., GCN 4363).  We have received data covering T-60
to T+120 seconds.  The ground-analysis position is
RA,Dec 328.711,+16.892 {21h 54m 50.7s,16d 53' 31.9"} (J2000)
with an uncertainty of 0.8 arcmin (radius, 90%, stat+sys).
T90 is 1.4 +- 0.2 sec.  The partial coding fraction is 63%.
The lightcurve has an initial hard spike at T+0.3 sec (FWHM of ~0.25 sec)
followed by 3 softer peaks at T+0.4, T+0.8, and T+1.2 sec.
Fitting a simple power law over the time-averaged interval (T+0.2 to T+2.4 sec),
gives a hard spectrum with a photon index of 1.39 +/- 0.06 and a fluence
of 1.16 +/- 0.04 X 10^-6 erg/cm^2.  The peak flux in a 1-sec wide window
starting at T+0.2 sec is 12.1 +/- 0.4 ph/cm^2/sec.  All values are
in the 15-150 keV band at the 90% confidence level.