TITLE:   GCN CIRCULAR
NUMBER:  4388
SUBJECT: GRB 051221A:  Refined spectral and temporal analysis of the Swift-BAT short hard burst
DATE:    05/12/22 17:02:29 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott@lheamail.gsfc.nasa.gov>

J. Norris (GSFC), T. Sakamoto (GSFC/NRC), D. Band (GSFC/UMBC), S. Barthelmy (GSFC)
on behalf of the Swift-BAT team:

The time profile of GRB 051221A binned to 1 ms resolution reveals
that the initial pulse structure (GCN 4363 & 4365) comprises 3 separate pulses
of FWHM ~10-15 ms with peak intensities of ~175,000 counts per sec.
We note for comparison that GRB 050525A, the brightest long burst
so far detected by BAT (in one year) had a peak count rate
of 101,000 cts/sec (corrected to match the same partial-coding as 051221A
of 63%).  The GRB 051221A time profile is available at:
http://swift.gsfc.nasa.gov/docs/swift/results/releases/images/GRB051221A/

No extended emission is evident in this burst in the interval 30-120 sec
after the initial pulse structure.  The 3 sigma upper limit is 1.16 counts/cm2.
The ratio of (extended emission, 15-50 keV) / (initial pulse complex 15-150 keV)
is < 0.1 (3 sigma).  The same ratio for GRB 050724 is 1.9.

The spectral lag is negligible, 0.0+-0.4 ms (0.8+-0.5 ms), between the
15-25 and 50-100 keV (25-50 and 100-350 keV) energy bands -- typical
of spectral lags in short bursts (Norris & Bonnell, submitted to ApJ).

We estimate roughly that the peak flux of GRB 051221A lies in the upper 3%
of short bursts detectable by the BAT.