TITLE:   GCN CIRCULAR
NUMBER:  31312
SUBJECT: GRB 211225B: Swift-BAT refined analysis
DATE:    21/12/27 00:17:54 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC),
T. Sakamoto (AGU), M. Stamatikos (OSU)
(i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 211225B (trigger #1090853)
(Page et al., GCN Circ. 31305).  The BAT ground-calculated position is
RA, Dec = 258.545, 17.277 deg which is
   RA(J2000)  =  17h 14m 10.9s
   Dec(J2000) = +17d 16' 37.2"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a multi-peaked structure that starts
at ~T-25 s and likely lasts beyond ~T+152 s when the burst went out of the
BAT
field of view. The main peak occurs at ~T+50 s. T90 (15-350 keV)
estimated from the available data when the burst was inside the BAT field
of view is 121.54 +- 2.94 sec (estimated error including systematics).

The time-averaged spectrum from T-24.78 to T+139.82 sec is best fit by
a power law with an exponential cutoff.  This fit gives a photon
index 1.31 +- 0.14, and Epeak of 139.9 +- 51.5 keV (chi squared 52.29
for 56 d.o.f.).  For this model the total fluence in the 15-150 keV
band is 1.5 +- 0.03 x 10^-5 erg/cm2 and the 1-sec peak flux measured
from T+50.19 sec in the 15-150 keV band is 6.1 +- 0.3 ph/cm2/sec.
A fit to a simple power law gives a photon index of 1.59 +- 0.03
(chi squared 64.87 for 57 d.o.f.).  All the quoted errors are at
the 90% confidence level.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/1090853/BA/