TITLE:   GCN CIRCULAR
NUMBER:  43234
SUBJECT: GRB 251221A: Swift-BAT refined analysis
DATE:    25/12/25 05:09:07 GMT
FROM:    Mike Moss at NASA GSFC <mikejmoss3@gmail.com>

A. Y. Lien (U Tampa), E. Ambrosi (INAF-IASFPA),
S. D. Barthelmy (GSFC), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 251221A (trigger #1426088)
(Ambrosi, et al., GCN Circ. 43166).  The BAT ground-calculated position is
RA, Dec = 101.422, 0.738 deg which is 
   RA(J2000)  =  06h 45m 41.4s 
   Dec(J2000) = +00d 44' 15.8" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 42%.
 
The mask-weighted light curves displays a complicated multi-pulsed light curve.
The T90 (15-350 keV) is 56.86 +- 7.05 sec (estimated error including systematics).
 
The time-averaged spectrum from T-38.10 to T+27.74 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.25 +- 0.13.  The fluence in the 15-150 keV band is 2.0 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+2.91 sec in the 15-150 keV band
is 1.1 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1426088