GCN Circular 37539

T. Parsotan (GSFC), S. D. Barthelmy (GSFC), 
P. A. Evans (U Leicester), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(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 240912A (trigger #1253910)
(P. A. Evans, et al., GCN Circ. 37466
Not found
 
 
).  The BAT ground-calculated position is
RA, Dec = 124.625, 33.999 deg which is 
   RA(J2000)  =  08h 18m 29.9s 
   Dec(J2000) = +33d 59' 56.2" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 86%.
 
The light curve displays an interval of weak flat emission for ~45 seconds before
a bright multi-pulsed emission period. There is a possible shallow rise of emission
at ~T0+280 seconds coincident with the flare seen in the XRT afterglow. 
T90 (15-350 keV) is 113.24 +- 7.83 sec (estimated error including systematics).
 
The time-averaged spectrum from T-2.35 to T+373.02 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.43 +- 0.11, 
and Epeak of 140.5 +- 41.0 keV (chi squared 27.79 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 3.4 +- 0.0 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T+76.05 sec in the 15-150 keV band is
15.8 +- 0.5 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.68 +- 0.03 (chi squared 43.74 for 57 d.o.f.).  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/1253910