TITLE:   GCN CIRCULAR
NUMBER:  36291
SUBJECT: GRB 240421B: Swift-BAT refined analysis
DATE:    24/04/24 21:46:59 GMT
FROM:    Sibasish Laha at GSFC <sibasish.laha@nasa.gov>

S. D. Barthelmy (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), T. Parsotan (GSFC),
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 240421B (trigger #1223470)
(Williams, et al., GCN Circ. 36208).  The BAT ground-calculated position is
RA, Dec = 299.685, -14.878 deg which is
   RA(J2000)  =  19h 58m 48.03s
   Dec(J2000) =  -14d 51' 36.4"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 4%.

The BAT light curve shows a complex structure with a duration of ~ 40 sec.
T90 (15-350 keV) is 24.00 +- 9.22 sec (estimated error including systematics).

The time-averaged spectrum from T-30.4 to T+7.6 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.63 +- 0.52,
and Epeak of 81.3 +- 22.3 keV (chi squared 50.17 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.4 +- 0.5 x 10^-06 erg/cm2
and the 1-sec peak flux measured from T-0.36 sec in the 15-150 keV band is
14.1 +- 1.6 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.55 +- 0.10 (chi squared 61.45 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/1223470