TITLE:   GCN CIRCULAR
NUMBER:  29975
SUBJECT: GRB 210509A: Swift-BAT refined analysis
DATE:    21/05/10 04:33:00 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

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

Using the data set from T-60 to T+243 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 210509A (trigger #1047531)
(Page et al., GCN Circ. 29970).  The BAT ground-calculated position is
RA, Dec = 350.363, 6.650 deg which is
   RA(J2000)  =  23h 21m 27.2s
   Dec(J2000) = +06d 39' 01.1"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 95%.

The mask-weighted light curve shows several overlapping pulses that
start at ~T-2 s and end at ~T+8 s. The burst went out of the BAT FOV
at ~T+77 s. T90 (15-350 keV) is 8.22 +- 0.75 sec (estimated error
including systematics).

The time-averaged spectrum from T-1.65 to T+7.63 sec is best fit by
a power law with an exponential cutoff.  This fit gives a photon
index 0.76 +- 1.00, and Epeak of 33.0 +- 8.3 keV (chi squared 70.16
for 56 d.o.f.).  For this model the total fluence in the 15-150 keV band
is 3.1 +- 0.4 x 10^-7 erg/cm2 and the 1-sec peak flux measured from
T+3.24 sec in the 15-150 keV band is 1.2 +- 0.2 ph/cm2/sec.  A fit to
a simple power law gives a photon index of 2.18 +- 0.17 (chi squared
78.23 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/1047531/BA/