TITLE:   GCN CIRCULAR
NUMBER:  21370
SUBJECT: GRB 170728A: Swift-BAT refined analysis 
DATE:    17/07/28 22:55:07 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),
J. K. Cannizzo (NASA/UMBC), J. R. Cummings (CPI),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), T. N. Ukwatta (LANL)
(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 170728A (trigger #765008)
(Cannizzo et al., GCN Circ. 21367).  The BAT ground-calculated position is
RA, Dec = 58.901, 12.169 deg which is
  RA(J2000)  =  03h 55m 36.3s
  Dec(J2000) = +12d 10' 07.3"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 73%.

The mask-weighted light curve shows a weak pulse that starts at ~T0 and ends
at ~T+1.5. There is no evidence of extended emission. T90 (15-350 keV)
is 1.25 +- 0.23 sec (estimated error including systematics).

The time-averaged spectrum from T+0.17 to T+1.56 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -0.49 +- 1.81,
and Epeak of 38.6 +- 7.3 keV (chi squared 51.37 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 7.1 +- 1.4 x 10^-8 erg/cm2
and the 1-sec peak flux measured from T+0.20 sec in the 15-150 keV band is
1.1 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.02 +- 0.26 (chi squared 60.76 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level.

The spectrum is on the very soft end of the BAT-detected short GRB, only
a handful of BAT-detected short GRBs have comparable hardness ratio as
this burst (Lien & Sakamoto et al. 2016).

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/765008/BA/