TITLE:   GCN CIRCULAR
NUMBER:  17776
SUBJECT: GRB 150428B: Swift-BAT refined analysis 
DATE:    15/04/28 13:45:43 GMT
FROM:    Takanori Sakamoto at AGU  <tsakamoto@phys.aoyama.ac.jp>

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), 
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (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-239 to T+963 sec from the recent telemetry downlink, 
we report further analysis of BAT GRB 150428B (trigger #639278) 
(Page, et al., GCN Circ. 17766).   The BAT ground-calculated position is 
RA, Dec = 292.661, 4.114 deg which is 
 RA(J2000)  =  19h 30m 38.8s 
 Dec(J2000) = +04d 06' 52.2” 
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).  
The partial coding was 40%. 

The mask-weighted light curve shows three peaks.  The first peak starts at T-40 sec, 
peaks at T-25 sec and ends at T-20 sec.  The second peak starts at T-15 sec, peaks 
at T+5 sec and ends at T+50 sec.  And the third peak starts at T+55 sec, peaks at T+65 sec 
and ends at T+120 sec.  T90 (15-350 keV) is 130.9 +- 15.6 sec 
(estimated error including systematics).  

The time-averaged spectrum from T-40.1 to T+121.7 sec is best fit by a power law 
with an exponential cutoff.  This fit gives a photon index 1.0 +- 0.5, and Epeak 
of 55 +- 10 keV (chi squared 37.44 for 56 d.o.f.).  For this model the total 
fluence in the 15-150 keV band is 3.7 +- 0.3 x 10^-6 erg/cm2 and the 1-sec peak flux 
measured from T+1.36 sec in the 15-150 keV band is 1.5 +- 0.3 ph/cm2/sec.  A fit 
to a simple power law gives a photon index of 1.87 +- 0.10 
(chi squared 49.44 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/639278/BA/