TITLE: GCN CIRCULAR NUMBER: 17774 SUBJECT: GRB 150428A: Swift-BAT refined analysis DATE: 15/04/28 12:49:41 GMT FROM: Takanori Sakamoto at AGU W. H. Baumgartner (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (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 150428A (trigger #639275) (Page, et al., GCN Circ. 17765). The BAT ground-calculated position is RA, Dec = 188.539, 6.967 deg which is RA(J2000) = 12h 34m 09.3s Dec(J2000) = +06d 58' 00.8” with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 16%. The mask-weighted light curve shows a multi-peaked structure that starts at ~ T-40 sec and ends at ~ T+60 sec. T90 (15-350 keV) is 53.2 +- 15.1 sec (estimated error including systematics). The time-averaged spectrum from T-39.0 to T+38.6 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.28 +- 0.09. The fluence in the 15-150 keV band is 6.3 +- 0.3 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+17.4 sec in the 15-150 keV band is 3.0 +- 0.5 ph/cm2/sec. 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/639275/BA/