TITLE: GCN CIRCULAR NUMBER: 18752 SUBJECT: GRB 151228B: Swift-BAT refined analysis DATE: 15/12/29 14:29:59 GMT FROM: Scott Barthelmy at NASA/GSFC H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), 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-61 to T+243 sec from the recent telemetry downlinks, we report further analysis of BAT GRB 151228B (trigger #668641) (Lien, et al., GCN Circ. 18739). The BAT ground-calculated position is RA, Dec = 344.431, 8.081 deg, which is RA(J2000) = 22h 57m 43.4s Dec(J2000) = +08d 04' 52.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The mask-weighted light curve shows a FreD-like peak starting at ~T-10 sec, peaking at ~T+3 sec, and ending at ~T+65 sec. T90 (15-350 keV) is 48.0 +- 16.0 sec (estimated error including systematics). The time-averaged spectrum from T-12.56 to T+67.44 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.09 +- 0.34, and Epeak of 78.1 +- 27.0 keV (chi squared 52.3 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.4 +- 0.1 x 10^-6 erg/cm2 and the 1-sec peak flux measured from T+10.94 sec in the 15-150 keV band is 1.0 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.66 +- 0.08 (chi squared 61.6 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/668641/BA/