TITLE: GCN CIRCULAR NUMBER: 17562 SUBJECT: GRB 150309A: Swift-BAT refined analysis DATE: 15/03/10 19:58:42 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift W. H. Baumgartner (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-239 to T+471 sec from the recent telemetry downlink, we report further analysis of BAT GRB 150309A (trigger #634200) (Cummings, et al., GCN Circ. 17553). The BAT ground-calculated position is RA, Dec = 277.004, 86.417 deg which is RA(J2000) = 18h 28m 01.0s Dec(J2000) = +86d 25' 02.7" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%. The mask-weighted light curve shows that the main peak of the burst, which occurred during a Swift preplanned slew maneuver, started at about T-200 seconds. The roughly symmetrical peak with multiple maxima lasted until about T-160 seconds. A weaker overlapping peak followed at about T-150 seconds and declined to background by about T-60 seconds. BAT triggered on a smaller peak that lasted from about T+0 seconds to about T+120 seconds. T90 (15-350 keV) is 242 +- 6 sec (estimated error including systematics). Because the source came into the BAT field of view during the slew, it is possible that there was some additional low-level emission before T-200 seconds. The time-averaged spectrum from T-197.32 to T+119.08 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.05 +- 0.16, and Epeak of 175.8 +- 76.0 keV (chi squared 37.47 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.7 +- 0.0 x 10^-05 erg/cm2 and the 1-sec peak flux measured from T-177.49 sec in the 15-150 keV band is 9.0 +- 0.3 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.36 +- 0.04 (chi squared 49.55 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/634200/BA/