TITLE: GCN CIRCULAR NUMBER: 18292 SUBJECT: GRB 150911A: Swift-BAT refined analysis DATE: 15/09/12 14:47:18 GMT FROM: Tilan Ukwatta at LANL C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), R. L. C. Starling (U Leicester), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 150911A (trigger #655262) (Starling, et al., GCN Circ. 18280). The BAT ground-calculated position is RA, Dec = 67.428, 5.731 deg which is RA(J2000) = 04h 29m 42.6s Dec(J2000) = +05d 43' 50.2" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 67%. The mask-weighted light curve shows a broad peak starting around T-2 sec and ending around T+10 sec. T90 (15-350 keV) is T90 (15-350 keV) is 7.2 +- 0.6 sec (estimated error including systematics). The time-averaged spectrum from T-1.04 to T+9.61 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.02 +- 0.26, and Epeak of 51.6 +- 3.8 keV (chi squared 52.34 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.1 +- 0.1 x 10^-06 erg/cm2 and the 1-sec peak flux measured from T+3.90 sec in the 15-150 keV band is 5.2 +- 0.3 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.93 +- 0.05 (chi squared 97.49 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/655262/BA/