TITLE: GCN CIRCULAR NUMBER: 9460 SUBJECT: GRB 090531: Swift-BAT refined analysis DATE: 09/05/31 16:36:42 GMT FROM: Scott Barthelmy at NASA/GSFC T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU) (i.e. the Swift-BAT team): Using the data set from T-60 to T+243 sec from recent telemetry downlinks, we report further analysis of BAT GRB 090531 (trigger #353627) (Stamatikos, et al., GCN Circ. 9454). The BAT ground-calculated position is RA, Dec = 178.672, 7.824 deg, which is RA(J2000) = 11h 54m 41.4s Dec(J2000) = +07d 49' 27.0" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 73%. The mask-weighted light curve shows a single peak starting at ~T-25 sec, peaking at T+9 sec, and ending at ~T+60 sec. There is a possible (2.5 sigma CL) episode of emission from T+80 to ~T+140 sec. T90 (15-350 keV) is 32 +- 6 sec (estimated error including systematics). The time-averaged spectrum from T-12.6 to T+29.4 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.86 +- 0.42, and Epeak of 64.7 +- 14.3 keV (chi squared 51.91 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2 and the 1-sec peak flux measured from T+9.29 sec in the 15-150 keV band is 1.3 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.68 +- 0.09 (chi squared 65.50 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/353627/BA/