TITLE: GCN CIRCULAR NUMBER: 9465 SUBJECT: GRB 090531B: Swift-BAT refined analysis DATE: 09/06/01 00:03:02 GMT FROM: Scott Barthelmy at NASA/GSFC T. N. Ukwatta (GWU), 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), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC) (i.e. the Swift-BAT team): Using the data set from T-61 to T+242 sec from the recent telemetry downlink, we report further analysis of BAT GRB 090531B (trigger #353728) (Cummings, et al., GCN Circ. 9461). The BAT ground-calculated position is RA, Dec = 252.070, -36.015 deg, which is RA(J2000) = 16h 48m 16.8s Dec(J2000) = -36d 00' 53.4" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The mask-weighted light curve shows an initial bright spike with a total width of 1.3 sec. It returns to baseline and then starts (!T+15 sec) a long smooth rise to a peak around T+40 sec and a smooth decline ending at ~T+110 sec. The spike is spectrally harder than the long smooth peak. This lightcurve is consistance with a short hard burst with an extended emission. T90 (15-350 keV) is 80 +- 23 sec (estimated error including systematics). The time-averaged spectrum from T-13.0 to T+67.0 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.51 +- 0.22. The fluence in the 15-150 keV band is 7.1 +- 0.9 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+0.2 sec in the 15-150 keV band is 1.25 +- 0.1 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/353728/BA/