TITLE: GCN CIRCULAR NUMBER: 10051 SUBJECT: GRB 091020, Swift-BAT refined analysis DATE: 09/10/21 01:28:59 GMT FROM: Hans Krimm at NASA-GSFC D. M. Palmer (LANL), 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), A. M. Parsons (GSFC), J. L. Racusin (NASA/GSFC/ORAU), T. Sakamoto (GSFC/UMBC), 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-61 to T+242 sec from the recent telemetry downlink, we report further analysis of BAT GRB 091020 (trigger #373458) (Racusin, et al., GCN Circ. 10048). The BAT ground-calculated position is RA, Dec = 175.727, 50.977 deg which is RA(J2000) = 11h 42m 54.4s Dec(J2000) = +50d 58' 36.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 43%. The mask-weighted light curve began with a small hump at T-10 sec, rising sharply to a peak at T+2 sec and decaying exponentially out to T+50 sec, with a much smaller peak superimposed at T+30 sec. T90 (15-350 keV) is 34.6 +- 2.7 sec (estimated error including systematics). The time-averaged spectrum from T-8.7 to T+38.3 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.53 +- 0.07. The fluence in the 15-150 keV band is 3.7 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+1.14 sec in the 15-150 keV band is 4.2 +- 0.3 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/373458/BA/