GRB 200909B: Swift-BAT refined analysis

Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 200909B (trigger #995288) (Melandri, et al., GCN Circ. 28406). The BAT ground-calculated position is RA, Dec = 175.171, -31.199 deg which is RA(J2000) = 11h 40m 41.1s Dec(J2000) = -31d 11' 54.7" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 35%. The mask-weighted light curve shows a single peak that stars at T-6 sec, peaks at T+1 sec, and ends at T+20 sec. A hint of an extended emission is visible until T+40 sec. T90 (15-350 keV) is 16.9 +- 2.2 sec (estimated error including systematics). The time-averaged spectrum from T-5.6 to T+14.2 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.60 +- 0.14. The fluence in the 15-150 keV band is 1.2 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T-0.44 sec in the 15-150 keV band is 1.3 +- 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/995288/BA/