GRB 240809A: Swift-BAT refined analysis

Using the data set from T-240 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 240809A (trigger #1247745) (Evans, et al., GCN Circ. 37110). The BAT ground-calculated position is RA, Dec = 237.545, -2.331 deg which is RA(J2000) = 15h 50m 10.8s Dec(J2000) = -02d 19' 51.1" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 72%. The mask-weighted light curve shows a single peaked fast rise exponential decay structure with a tail that lasts until ~100 seconds after the trigger time. T90 (15-350 keV) is 72.86 +- 19.39 sec (estimated error including systematics). The time-averaged spectrum from -10.876 to 180.52 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.17 +- 0.05. The fluence in the 15-150 keV band is 9.7 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T+2.68 sec in the 15-150 keV band is 12.8 +- 0.4 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1247745