TITLE: GCN CIRCULAR NUMBER: 23123 SUBJECT: GRB 180809B, Swift-BAT refined analysis DATE: 18/08/10 18:20:37 GMT FROM: Amy Lien at GSFC M. Stamatikos (OSU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), M. J. Moss (GWU), D. M. Palmer (LANL), T. Sakamoto (AGU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-240 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 180809B (trigger #852553) (Moss et al., GCN Circ. 23105). The BAT ground-calculated position is RA, Dec = 299.712, -15.293 deg which is RA(J2000) = 19h 58m 50.8s Dec(J2000) = -15d 17' 34.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 63%. The mask-weighted light curve shows a multi-peaked structure that starts at ~T-36 s and ends at ~T+80 s, followed by another pulse that starts at ~T+200 s and ends at ~T+250 s. The total burst emission ends at ~T+300 s. T90 (15-350 keV) is 233.2 +- 1.1 sec (estimated error including systematics). We note that the later pulse overlaps with the XRT flare and some ground-based optical observations (e.g. Tiurina et al.; GCN #23113), which indicates an increase in the optical magnitude from Mag 16.5 to 15.5 at the pulse time. The time-averaged spectrum from T-36.1 to T+302.3 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.26 +- 0.02. The fluence in the 15-150 keV band is 7.3 +- 0.1 x 10^-5 erg/cm2. The 1-sec peak photon flux measured from T+35.10 sec in the 15-150 keV band is 26.3 +- 0.6 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/852553/BA/