TITLE: GCN CIRCULAR NUMBER: 21912 SUBJECT: GRB 170921A: Swift-BAT refined analysis DATE: 17/09/22 14:27:15 GMT FROM: Amy Lien at GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), A. D'Ai (INAF-IASFPA) H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 170921A (trigger #773509) (D'Ai et al., GCN Circ. 21901). The BAT ground-calculated position is RA, Dec = 26.840, 30.880 deg which is RA(J2000) = 01h 47m 21.6s Dec(J2000) = +30d 52' 47.1" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 86%. The mask-weighted light curve shows a single-peaked structure that starts at ~T-22 s, peaks at ~T+3 s, and ends at ~T+13 s. T90 (15-350 keV) is 28.9 +- 6.0 sec (estimated error including systematics). The time-averaged spectrum from T-21.82 to T+13.00 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.76 +- 0.17. The fluence in the 15-150 keV band is 6.7 +- 0.8 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+2.74 sec in the 15-150 keV band is 0.8 +- 0.2 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/773509/BA/