TITLE: GCN CIRCULAR NUMBER: 26562 SUBJECT: GRB 191221B: Swift-BAT refined analysis DATE: 19/12/22 21:07:26 GMT FROM: Sibasish Laha at GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), 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 191221B (trigger #945521) (Laha et al., GCN Circ. 26534). The BAT ground-calculated position is RA, Dec = 154.831, -38.152 deg which is RA(J2000) = 10h 19m 19.3s Dec(J2000) = -38d 09' 07.1" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 12%. The mask weighted light curve shows complex structures with a duration of ~30 s. The overall structure starts at ~T-10 s and lasts till ~T+20s. There is some hint of weak emission extending up to ~ T+60 s. T90 (15-350 keV) is 48.00 +- 16.00 sec (estimated error including systematics). The time-averaged spectrum from T-15.28 to T+96.72 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.24 +- 0.05. The fluence in the 15-150 keV band is 1.9 +- 0.1 x 10^-05 erg/cm2. the fluence of this burst is among the brightest ~3% of all BAT-detected GRBs (based on fluences from the BAT GRB catalog https://swift.gsfc.nasa.gov/results/batgrbcat/). The 1-sec peak photon flux measured from T+8.22 sec in the 15-150 keV band is 4.8 +- 0.7 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/945521/BA/