TITLE: GCN CIRCULAR NUMBER: 26535 SUBJECT: GRB 191220A: Swift-BAT refined analysis DATE: 19/12/21 21:19:35 GMT FROM: Sibasish Laha at GSFC D. M. Palmer (LANL), 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), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 191220A (trigger #945344) (Laha et al., GCN Circ. 26516). The BAT ground-calculated position is RA, Dec = 281.335, 26.679 deg which is RA(J2000) = 18h 45m 20.3s Dec(J2000) = +26d 40' 43.1" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 19%. The mask weighted light curve shows complex structures with a duration of ~50 sec, with a hint of some weak emission lasting till ~ T+ 240 s. The overall structure starts at ~ T-2 sec and lasts till ~ T+60 sec. T90 (15-350 keV) is 175.55 +- 125.93 sec (estimated error including systematics). The time-averaged spectrum from T-2.46 to T+248.42 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.12 +- 0.40. The fluence in the 15-150 keV band is 1.1 +- 0.3 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+6.69 sec in the 15-150 keV band is 1.1 +- 0.4 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/945344/BA/