TITLE: GCN CIRCULAR NUMBER: 31289 SUBJECT: GRB 211221A: Swift-BAT refined analysis DATE: 21/12/23 15:01:27 GMT FROM: Tyler Parsotan at UMBC/GSFC/CRESST II S. D. Barthelmy (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC), T. Sakamoto (AGU), M. Stamatikos (OSU), (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec sec from the recent telemetry downlink, we report further analysis of BAT GRB 211221A (trigger #1090472) (Page et al., GCN 31273). The BAT ground-calculated position is RA, Dec = 78.906, 0.902 deg which is RA(J2000) = 05h 15m 37.4s Dec(J2000) = +00d 54' 06.2" with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%. The BAT light curve showed a complex peak structure with a duration of about 20 sec. There may be hints of precursor activity and low-level post-peak emission from the GRB as seen in the mask weighted light curve. The post-peak emission lasts beyond the available event data (which ends at T+963). T90 (15-350 keV) is 671.67 +- 105.96 sec (estimated error including systematics). The time-averaged spectrum from T-129.40 to T+688.36 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.48 +- 0.13. The fluence in the 15-150 keV band is 4.0 +- 0.3 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T-0.27 sec in the 15-150 keV band is 0.5 +- 0.1 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/1090472/BA/