TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4382 SUBJECT: GRB 051221B: Refined analysis of the Swift-BAT burst DATE: 05/12/22 05:46:05 GMT FROM: Scott Barthelmy at NASA/GSFC E. Fenimore (LANL), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), T. Takahashi (ISAS), J. Tueller (GSFC) on behalf of the Swift-BAT team: Using the full data set from the recent telemetry downlink covering T-300 to T+300 sec, we report further analysis of Swift-BAT GRB 051221B (trigger #173904) (Boyd, et al., GCN 4376). The ground-analysis position is RA,Dec 312.359,+53.040 {20h 49m 26.1s,+53d 02' 23.4"} (J2000) with an uncertainty of 1.6 arcmin (radius, 90%, stat+sys). The partial coding fraction is 45%. The lightcurve has two, possibly 3, peaks starting at T+49 sec with a total duration of 70 sec. T90 is 61 +- 1 sec. Fitting a simple power law over the full interval from T+49 to T+119 sec, the photon index is 1.48 +/- 0.18 with a fluence of 1.13 +/- 0.13 X 10^-6 erg/cm^2. The peak flux in a 1-sec wide window starting at T+59 sec is 0.54 +/- 0.20 ph/cm^2/sec. All values are in the 15-150 keV band at the 90% confidence level. A final note: we think it is very likely that this is a GRB, but we can not rule out the possibility that it is a hard x-ray transient.