TITLE:   GCN CIRCULAR
NUMBER:  10291
SUBJECT: GRB 091221 Swift-BAT refined analysis
DATE:    09/12/22 16:44:09 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <james.r.cummings@nasa.gov>

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC),
T. N. Ukwatta (GWU)
(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 091221 (trigger #380311)
(Krimm, et al., GCN Circ.#10283).  The BAT ground-calculated position is
RA, Dec = 55.798, 23.243 deg which is
    RA(J2000)  =  03h 43m 11.6s
    Dec(J2000) = +23d 14' 35.2"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 62%.

The mask-weighted light curve shows a weak peak from T-45 to T-20 sec 
and a stronger, slow-rise, complex peak from T-10 to T+42 sec.  T90
(15-350 keV) is 68.5 +- 5.5 sec (estimated error including systematics).

The time-averaged spectrum from T-43.6 to T+41.1 sec is best fit by a 
simple power-law model.  The power law index of the time-averaged
spectrum is 1.59 +- 0.06.  The fluence in the 15-150 keV band is
5.7 +- 0.2 x 10^-06 erg/cm2.  The 1-sec peak photon flux measured from
T+20.70 sec in the 15-150 keV band is 3.0 +- 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/380311/BA/