GCN Circular 10283

H. A. Krimm (CRESST/GSFC/USRA), D. N. Burrows (PSU),
P.A. Curran (MSSL-UCL), A. de Ugarte Postigo (INAF-OAB),
N. Gehrels (NASA/GSFC), D. Grupe (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
N. P. M. Kuin (MSSL), J. Mao (INAF-OAB),
R. Margutti (Univ Bicocca&OAB), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-IASFPA),
M. H. Siegel (PSU), M. C. Stroh (PSU), E. Troja (NASA/GSFC/ORAU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 20:52:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091221 (trigger=380311).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 55.754, +23.212 which is 
   RA(J2000) = 03h 43m 01s
   Dec(J2000) = +23d 12' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a peak from T+0 to T+15
seconds with a second, larger peak from T+15 to T+40. There is
some complexity in the light curve from A0535+262 (currently in outburst
at ~5 Crabs of flux) in the Field Of View, but the GRB peaks are
distinguishable from the softer A0535+262 emission.  The peak count
rate was 3500 counts/sec at ~30 sec after the trigger. 

The XRT began observing the field at 20:54:04.7 UT, 72.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 55.79831, 23.24030
which is equivalent to:
   RA(J2000)  = 03h 43m 11.59s
   Dec(J2000) = +23d 14' 25.1"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at http://www.swift.ac.uk/sper.  This position is 
170 arcseconds from the BAT position. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.09e+21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.47e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 79 seconds after the BAT trigger. No credible
afterglow candidate has  been found in the initial data products. The
2.7'x2.7' sub-image covers 100% of  the XRT error circle. The typical
3-sigma upper limit has been about 19.6 mag.  The 8'x8' region for the
list of sources generated on-board covers 100% of the  XRT error
circle. The list of sources is typically complete to about 18 mag. No 
correction has been made for the expected extinction corresponding to
E(B-V) of  0.22.  We note the possibility of a faint source on the
northern edge of the XRT error circle.  Further analysis is
forthcoming. 

Burst Advocate for this burst is H. A. Krimm (krimm AT milkyway.gsfc.nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)