GCN Circular 11181

T. Sakamoto (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
J. M. Gelbord (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
W.B Landsman (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC),
R. L. C. Starling (U Leicester) and M. C. Stroh (PSU) report on behalf
of the Swift Team:

At 19:31:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100902A (trigger=433160).  BAT slewed to the location
after a brief delay due to observing constraints. 
The BAT on-board calculated location is 
RA, Dec 48.646, +30.973 which is 
   RA(J2000) = 03h 14m 35s
   Dec(J2000) = +30d 58' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a small spike 
around the trigger time.  There are brighter spikes around T+150 sec 
and T+200 sec with a duration of 50 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~210 sec after the trigger. 

The XRT began observing the field at 19:37:10.1 UT, 316.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
48.6294, 30.9791 which is equivalent to:
   RA(J2000)  = 03h 14m 31.05s
   Dec(J2000) = +30d 58' 44.8"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 56 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.13e+21
cm^-2, Kalberla et al. 2005), with an excess column of 4.1
(+2.41/-2.03) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 2.20e-07 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 150 seconds with the
White filter  starting 325 seconds after the BAT trigger. No credible
afterglow candidate has  been found in the initial data products. Data
from the 2.7'x2.7' sub-image are  not available at this time. 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 0.31. 

Burst Advocate for this burst is T. Sakamoto (Taka.Sakamoto AT 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.)