GCN Circular 11227

C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU), V. D'Elia (ASDC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
J. M. Gelbord (PSU), O. Godet (U Leicester), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), W.B Landsman (GSFC),
O. M. Littlejohns (U Leicester), F. E. Marshall (NASA/GSFC),
D. C. Morris (GWU/GSFC), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), R. L. C. Starling (U Leicester) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 13:49:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100906A (trigger=433509).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 28.709, +55.614, which is 
   RA(J2000)  =  01h 54m 50s
   Dec(J2000) = +55d 36' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple bright peaks
with a duration of about 130 sec.  The peak count rate
was ~11000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 13:50:47.6 UT, 80.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 28.68305, 55.62985
which is equivalent to:
   RA(J2000)  = 01h 54m 43.93s
   Dec(J2000) = +55d 37' 47.5"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 78 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 (2.21 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 2.6
(+2.21/-1.90) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 2.34e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
146 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	01:54:44.11 =  28.68380
  DEC(J2000) = +55:37:49.6  =  55.63045
with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.90 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.36. 

A bright XRT flare at ~120 s corresponds to the last peak in the prompt emission. 
The UVOT bright source is probably also associated in time with the flare.  More
information to follow. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)