GCN Circular 9854

P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and M. A. Stark (PSU) report on behalf
of the Swift Team:

At 21:30:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090831C (trigger=361489).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 108.278, -25.100 which is 
   RA(J2000)  =  07h 13m 07s
   Dec(J2000) = -25d 06' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 10 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 21:32:23.5 UT, 117.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 108.29307, -25.11934 which
is equivalent to:
   RA(J2000)  = 07h 13m 10.34s
   Dec(J2000) = -25d 07' 09.6"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 85 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 consistent with the Galactic value of
3.24e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. Results from the 2.7'x2.7' sub-image
are not available at this time. The coverage of the XRT error circle by the
8'x8' region for the list of sources generated on-board is uncertain because
the large number of sources filled the available telemetry. No correction has
been made for the expected extinction corresponding to E(B-V) of 0.38. 




Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.ac.uk). 
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.)