GCN Circular 13796

V. N. Yershov (UCL-MSSL), S. D. Barthelmy (GSFC),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
C. J. Mountford (U Leicester), S. R. Oates (UCL-MSSL),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 05:16:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120923A (trigger=534402).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 303.795, +6.235 which is 
   RA(J2000)  =  20h 15m 11s
   Dec(J2000) = +06d 14' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several overlapping peaks
with a total duration of about 35 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 05:18:26.0 UT, 139.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 303.79631, 6.22039 which is equivalent
to:
   RA(J2000)  = 20h 15m 11.11s
   Dec(J2000) = +06d 13' 13.4"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 52 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 9.80
x 10^20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 143 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
0.16. 

Burst Advocate for this burst is V. N. Yershov (vny 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.)