GCN Circular 12279

C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
T. A. Pritchard (PSU), A. Rowlinson (U Leicester),
T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Swift
Team:

At 20:37:49 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110818A (trigger=500914).  Because of an Earth-limb
observing constraint, the slew to the burst was delayed by ~4 min. 
The BAT on-board calculated location is 
RA, Dec 317.384, -63.970 which is 
   RA(J2000)  =  21h 09m 32s
   Dec(J2000) = -63d 58' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks
with a total duration of about 60 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 20:44:09.7 UT, 380.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 317.3382,
-63.9814 which is equivalent to:
   RA(J2000)  = 21h 09m 21.17s
   Dec(J2000) = -63d 58' 53.1"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 83 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 (2.62 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2
(+1.14/-1.05) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the U filter starting
856 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 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.04. 

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.)