GCN Circular 13885

S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. J. Mountford (U Leicester),
P. Romano (INAF-IASFPA) and M. C. Stroh (PSU) report on behalf of the
Swift Team:

At 18:08:06 UT, the Swift Burst Alert Telescope (BAT) triggered
(536502) on a noise fluctuation in the image domain which we likely
believe was a chance coincidence with the source SAX J1818.6-1703 
in the on-board catalog.  As is typical for image triggers (64 sec,
image significance=5.81 sigma), the real-time TDRSS lightcurve
does not show anything significant. 

The XRT began observing the field at 18:16:45.8 UT, 519.7 seconds after
the BAT trigger. No source was detected in 441 s of promptly downlinked
data, which covered 87% of the BAT error circle. We are waiting for the
full dataset to detect and localise any possible XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 546 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 25% of
the BAT 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
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain extinction expected.