TITLE:   GCN CIRCULAR
NUMBER:  22406
SUBJECT: Trigger 809374: Swift detection of a possible non-burst faint source
DATE:    18/02/10 01:05:50 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

J. R. Cummings (NASA/UMBC), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
A. Deich (PSU), J.D. Gropp (PSU), B. Sbarufatti (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:09:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible faint non-burst source (trigger=809374).  Swift slewed 
immediately to the location of the source. 
The BAT on-board calculated location is 
RA, Dec 299.747, +35.038 which is 
   RA(J2000)  =  19h 58m 59s
   Dec(J2000) = +35d 02' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows nothing significant, 
which is as expected for a 5-minute image trigger. 

The XRT began observing the field at 00:16:09.6 UT, 414.1 seconds after
the BAT trigger. No credible source was detected in 850 s of promptly 
downlinked data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 417 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 coverage of the BAT 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. The list of sources is typically complete to
about 18 mag. No correction has been made for the large, but uncertain,
extinction expected. 

Since the nature of this trigger is uncertain at the moment we will
wait for the full dataset to determine if the source is real.