TITLE:   GCN CIRCULAR
NUMBER:  22449
SUBJECT: Swift Trigger 811686 is probably not an astrophysical source. 
DATE:    18/02/25 20:44:51 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 20:30:51 UT, the Swift Burst Alert Telescope (BAT) triggered on a noise peak
(trigger=811686).  Swift slewed immediately. 
The BAT on-board calculated location is 
RA, Dec 185.609, +29.293, which is 
   RA(J2000)  =  12h 22m 26s
   Dec(J2000) = +29d 17' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, there is nothing
significant in the real-time light curve. 

The XRT began observing the field at 20:33:42.7 UT, 171.3 seconds after
the BAT trigger. No source was detected in 406 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 178 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 expected extinction corresponding to E(B-V) of
0.02. 

Swift followed up on this location due to a program of verifying
marginal detections that happen to be near the locations of
nearby galaxies.  However, due to the low significance (6.2 sigma)
of the image peak, the lack of a corresponding count rate increase,
and the non-detection of an XRT counterpart, we believe that this
is probably a statistical fluctuation and not an astrophysical source. 

Further determination of the reality of this event will require the 
full downlinked data.