TITLE:   GCN CIRCULAR
NUMBER:  21043
SUBJECT: Swift Trigger 750337 is probably a noise fluctuation
DATE:    17/04/28 15:02:16 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

J. A. Kennea (PSU), H. A. Krimm (CRESST/NSF/USRA),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 14:50:53 UT, the Swift Burst Alert Telescope (BAT) detected an image
peak in the vicinity of a nearby galaxy (trigger=750337).  Swift slewed 
immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 187.781, -7.972 which is 
   RA(J2000) = 12h 31m 08s
   Dec(J2000) = -07d 58' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows no significant activity. 

The XRT began observing the field at 14:53:39.5 UT, 166.5 seconds after
the BAT trigger. No source was detected in 288 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 169 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 97% 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. 

Due to the low significance of the image peak (5.80 sigma), the 
large distance to the potential associated galaxy (5.0 arcminutes),
the lack of activity in the BAT count rates, and the non-detection 
by XRT, we believe that this is merely a noise fluctuation in
the image plane and not an astrophysical source.