TITLE:   GCN CIRCULAR
NUMBER:  21358
SUBJECT: Trigger 763539 from Swift is probably not astrophysical
DATE:    17/07/20 04:54:34 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 04:41:08 UT, the Swift Burst Alert Telescope (BAT) detected a marginal peak 
in the image domain in the vicinity of a known source (trigger=763539).  
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 185.367, +5.029 which is 
   RA(J2000) = 12h 21m 28s
   Dec(J2000) = +05d 01' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). As is usual to an image trigger, the BAT light curve 
shows no significant activity. 

The XRT began observing the field at 04:43:26.4 UT, 137.9 seconds after
the BAT trigger. No source was detected in 305 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 142 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 triggered and slewed to this location as part of program
that reduces trigger thresholds in the vicinity of known sources
to either confirm or refute the detection. 

Due to the marginal significance of the BAT detection (5.98 sigma),
the lack of a rate trigger, the distance to the putative source 
(9.5 arcmin), and the lack of an XRT detection, we believe that 
this is merely a statistical fluctuation in the image and
not an astrophysical source.