[vsnet-grb-info 16431] Swift Trigger 638217 is probably not an astrophysical source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17712
SUBJECT: Swift Trigger 638217 is probably not an astrophysical source
DATE:    15/04/17 05:09:59 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

N. Gehrels (NASA/GSFC), H. A. Krimm (CRESST/GSFC/USRA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 04:52:50 UT, the Swift Burst Alert Telescope (BAT) detected a
marginal significance peak in an non-rate-triggered image
(trigger=638217). Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 186.582, +15.433 which is 
   RA(J2000) = 12h 26m 20s
   Dec(J2000) = +15d 25' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, 
the available BAT light curve shows no significant structure. 

The XRT began observing the field at 04:55:03.4 UT, 132.9 seconds after
the BAT trigger. No source was detected in 147 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 135 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. 

This event triggered as part of program to follow-up marginal
image triggers in proximity to nearby galaxies to confirm or refute
weak events.  In this case, the low significance (5.96 sigma),
lack of a rate trigger, distance from the potential host galaxy
(10 arcminutes from NGC 4421, which has a nominal diameter of 2.5 arcmin)
and non-detection of any afterglow by XRT and UVOT implies that 
this is a noise fluctuation in the image, and not an astrophysical source.