[vsnet-grb-info 19342] Swift Trigger 750096 is probably a noise fluctuation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21039
SUBJECT: Swift Trigger 750096 is probably a noise fluctuation
DATE:    17/04/26 23:00:26 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 22:44:21 UT, the Swift Burst Alert Telescope (BAT) detected an 
image peak in the vicinity of a nearby galaxy (trigger=750096).  
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 184.880, +5.656 which is 
   RA(J2000) = 12h 19m 31s
   Dec(J2000) = +05d 39' 21"
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 22:46:18.1 UT, 117.1 seconds after
the BAT trigger. No source was detected in 513 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
117 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image processing FAILED
because of no aspect solution. Results from the list of sources generated
on-board are not available at this time. 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 source (7.7 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. 

This trigger, like the earlier Trigger #750044, was the result
of a triggering strategy that locally depresses the threshold 
significance in the vicinity of known sources and nearby galaxies 
in order to gain greater sensitivity to such sources (at the
expense of a higher proportion of false triggers).