[vsnet-grb-info 18653] Swift Trigger 718919 is not an astrophysical event.

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20101
SUBJECT: Swift Trigger 718919 is not an astrophysical event. 
DATE:    16/10/23 17:26:14 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC),
D. Malesani (DARK/NBI and DTU Space), K. L. Page (U Leicester),
D. M. Palmer (LANL) and T. N. Ukwatta (LANL) report on behalf of the
Swift Team:

At 17:11:02 UT, the Swift Burst Alert Telescope (BAT) triggered on noise near to
NGC 5023 (trigger=718919).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 197.822, +43.936, which is 
   RA(J2000)  =  13h 11m 17s
   Dec(J2000) = +43d 56' 09"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, there is nothing
visible in the real-time telemetry light curve.  We do not believe this trigger
is astrophysical -- it is very likely to be a noise trigger. 

The XRT began observing the field at 17:13:14.8 UT, 131.9 seconds after
the BAT trigger. No source was detected in 270 s of promptly downlinked
data. 

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. 

BAT selected this trigger based on its proximity to the
direction to a known nearby galaxy, despite its low significance (5.8 sigma). 

Based on the lack of a corresponding rate increase in BAT and the
non-detection by XRT of any counterpart, we believe that this event
is a statistical fluctuation in the image.