TITLE:   GCN CIRCULAR
NUMBER:  18631
SUBJECT: Swift Trigger 664610: is unlikely a GRB
DATE:    15/11/22 09:15:03 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

K. L. Page (U Leicester), S. D. Barthelmy (GSFC) and
J. R. Cummings (NASA/UMBC) report on behalf of the Swift Team:

At 08:49:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible source(trigger=664610).  Swift slewed immediately to the target. 
The BAT on-board calculated location is 
RA, Dec 184.205, +17.599, which is 
   RA(J2000)  =  12h 16m 49s
   Dec(J2000) = +17d 35' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, there is nothing
significant in the real-time TDRSS light curve.  This is a 5.9 sigma trigger. 

The XRT began observing the field at 08:51:23.6 UT, 139.1 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of 144 seconds with the White filter
starting 143 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
0.04. 

We note that this is a low significance trigger (5.9 sigma); part of the
sub-threshold program within BAT.  Given that the initial XRT exposure
did not produce a source (time-on-target was only 2.9 min), it is unlikely
that this source is real, but we can not rule it out at this time.  We will have
to wait for more data in the next orbit. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)