TITLE:   GCN CIRCULAR
NUMBER:  23425
SUBJECT: Swift Trigger 872470 is probably not a GRB
DATE:    18/11/18 18:04:34 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
M. J. Moss (George Washington University), B. Sbarufatti (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:33:34 UT, the Swift Burst Alert Telescope (BAT) triggered 
on count rate rise near the SAA passage and produced an image with 
a marginal-significance peak of 6.55 sigma (trigger=872470).  
Swift slewed immediately to the event. 
The BAT on-board calculated location is 
RA, Dec 299.270, -15.665 which is 
   RA(J2000) = 19h 57m 05s
   Dec(J2000) = -15d 39' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).   The BAT light curve shows a gradual rise and 
decrease of high background count rate due to the SAA, but 
no obvious peak at the trigger time. 

The XRT began observing the field at 17:37:56.5 UT, 262.2 seconds after
the BAT trigger. No source was detected in 451 s of promptly downlinked
data, which covered 94% of the BAT error circle. 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 1073 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 0.16. 

Due to the lack of a clear peak in the BAT lightcurve, the
marginal (6.55 sigma) BAT image peak, and the lack of an
XRT counterpart, we believe that this event was due to particle
background near the SAA and is not an astrophysical event. 

Further analysis will require the full downlinked dataset.