[vsnet-grb-info 12405] Swift Trigger 534017 is probably not real.

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13780
SUBJECT: Swift Trigger 534017 is probably not real. 
DATE:    12/09/18 12:25:19 GMT
FROM:    Caryl Gronwall at PSU/Swift-UVOT  <caryl at astro.psu.edu>

B. N. Barlow (PSU), W. H. Baumgartner (GSFC/UMBC), V. D'Elia (ASDC),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), D. Grupe (PSU),
E. A. Helder (PSU), J. A. Kennea (PSU) and B.-B. Zhang (PSU) report on
behalf of the Swift Team:

At 11:58:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger=534017.  Swift slewed immediately to the trigger location. 
The BAT on-board calculated location is 
RA, Dec 345.700, -39.715 which is 
   RA(J2000) = 23h 02m 48s
   Dec(J2000) = -39d 42' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This event resulted from a 64s image trigger, 
and as usual for these events nothing is seen in the BAT light curve.  This
trigger was a sub threshold trigger (5.8 sigma significance), and is 
spatially coincident with a nearby galaxy (11 arc minutes from NGC 7456). 
Further analysis of data on the ground will be required to verify the 
reality of this event. 

The XRT began observing the field at 12:00:15.4 UT, 129.1 seconds after
the BAT trigger. No source was detected in 157 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 132 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
0.01.