[vsnet-grb-info 19980] Swift Trigger 783893 is probably not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22067
SUBJECT: Swift Trigger 783893 is probably not a GRB
DATE:    17/10/29 06:52:04 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

P. D'Avanzo (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL)
and B. Sbarufatti (PSU) report on behalf of the Swift Team:

At 06:28:24 UT, the Swift Burst Alert Telescope (BAT) triggered on
a low-significance image peak (trigger=783893).  Swift slewed 
immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 335.758, +67.859 which is 
   RA(J2000) = 22h 23m 02s
   Dec(J2000) = +67d 51' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows the strong ~10 
second periodicity due to Swift J0243.6+6124, but no variation
that would be indicative of a GRB. 

The XRT began observing the field at 06:29:44.1 UT, 79.5 seconds after
the BAT trigger. No source was detected in 684 s of promptly downlinked
data, which covered 97% 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 81 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
0.84. 

Swift J0243.6+6124 continues to rise in intensity, now reaching ~4 Crabs
in average flux, in the form of a ~10 second periodic lightcurve. 
(See https://swift.gsfc.nasa.gov/results/transients/weak/SwiftJ0243.6p6124/ 
for a BAT flux history.)  As a result, BAT is continually producing
rate triggers when it is in the FOV, allowing many opportunities
for false triggers in the image plane.  

This low-significance (~6.62 sigma) detection is probably a 
statistical fluctuation in the image plane, given the lack of
an XRT counterpart.