[vsnet-grb-info 18750] Swift Trigger 723124 is probably not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20199
SUBJECT: Swift Trigger 723124 is probably not a GRB
DATE:    16/11/21 02:40:22 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

F. E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 02:17:34 UT, the Swift Burst Alert Telescope (BAT) triggered on
a peak in a waiting mode image (trigger=723124).  Swift slewed immediately 
to the location.  The BAT on-board calculated location is 
RA, Dec 176.394, -6.896 which is 
   RA(J2000) = 11h 45m 35s
   Dec(J2000) = -06d 53' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, no
BAT count rate variation is visible in the immediately available data. 

The XRT began observing the field at 02:19:29.6 UT, 114.9 seconds after
the BAT trigger. No source was detected in 599 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise any
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 116 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 0.03. 

Due to the marginal significance of the peak in the BAT image (7.16 sigma)
the lack of an increase in the BAT count rates, and the non-detection
of an afterglow by XRT, we believe that this is probably a statistical
fluctuation and not an astrophysical event. 

Determination of the reality of this trigger will require the complete
downlinked dataset. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). 
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.)