[vsnet-grb-info 18738] Swift Trigger 722681 is probably not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20187
SUBJECT: Swift Trigger 722681 is probably not a GRB
DATE:    16/11/17 15:36:51 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

C. B. Markwardt (NASA/GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
P.A. Evans (U Leicester), H. A. Krimm (CRESST/NSF/USRA),
F. E. Marshall (NASA/GSFC), B. Mingo (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 15:13:58 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an image peak (trigger=722681).  Swift slewed immediately to 
the location.  The BAT on-board calculated location is 
RA, Dec 140.040, -51.144 which is 
   RA(J2000) = 09h 20m 10s
   Dec(J2000) = -51d 08' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the BAT 
light curve shows no clear structure. 

The XRT began observing the field at 15:16:31.9 UT, 153.2 seconds after
the BAT trigger. No source was detected in 789 s of promptly downlinked
data. 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 157 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the BAT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the large, but uncertain extinction
expected. 

Although this was seen as >7 sigma peak in the on-board image,
ground analysis of the immediately available data reduces this
to 6.0 sigma.  This, combined with the lack of a BAT count rate
trigger and the non-detection by XRT indicates that this is
probably not an astrophysical event. 

A final determination of the reality of this trigger will
require the full downlinked data set. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt AT 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.)