[vsnet-grb-info 20649] Swift Trigger 834937 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22747
SUBJECT: Swift Trigger 834937 is not a GRB
DATE:    18/05/31 04:23:46 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), A. Deich (PSU),
J.D. Gropp (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), M. H. Siegel (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 04:02:10 UT, the Swift Burst Alert Telescope (BAT) triggered on noise
(trigger=834937).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 210.508, +11.985, which is 
   RA(J2000)  =  14h 02m 02s
   Dec(J2000) = +11d 59' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the real-time light
curve does not show anything significant. 

The XRT began observing the field at 04:04:17.0 UT, 126.1 seconds after
the BAT trigger. No source was detected in 962 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 129 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
0.02. 

Ground analysis of the scaled map data does not find a significant
peak at the reported position.  This, combined with the lack of a 
BAT rate trigger, the marginal significance of the original BAT 
image peak (7.1 sigma), and the lack of an XRT counterpart,
leads us to believe that this is a statistical fluctuation in
the original image and not an astrophysical source.