[vsnet-grb-info 26153] Swift trigger 990358 is not an astrophysical source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28253
SUBJECT: Swift trigger 990358 is not an astrophysical source
DATE:    20/08/17 02:14:59 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), T. Sbarrato (INAF-OAB), M. H. Siegel (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:53:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on an image peak that is likely due to a noisy detector element
(trigger=990358).  Swift slewed immediately to the event. 
The BAT on-board calculated location is 
RA, Dec 154.131, -33.921 which is 
   RA(J2000) = 10h 16m 31s
   Dec(J2000) = -33d 55' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve shows a single spike 
that appears only in the 25-50 keV band, which is likely due to
the presence of the noisy detector element. The full downlinked
data will allow the noisy element to be removed from analysis. 

The XRT began observing the field at 01:54:44.4 UT, 83.4 seconds after
the BAT trigger. No source was detected in 769 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 87 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
0.10. 

This marginal significance burst (6.58 sigma) was not confirmed by
ground imaging of the scaled detector map.  This, combined with the
presence of a hot detector pixel in the data and the non-detection
of a source by XRT leads us to believe that this is not an
astrophysical source.