[vsnet-grb-info 16443] Swift trigger 638589 is probably not an astrophysical source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17724
SUBJECT: Swift trigger 638589 is probably not an astrophysical source
DATE:    15/04/20 17:30:30 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (NASA/UMBC),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 17:00:10 UT, the Swift Burst Alert Telescope (BAT) detected a
marginal significance peak in an non-rate-triggered image (trigger=638589).  
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 87.815, -10.422 which is 
   RA(J2000) = 05h 51m 16s
   Dec(J2000) = -10d 25' 20"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, 
the available BAT light curve shows no significant structure. 

The XRT began observing the field at 17:09:20.0 UT, 549.5 seconds after
the BAT trigger. No source was detected in 416 s of promptly downlinked
data, which covered 85% 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 1267 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 0.87. 

Because this is a marginal significance detection of a peak
in a non-rate-triggered image, with no corresponding source
in the XRT data, we believe that this is noise fluctuation
and not an astrophysical source.