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GCN Circular 17862

Subject
Swift trigger 641332 is probably not an astrophysical source
Date
2015-05-23T09:10:08Z (9 years ago)
From
Caryl Gronwall at PSU/Swift-UVOT <caryl@swift.psu.edu>
C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 08:49:06 UT, the Swift Burst Alert Telescope (BAT) detected a
marginal significance peak in an non-rate-triggered image 
(trigger=641332).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 353.476, -35.987 which is 
   RA(J2000) = 23h 33m 54s
   Dec(J2000) = -35d 59' 13"
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 08:51:27.7 UT, 141.3 seconds after
the BAT trigger. No source was detected in 348 s of promptly downlinked
data, which covered 98% 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 145 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. 

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.
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