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GCN Circular 23509

Subject
Swift Trigger 874754 is likely not an astrophysical event
Date
2018-12-04T20:44:53Z (5 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
J.D. Gropp (PSU), J. A. Kennea (PSU), S. J. LaPorte (PSU),
A. Y. Lien (GSFC/UMBC), B. Sbarufatti (PSU), K. K. Simpson (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:28:17 UT, the Swift Burst Alert Telescope (BAT) triggered detected a
marginal-significance peak in an untriggered image near the
line of sight to a nearby galaxy (trigger=874754).  
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 188.083, +16.309 which is 
   RA(J2000) = 12h 32m 20s
   Dec(J2000) = +16d 18' 32"
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 20:30:40.8 UT, 142.8 seconds after
the BAT trigger. No source was detected in 368 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 147 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
0.03. 

Swift alerted and followed up on this event due to a program of
lowering thresholds for image peaks in the vicinity of nearby galaxies,
trading an increased false event rate for greater sensitivity. 

Given to the lack of a BAT rate increase, the marginal significance
of the image peak (5.99 sigma), the distance between the
image peak and the putative associated galaxy (11.1 arcmin),
and the lack of a new source in the XRT data, we believe that
this event is a spurious statistical fluctuation in the image. 

We do not believe that this is an astrophysical event.  Unless
the full downlinked dataset refutes this belief, no further
observations are planned.
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