[vsnet-grb-info 13588] Swift Trigger 558631 is not a real source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14917
SUBJECT: Swift Trigger 558631 is not a real source
DATE:    13/06/20 10:53:11 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

B. N. Barlow (PSU), D. N. Burrows (PSU), S. T. Holland (STScI),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), C. A. Swenson (PSU) and B.-B. Zhang (PSU) report
on behalf of the Swift Team:

At 02:21:57 UT, the Swift Burst Alert Telescope (BAT) triggered on
a likely noise fluctuation (trigger=558631).  Swift slewed immediately 
to the location.  The BAT on-board calculated location is 
RA, Dec 271.805, -22.291, which is 
   RA(J2000)  =  18h 07m 13s
   Dec(J2000) = -22d 17' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Since it is an image trigger, the light
curve shows no significant features. 

The XRT began observing the field at 02:29:59.5 UT, 482.3 seconds after
the BAT trigger. No source was detected in 376 s of promptly downlinked
data, which covered 39% of the BAT error circle. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 486 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 large, but uncertain extinction expected. 

This was a marginal significance (5.8 sigma) image peak which was
within 0.2 degrees (actual distance 0.176 degrees) of a known source. 
Under this set of conditions, Swift is programmed to slew to determine
the reality of the detection using the XRT and UVOT. 

Due to the lack of an XRT detection, and the lack of a rate trigger,
we believe that this is a statistical fluctuation and not a real
source detection. 

[GCN OPS NOTE(20jun13):  This Circular was delayed in distribution (~8 hrs)
because none of the "magic" strings (eg "GRB", "Swift") was present
in the Subject-line.]