[vsnet-grb-info 15773] Swift Trigger 618402: probable noise event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17061
SUBJECT: Swift Trigger 618402: probable noise event
DATE:    14/11/12 10:14:17 GMT
FROM:    Craig Markwardt at NASA/GSFC  <Craig.Markwardt at nasa.gov>

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
D. Malesani (DARK/NBI) and C. B. Markwardt (NASA/GSFC) report on
behalf of the Swift Team:

At 09:06:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located image trigger 618402.  Swift slewed immediately 
to the location. The BAT on-board calculated location is 
RA, Dec 189.977, +15.479 which is 
   RA(J2000) = 12h 39m 55s
   Dec(J2000) = +15d 28' 44"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve does not show any significant
variation, which is typical for an image trigger.  The on-board image 
significance is 6.3 sigma, which is below the threshold for declaring
a burst.  However, ground-based processing does find a slightly improved
significance of 6.5 sigma.  More information will be known after processing
the full BAT data set. 

The XRT began observing the field at 09:08:42.0 UT, 134.8 seconds after
the BAT trigger. 
No X-ray source was detected in 590 s of promptly-downlinked data, 
suggesting that the initial on-board centroid was likely due to a cosmic ray. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 142 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products.  Results from the list of sources
generated on-board are not available at this time. 

This was a low-significance image trigger in the direction of NGC 4595. 
However, NGC 4595 is 11 arc minutes from the center of the 
BAT error circle (error circle radius 3 arcminutes), so the association is 
unlikely.  Given the lack of detection of XRT or UVOT counterparts, this
trigger is likely the result of a noise fluctuation.