[vsnet-grb-info 16320] Swift Trigger 635306 is probably not an astrophysical source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17601
SUBJECT: Swift Trigger 635306 is probably not an astrophysical source
DATE:    15/03/18 11:22:08 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. A. Breeveld (UCL-MSSL), P. A. Evans (U Leicester),
H. A. Krimm (CRESST/GSFC/USRA), C. J. Mountford (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 10:54:26 UT, the Swift Burst Alert Telescope (BAT) imaged a peak
with a projected location near IC 749 (trigger=635306). 
Swift slewed immediately to the location to confirm
or refute the presence of a source. 
The BAT on-board calculated location is 
RA, Dec 183.006, +12.037, which is 
  RA(J2000)  =  12h 12m 02s 
  Dec(J2000) = +12d 02' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for image triggers, there
is no significant variation in the immediately available BAT lightcurve. 

The XRT began observing the field at 10:56:52.9 UT, 146.3 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

No source is detected in the UVOT in the initial images. 

Because this is a low significance peak (5.48 sigma) in
a 64s image made without a rate trigger, it was only
considered to be of possible note because it was
in the proximity of the nearby galaxy IC 749. 
It had an offset of 9 arcminutes for this galaxy, which is well 
outside of its luminous region.  In such cases, Swift triggers
a follow-up observation to test whether the source is real. 
In the absence of an XRT or UVOT confirmation, it is unlikely that
this event is anything more than a noise fluctuation. 

Full determination of the reality of this event will require
the full downlinked dataset.