[vsnet-grb-info 13096] Swift Trigger 554135 is probably not a GRB or transient

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14433
SUBJECT: Swift Trigger 554135 is probably not a GRB or transient
DATE:    13/04/21 21:18:18 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), S. R. Oates (UCL-MSSL),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 21:02:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a peak in the resulting image (trigger=554135).  Swift slewed 
immediately to the location.  The BAT on-board calculated location is 
RA, Dec 235.706, +58.498 which is 
   RA(J2000) = 15h 42m 49s
   Dec(J2000) = +58d 29' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve is consistent with
random variation. 

The XRT began observing the field at 21:03:36.4 UT, 86.4 seconds after
the BAT trigger. No source was detected in 253 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 89 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
0.01. 

Because BAT's detection was marginal, and XRT found nothing at
the location, we believe that this is probably not a GRB. 
However, a final determination of the reality of the source
will require the full downlinked dataset.