[vsnet-grb-info 14821] Swift Trigger 596641 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16135
SUBJECT: Swift Trigger 596641 is not a GRB
DATE:    14/04/21 01:07:59 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), C. Gronwall (PSU),
S. T. Holland (STScI), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA),
D. M. Palmer (LANL) and T. Sakamoto (AGU) report on behalf of the
Swift Team:

At 00:37:24 UT, the Swift Burst Alert Telescope (BAT) triggered 
on something which is not likely astrophysical (trigger=596641). 
Swift slewed immediately to the location.  The BAT on-board calculated location
is RA, Dec 336.363, +3.636, which is 
   RA(J2000)  =  22h 25m 27s
   Dec(J2000) = +03d 38' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As per usual for an image trigger, the TDRSS
real-time light curve does not show anything significant. 

The XRT began observing the field at 00:39:53.6 UT, 149.5 seconds after
the BAT trigger. No source was detected in 862 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 153 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
0.10. 

Due to the lack of a BAT rate trigger, the marginal detection (7.0 sigma)
in the BAT image, and the lack of a source in the XRT and UVOT
data, we doubt that this is an astrophysical source. 
Further analysis will require the full ground downlinked data.