[vsnet-grb-info 7555] Swift-BAT trigger 348781: almost certainly not a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9113
SUBJECT: Swift-BAT trigger 348781: almost certainly not a burst
DATE:    09/04/09 04:13:59 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

D. Grupe (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL) and
J. L. Racusin (PSU) report on behalf of the Swift Team:

At 03:16:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a probable noise event.  Swift executed a delayed slew to the burst. 
The BAT on-board calculated location is 
RA, Dec 268.829, +20.664, which is 
   RA(J2000)  =  17h 55m 19s
   Dec(J2000) = +20d 39' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT TDRSS light curve shows noise
on a rising background while going into the SAA. 

The XRT began observing the field at 03:23:59.1 UT, 464.8 seconds after
the BAT trigger, while still in the SAA. 
No source was detected in the promptly available XRT data or in the 
single pixels event data with an exposure time of 47 seconds. 
We are waiting for the full dataset to detect and localize the possible
XRT counterpart. 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 1620 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 coverage of the BAT error circle by the 8'x8' region for the list of
sources generated on-board is uncertain because the large number of sources
filled the available telemetry. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.08. 




Due to the marginal significance of the BAT detection (6.6 sigma 
in the image),  the fact that it occurred during entry to the SAA, 
and the lack of an XRT or UVOT counterpart, we believe that this 
event is most likely not an astrophysical source.  A final 
determination of the reality of the GRB will require the full 
downlink of the Malindi data.