GCN Circular 8943

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC),
E. A. Hoversten (PSU), F. E. Marshall (NASA/GSFC), C. Pagani (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC),
M. H. Siegel (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 03:46:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a marginal peak (trigger=345551).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 244.979, -28.644, which is 
   RA(J2000)  = 16h 19m 55s
   Dec(J2000) = -28d 38' 38"
with an uncertainty of 4 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a slight increase
above background at T_0, but it is very weak. 

The XRT began observing the field at 03:48:52.38 UT, 135.5 seconds
after the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source located at RA, Dec 244.99412, -28.63366 which
is equivalent to:
   RA(J2000)  = 16h 19m 58.59s
   Dec(J2000) = -28d 38' 01.2"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). The source
does not appear to be fading at this time, with a limit of alpha ~0.5. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.14e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 128 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 100% of
the XRT 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
XRT 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
0.53. 




We are currently commissioning a new sub-threshold capability
for Swift, allowing the spacecraft to quickly slew to events
with marginal statistics (in the 5.5 - 6.5 sigma range) in 
the BAT image, and using XRT to test whether a source is 
detected at a consistent position.  If automatic ground
processing finds a consistent XRT source, the event is
automatically promoted to GRB status, and the original 
BAT position and other TDRSS messages are released
as delayed GCN Notices. 

We believe that this event was most likely to be a low-significance
statistical fluctuation in the BAT image, in coincidence with a 
serendipitous detection of a non-GRB astrophysical source by the 
XRT. After the XRT source was found, GCN Notices were transmitted,
7 minutes after the original trigger. 

The association of the initial BAT detection with a real GRB
will be determined using Malindi data, including analysis of the XRT
data to see if the XRT source exhibits GRB-like temporal and
spectral behavior.  The currently available XRT data does not
indicate fading.