GCN Circular 8582

J. Mao (INAF-OAB), P. J. Brown (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), R. Margutti (Univ Bicocca&OAB),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (GSFC/GWU) report on
behalf of the Swift Team:

At 21:41:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB, trigger=335971.  Swift slewed immediately to 
the location.  The BAT on-board calculated location is 
RA, Dec 324.330, +21.516 which is 
   RA(J2000) = 21h 37m 19s
   Dec(J2000) = +21d 30' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve does not show any significant 
peak, which is not unusual for an image trigger. 

The XRT began observing the field at 21:43:58.4 UT, 137.7 seconds after
the BAT trigger. No source was detected in the promptly available XRT
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of nominal 150 seconds with the
White filter  starting 141 seconds after the BAT trigger. No credible
afterglow candidate has  been found in the initial data products. The
overlap of the sub-image and the  BAT error circle is 25%.  The
typical limiting magnitude for the sub-image is 19.5.   The overlap
of the 8'x8' region for the list of  sources generated on-board and
the BAT error circle is 100%.  The typical limiting magnitude for the
source list is 18. No correction  has been made for the expected
extinction corresponding to E(B-V) of 0.10. 

Due to the marginal nature of the BAT detection,
and the lack of a confirmed counterpart, we are not
sure whether this is a real burst, although a final confirmation
will require the downloaded data from the next Malindi pass.