TITLE:   GCN CIRCULAR
NUMBER:  10737
SUBJECT: Trigger 421695: Swift detection of a possible burst
DATE:    10/05/11 14:20:12 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@lheamail.gsfc.nasa.gov>

B. Sbarufatti (INAF-OAB/IASFPA), W. H. Baumgartner (GSFC/UMBC),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
J. M. Gelbord (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), J. Mao (INAF-OAB),
R. Margutti (INAF-OAB), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (INAF-IASFPA),
A. Rowlinson (U Leicester), M. H. Siegel (PSU), M. A. Stark (PSU) and
L. Vetere (PSU) report on behalf of the Swift Team:  

At 11:50:39 UT, the Swift Burst Alert Telescope (BAT) triggered
on trigger=421695.  Swift slewed immediately to the location.
The BAT on-board calculated location is
RA, Dec 280.612, +18.328 which is
   RA(J2000) = 18h 42m 27s
   Dec(J2000) = +18d 19' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  The onboard significance is low (~6.5 sigma)
in both the rate and image domains. Also the initial BAT light curve
does not show any significant peaks, so we do not believe that this
is a real burst.

The XRT began observing the field at 11:51:50.7 UT, 71.7 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 any
XRT counterpart.

UVOT took a finding chart exposure of 150 seconds with the White
filter starting 75 seconds after the BAT trigger. No credible
afterglow candidate has been found in the initial data products.
Because part of the 2.7'x2.7' sub-image was not received, the overlap  
with the BAT error circle is uncertain. The overlap is at most 25%.
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 0.33.

Due to the marginal detection of this event in BAT, and the lack
of a detection in the early XRT data, we believe that this is
probably not a real astrophysical event.  Further determination
of its reality will require analysis of the ground-linked data
after ~17:00 UT.

Burst Advocate for this burst is B. Sbarufatti (sbarufatti AT ifc.inaf.it).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)