TITLE:   GCN CIRCULAR
NUMBER:  33313
SUBJECT: Swift Trigger 1153670 is probably not a GRB
DATE:    23/02/10 06:24:53 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

N. J. Klingler (GSFC/UMBC/CRESSTII), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), N. P. M. Kuin (UCL-MSSL),
D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII),
C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), M. H. Siegel (PSU)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 05:52:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an image peak (trigger=1153670).  Swift slewed immediately
to the location. 
The BAT on-board calculated location is 
RA, Dec 50.745, +7.153 which is 
   RA(J2000) = 03h 22m 59s
   Dec(J2000) = +07d 09' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows strong variation
with the 207s period of the known source LSV +44 17, which is currently
in strong outburst with an intensity of about 2 Crab and which was
in the BAT Field Of View at the time. One of the peaks corresponds
with the time of the trigger and has an amplitude of ~200 counts/sec
(15-350 keV), consistent with the previous peak of the source. 

The XRT began observing the field at 05:54:19.7 UT, 124.4 seconds after
the BAT trigger. No source was detected in 1.4 ks 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 126 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
0.298. 

Due to the BAT rate trigger being due to the known source  LSV +44 17,
the marginal significance of the image peak (7.17 in onboard imaging,
refined to 6.2 sigma in further ground processing), and the lack
of an XRT afterglow detection, we believe that this trigger is
not due to an astrophysical GRB. 

Further analysis to conclusively determine the origin of this
trigger will be performed on the ground-downlinked data. 

Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.edu). 
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/)