[vsnet-grb-info 14009] Trigger 574266 is not a Swift detection of a known source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15323
SUBJECT: Trigger 574266 is not a Swift detection of a known source
DATE:    13/10/11 22:55:55 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), A. Y. Lien (NASA/GSFC/ORAU),
C. J. Mountford (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 22:16:58 UT, the Swift Burst Alert Telescope (BAT) triggered on a rate 
increase  (trigger=574266), and produced an image.  This image included a 
statistically marginal peak which was 6 arcminutes from the RS CVn source 
VY Ari.  Swift slewed immediately to the peak location.  The BAT on-board 
calculated location is 
RA, Dec 42.286, +7.440 which is 
   RA(J2000) = 02h 49m 09s
   Dec(J2000) = +07d 26 24
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked 
structure with a duration of about 30 sec, starting gradually at about 
T-20.  The peak count rate was ~3386 counts/sec (15-350 keV), at ~0 
sec after the trigger. 

The XRT began observing the field at 22:18:47.5 UT, 109.3 seconds after
the BAT trigger. No source was detected in 957 s of promptly downlinked
data, which covered 84% of the BAT error circle. 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 113 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
0.22. 

When a marginal BAT image peak is found near to a previously-known
source, Swift is programmed to make follow-up observations
to test whether the detection is real. In this case, the strength of
the rate variation in the BAT light curve is inconsistent with the 
low significance of the image peak location, and the peak location
is outside of the expected BAT error circle for the VY Ari source. 

For these reasons, we believe that VY Ari is not the source of the BAT 
lightcurve variations, which may be due to a GRB outside of the BAT FOV. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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.)