GCN Circular 16095

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
S. B. Cenko (GSFC), V. D'Elia (ASDC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU),
M. H. Siegel (PSU) and T. N. Ukwatta (MSU) report on behalf of the
Swift Team:

At 17:20:28 UT, the Swift Burst Alert Telescope (BAT) triggered and found an image
peak in the vicinity of NGC 4457 (trigger=595345).  Swift slewed immediately to the 
BAT position. The BAT on-board calculated location is 
RA, Dec 187.132, +3.480 which is 
   RA(J2000) = 12h 28m 32s
   Dec(J2000) = +03d 28' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, nothing of note
is found in the BAT light curve. 

The XRT began observing the field at 17:22:21.6 UT, 113.5 seconds after
the BAT trigger. No source was detected in 1.2 ks of promptly
downlinked data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 117 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
0.02. 

Due to the low significance of the BAT peak (6.51 sigma) in an image trigger,
the lack of a corresponding rate increase, the large distance between 
the image peak and the potential host galaxy (0.149 degrees), and the lack of 
an XRT detection, we believe that this trigger was a noise peak and not 
an astrophysical source.