TITLE:   GCN CIRCULAR
NUMBER:  16639
SUBJECT: Swift Trigger 607506 is probably noise
DATE:    14/07/29 20:24:09 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

W. H. Baumgartner (GSFC/UMBC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), C. B. Markwardt (NASA/GSFC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 19:54:27.67 UT, the Swift Burst Alert Telescope (BAT) found a
low-significance peak in an untriggered image at a location near
NGC3109 (trigger=607506).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 150.766d, -26.028d which is 
 RA(J2000) = 10h 03m 04s
 Dec(J2000) = -26d 01' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT lightcurve shows a rapidly
increasing count rate, consistent with Swift's entry into the SAA. 

The XRT began observing the field at 19:57:03.3 UT, 155.6 seconds after
the BAT trigger. No source was detected in 37 s of promptly downlinked
data, which covered 94% 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 118 seconds with the White filter
starting 159 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
0.06. 

When BAT finds a marginal significance image peak in the vicinity
of a nearby galaxy, as in this case, Swift makes follow-up observations
with the narrow-field instruments to test the reality of the
event.  The expectation is that most such events will be noise. 

Due to the lack of a rate trigger, the marginal significance of the 
image peak (5.92 sigma), and the lack of a detection in the
narrow field instruments, we do not believe that this is an
astrophysical source.  A final determination will require
the full downlinked dataset.