[vsnet-grb-info 19930] Swift trigger 779171 is probably a noise fluctuation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22017
SUBJECT: Swift trigger 779171 is probably a noise fluctuation
DATE:    17/10/15 20:04:44 GMT
FROM:    Boris Sbarufatti at PSU  <bxs60 at psu.edu>

S. D. Barthelmy (GSFC), J.D. Gropp (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), B. Sbarufatti (PSU), A. Tohuvavohu (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Swift Team:

At 19:14:22 UT, the Swift Burst Alert Telescope (BAT) detected an image
peak in the vicinity of a nearby galaxy (trigger=779171).  Swift slewed
immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 78.485, +62.383 which is 
   RA(J2000) = 05h 13m 56s
   Dec(J2000) = +62d 22' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT raw (non mask-weighted) light curve 
shows some periodic behavior with a ~ 10 s period. However, because 
Swift J0243.6+6124, which has a periodicity of ~ 10 s, was also in the BAT FOV 
and currently shows significant increase in its brightness, we believe 
that the oscillation in the BAT light curve is caused by Swift J0243.6+6124. 

The XRT began observing the field at 19:15:58.6 UT, 96.0 seconds after
the BAT trigger. No source was detected in 829 s 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 100 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
0.31. 

Due to the low significance of the image peak (5.90 sigma) and the
possible contamination by Swift J0243.6+6124 and the non-detection by
XRT, we believe that this is merely a noise fluctuation in the image
plane and not an astrophysical source.