[vsnet-grb-info 18147] Swift Trigger 701668 is probably not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19595
SUBJECT: Swift Trigger 701668 is probably not an astrophysical event
DATE:    16/06/26 23:53:32 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

V. D'Elia (ASDC), C. Gronwall (PSU), L. Izzo (IAA-CSIC),
F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of
the Swift Team:

At 23:30:15 UT, the Swift Burst Alert Telescope (BAT) detected a 
marginal-significance peak in an image (trigger=701668). 
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 186.444, +12.747 which is 
   RA(J2000) = 12h 25m 46s
   Dec(J2000) = +12d 44' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, no
obvious variation is visible in the immediately-available lightcurve. 

The XRT began observing the field at 23:39:38.0 UT, 562.5 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 186.4463, 12.6611 which is
equivalent to:
   RA(J2000)  = 12h 25m 47.10s
   Dec(J2000) = +12d 39' 39.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at http://www.swift.ac.uk/sper. This position is 6.2
arcseconds from a known X-ray source: 1SXPS J122546.7+123942. This
source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV
count-rate of 0.1600 +/- 0.0025 ct/sec; see
http://www.swift.ac.uk/1SXPS/1SXPSJ122546.7%2B123942 for details of
these previous observations

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.86
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 567 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers none of the XRT error circle. No correction has been made for
the expected extinction corresponding to E(B-V) of 0.03. 

Because this location is in the approximate direction of a nearby
galaxy (4 arcmin from NGC4387) Swift made follow-up observations
to confirm or refute the existence of a source, despite the 
marginal significance (6.34 sigma).  Based on the non-detection
by XRT and UVOT of any previously uncatalogued source, we believe that
this is not an astrophysical event.