[vsnet-grb-info 21313] Swift Trigger 871103 is not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23412
SUBJECT: Swift Trigger 871103 is not an astrophysical event
DATE:    18/11/08 20:04:24 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 19:37:14 UT, the Swift Burst Alert Telescope (BAT) detected a
marginal-significance peak in an untriggered image near the
line of sight to a nearby galaxy (trigger=871103).  Swift 
slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 188.035, +13.338 which is 
   RA(J2000) = 12h 32m 08s
   Dec(J2000) = +13d 20' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers,
the immediately-available BAT lightcurve shows no variation. 

The XRT began observing the field at 19:40:39.9 UT, 205.8 seconds after
the BAT trigger. The XRT centroided on a cosmic ray and the initial XRT
position notice should be ignored. Using promptly downlinked data
we find an X-ray source located at RA, Dec 188.06304, 13.34248 which is
equivalent to:
   RA(J2000)  = 12h 32m 15.13s
   Dec(J2000) = +13d 20' 32.9"
with an uncertainty of 4.5 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 39.2
arcseconds from that of a known X-ray source: 1WGA J1232.2+1321 in the
ROSAT WGACAT catalogue. The catalogued count-rate of this source is
equivalent to approximately 0.038 XRT count/sec; the mean count-rate in
the promptly-available XRT data is 0.074 count/sec. This source is a 
Seyfert 1 galaxy, and not believed to be of interest in this case. We note 
that this AGN is not the same galaxy as NGC 4506 which led to the lowered 
BAT trigger threshold. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.43
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 212 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 100% of the XRT 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 0.03. 

Swift alerted and followed up on this event due to a program of
lowering thresholds for image peaks in the vicinity of nearby galaxies,
trading an increased false event rate for greater sensitivity. 

Given to the lack of a BAT rate increase, the marginal significance
of the image peak (6.0 sigma), the distance between the
image peak and the putative associated galaxy (295 arcsec),
and the lack of a new source in the XRT data, we believe that
this event is a spurious statistical fluctuation in the image. 

We do not believe that this is an astrophysical event.  Unless
the full downlinked dataset refutes this belief, no further
observations are planned.