[vsnet-grb-info 11986] Swift Trigger 524949: Noise fluctuation near a galaxy

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13375
SUBJECT: Swift Trigger 524949: Noise fluctuation near a galaxy
DATE:    12/06/23 06:48:53 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), V. D'Elia (ASDC), C. B. Markwardt (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 06:08:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible near-threshold image peak near the location of
the nearby galaxies NGC4405 and NGC 4383 (trigger=524949).  Swift 
slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 186.424, +16.302, which is 
   RA(J2000)  =  12h 25m 42s
   Dec(J2000) = +16d 18' 09"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As it typical with image trigger (11.7 min)
the TDRSS lightcurve does not show anything significant. 

The XRT began observing the field at 06:25:33.5 UT, 1029.9 seconds
after the BAT trigger. No source was detected in 631 s of promptly
downlinked data, which covered 82% of the BAT error circle. We are
waiting for the full dataset to detect and localise the XRT
counterpart if any. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 1033 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. 

As part of a sensitivity enhancement program for Swift, we now
do follow-up observations on weak fluctuations near to a
catalog of ~600 nearby galaxies and other objects.  Most of these
follow-ups are expected to be noise fluctuations.  Given the 
lack of an XRT confirmation it is most likely that this source
one of those false triggers.