TITLE:   GCN CIRCULAR
NUMBER:  18185
SUBJECT: Swift Trigger 652824: Is most likely a noise fluctuation in the direction of M31
DATE:    15/08/21 02:51:09 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), A. Melandri (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 02:17:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a noise fluctuation in the direction of M31 (trigger=652824). 
There was no immediate slew because of an observing constraint. 
The BAT on-board calculated location is
RA, Dec 10.241, +41.081, which is 
   RA(J2000)  =  00h 40m 58s
   Dec(J2000) = +41d 04' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers (7 min),
there is nothing significant in the real-time TDRSS light curve. 

This highly marginal image trigger (6.04 sigma) had a nominal
direction in the vicinity of M31, and so the onboard software
alerted the ground and requested Swift to make follow-up 
observations to confirm or refute the existence of an 
astrophysical source. 

However, due to the observing constraint,  Swift will 
not able to follow up on the trigger until around 03:30 UT. 
Based on past experience with marginal detection follow-ups,
it is most likely that this is a noise fluctuation rather
than an astrophysical source. 

A final determination of the reality of this source will
require the XRT follow-up observations, and analysis of the 
full downlinked data.  We further caution that it is likely
that XRT will find multiple unrelated source in its FOV due
to the location in M31.  If so it will not necessarily be
the case that any of these are correlated with the image trigger.