[vsnet-grb-info 17715] Swift trigger 677890 is noise

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19153
SUBJECT: Swift trigger 677890 is noise
DATE:    16/03/05 21:03:09 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester)
and D. M. Palmer (LANL) report on behalf of the Swift Team:

At 20:42:32 UT, the Swift Burst Alert Telescope (BAT) triggered on a noise event
that was spatially near Ser X-1 (trigger=677890).  Swift slewed immediately to the
location.  The BAT on-board calculated location is 
RA, Dec 279.868, +5.113, which is 
   RA(J2000)  =  18h 39m 28s
   Dec(J2000) = +05d 06' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for images triggers (64 sec in this case)
the real-time light curve does not show anything.  We believe this trigger is
non-astrophysical. 

The XRT began observing the field at 20:44:52.5 UT, 140.4 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. The source in the promptly downlinked 
data is only a marginal detection and is likely due to background variations. 
We do not believe it is related to this trigger. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
146 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
BAT error circle. The overlap of the 8'x8' region for the list of sources
generated on-board and the XRT error circle is uncertain. No correction has
been made for the large, but uncertain extinction expected. 

This trigger was due to a low-significance image fluctuation
in an image without a corresponding rate trigger.  Although
the derived location of the image peak was close enough
to Ser X-1 to trigger automated follow-up observations,
its 9 arc minute offset is inconsistent with being a 
true detection of the source. 

Therefore, we believe that this event is merely a statistical
fluctuation in the image plane, and not an astrophysical event.