[vsnet-grb-info 16233] Swift Trigger 633173 is probably not an astrophysical source

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17514
SUBJECT: Swift Trigger 633173 is probably not an astrophysical source
DATE:    15/03/01 18:25:04 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), M. M. Chester (PSU),
M. De Pasquale (INAF-IASFPA), N. Gehrels (NASA/GSFC),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 17:58:50 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a likely noise event (trigger=633173).  Swift slewed immediately
to the location. The BAT on-board calculated location is 
RA, Dec 139.480, -22.437, which is 
   RA(J2000)  =  09h 17m 55s
   Dec(J2000) = -22d 26' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, nothing shows up
in the real-time TDRSS light curve. 

No XRT counterpart was detected in 980 s of promptly downlinked data. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
341 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 21% of the
BAT error circle. The typical 3-sigma upper limit has been about 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.10. 

This event alerted as part of a program to follow up low-significance
possible detections in the direction of nearby galaxies. 
Due to the marginal significance of the detection (6.16 sigma)
the distance of the event from the putative host galaxy (5 arcminutes,
about twice the galaxy's nominal radius) and the lack of a detection
by the XRT and UVOT, we believe that this is not an astrophysical
detection.