[vsnet-grb-info 9259] Swift-BAT trigger 421695 is likely not astrophysical

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10742
SUBJECT: Swift-BAT trigger 421695 is likely not astrophysical 
DATE:    10/05/11 20:46:36 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti at brera.inaf.it>

B. Sbarufatti (INAF-OAB/IASFPA), R. Margutti (INAF-OAB), S. D. Barthelmy
(GSFC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), S.T. Holland
(GSFC/USRA), H. A. Krimm (GSFC/USRA)

The BAT data set from T-61 to T+302 sec from the recent telemetry downlink
suggests that trigger 421695 (Sbarufatti et al., GCN Circ 10737) is likely
not of astrophysical origin, but is instead most likely due to a chance
superposition of a rate increase with an upward fluctuation in the image
domain.  The best fit image significance is 7.1 sigma. The excess rate is
seen only in the lowest energy band, which is inconsistent with a short,
but faint, GRB.  The galactic latitude (+10.1 degrees) and short duration
argue against its being a galactic transient and the lack of association
with any of the 370 nearby galaxies in the GCN ground catalog is an
argument against its being an SGR giant flare from a nearby galaxy. 
However, since the trigger has neither the spectral nor temporal signature
of an electronic noise or cosmic ray event, we cannot absolutely rule out
the possibility that the trigger is of astrophysical origin.

Swift-XRT obtained 2 ks of Photon Counting (PC) mode data, from T0+72 s to
T0+2 ks. No source is found in the BAT error circle, with a 3-sigma upper
limit of 5 x 10-3 ct/sec; for a typical GRB, this would correspond to an
upper limit on the observed 0.3-10 keV flux of 2 x 10-13 erg cm-2 s-1.

No credible afterglow candidate has been found in the UVOT data products
relative to the USNO B1.0 catalogue. The estimated 3-sigma upper limit is
white > 20.8 mag. No correction has been made for the expected Galactic
extinction corresponding to a reddening of 0.33 mag. Photometry is on the
UVOT photometry system of Poole et al. (2008).

Recent problems within the Goddard network (that blocked email to and from
the circulars-processing computer), a duplicated (GCN Circ 10741) of the
original Swift circular on trigger 421695 (GCN Circ 10737) was
distributed.  GCN Circ 10741 has been removed from the archives. Apologies
for the confusion.