[vsnet-grb-info 8716] Swift trigger 377436 (possible GRB 091130A) is not a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10217
SUBJECT: Swift trigger 377436 (possible GRB 091130A) is not a burst 
DATE:    09/11/30 22:38:10 GMT
FROM:    Hans Krimm at NASA-GSFC  <hans.krimm at nasa.gov>

H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), J. R. Cummings (CRESST/GSFC/UMBC),
E. Hoversten (PSU), on behalf of the Swift team.

Using the Swift/BAT data set from T-60 to T+236 sec and the
XRT data set beginning at T+58 minutes from recent telemetry
downlinks, we report that BAT trigger 356516 (reported as
possible GRB 091130A, Krimm et al, Circ 10212) is probably
not a GRB.  We base this conclusion on the low significance
in the BAT and the lack of a counterpart in the XRT.
However, we can not rule out that this is an astrophysical
event which was extended in duration in the BAT (the
original trigger was a 400-sec integration), but rapidly
fading prior to the XRT observations.

The BAT image shows a 3.5 sigma bump and the mask-weighted
light curve shows an integrated 5-sigma positive excess out
to T+236 sec.  If the source is real, it is at RA, Dec =
161.854, +11.580 deg with an uncertainty of 4.5 arcmin,
(radius, sys+stat, 90% containment). The partial coding is 97%.

In 2.57 ks of Swift-XRT photon counting mode data beginning
at T+58 minutes, we do not detect any X-ray source in the
BAT error circle to a 3 sigma upper limit count rate
of 8.0e-3 count/s. This corresponds to a  0.3-10 keV
observed flux limit of 3.1e-13 erg cm-2 s-1, assuming a
typical counts-to-flux conversion factor
of 3.9e-11 erg cm-2 count-1.

UVOT took a finding chart exposure of 150 seconds with
the White filter starting 3592 seconds after the BAT trigger.
No credible afterglow candidate has been found in the initial
data products. The 2.7'x 2.7' 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 0.03.