[vsnet-grb-info 8782] Swift trigger 379449 (possible GRB) is not a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10282
SUBJECT: Swift trigger 379449 (possible GRB) is not a burst
DATE:    09/12/15 22:36:03 GMT
FROM:    Takanori Sakamoto at NASA/GSFC  <takanori at milkyway.gsfc.nasa.gov>

T. Sakamoto (NASA/UMBC), J. R. Cummings (CRESST/GSFC/UMBC),
D. M. Palmer (LANL),  G. Stratta (ASDC), V. D'Elia (ASDC), 
M. Perri (ASDC), F.E. Marshall (NASA/GSFC), on behalf of the Swift team.

Using the Swift/BAT data set from T-60 to T+243 sec from the recent 
telemetry downlink, we report that BAT trigger 379449 (reported as 
possible GRB, Sakamoto et al, GCN Circ. 10280) 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 and the UVOT.  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 64-sec integration), but rapidly 
fading prior to the XRT/UVOT observations.

The BAT image shows a 5.8 sigma bump in the 15-50 keV band and the 
mask-weighted light curve shows an integrated 5-sigma positive excess out
to T+64 sec.  If the source is real, it is at RA, Dec =
242.038, +51.399 deg with an uncertainty of 4 arcmin, 
(radius, sys+stat, 90% containment). The partial coding is 77%.

We analyzed the first 3.2 ks of the XRT data.  Data are in Photon Counting (PC) 
mode and start 144.5 seconds after the BAT trigger.  No source is detected 
within the BAT error circle. The 3-sigma upper limit on the count rate is 
3e-3 count s-1, corresponding to an approximate 0.3-10 keV flux of 
1.5e-13 erg cm-2 s-1 (using a standard counts to flux conversion factor).

UVOT took an exposure of 150 seconds with the White filter starting 150 seconds 
after the BAT trigger.  No credible afterglow candidate has been found in the image, 
which covers about 95% of the BAT error circle.  The estimated 3-sigma upper limit 
is 20.2 mag.  A subsequent exposure of 250 seconds with the U filter started
308 seconds after the trigger and covers all of the BAT error circle.
The estimated 3-sigma upper limit for this image is 19.8 mag. 
No correction has been made for the expected extinction corresponding 
to E(B-V) of 0.02.