[vsnet-grb-info 18422] GRB 160825A: not localized by Swift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19870
SUBJECT: GRB 160825A: not localized by Swift
DATE:    16/08/26 18:52:44 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

D. M. Palmer (LANL) reports on behalf of the Swift team:

On 2016-08-25 at 09:39:29, INTEGRAL SPI ACS reported a 
trigger (#7539) via GCN.  Examination of the Swift/BAT count rates
at that time showed a simultaneous detection of a short hard
burst (duration ~0.1 s, with the detected flux predominantly 
above 50 keV).

On-board analysis of the data showed that the highest-significance
peak in the corresponding image was at a location 
ra,dec = 251.982, -7.833 (J2000) with a ~5 arcmin error radius.  
This peak is of marginal significance (5.6 sigma) and did not trigger
an automated follow-up observation.  

A preliminary analysis of the Time-of-Arrival difference between 
INTEGRAL and BAT lightcurves did not exclude this location.
Therefore, this was considered a possible GRB localization
and a ToO observation was requested.

Swift-XRT performed follow-up observations of this location, collecting
2.0 ks of Photon Counting (PC) mode data between T0+55.9 ks and 
T0+62.2 ks.

No X-ray sources have been detected within the 296 arcsec radius BAT 
eror circle. The 3-sigma upper limit in the field is 0.004 ct s^-1, 
corresponding to a 0.3-10 keV observed flux of 1.5e-13 erg cm^-2 s^-1 
(assuming a typical GRB spectrum).

An uncatalogued was detected, however this was too far from the GRB
position to be the afterglow.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
http://www.swift.ac.uk/ToO_GRBs/00020686.

A more detailed analysis of the light curve TOA difference also
conclusively excludes the  BAT location as the true burst position.
(Dmitry Svinkin, private communication).

Based on these observations and analyses, we can exclude the
marginal image peak location as being the GRB location.

The simultaneous INTEGRAL and BAT detections proves that
there was a short GRB at that time, but it was most likely outside
of the BAT's imaging field of view.