GCN Circular 19585

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N.
Burrows (PSU), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), S.L. Gibson
(U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 160625B (Dirirsa et al. GCN Circ. 19580
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in a series of observations tiled on the sky. The total exposure time
is 2.7 ks, distributed over 8 tiles; the maximum exposure at a single
sky location was 1.1 ks. The data were collected between T0+9.6 ks and
T0+10.0 ks, and are entirely in Photon Counting (PC) mode. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow.  The position of this source
is RA, Dec=308.5969, +6.9196 which is equivalent to:

RA (J2000): 20:34:23.25
Dec(J2000): +06:55:10.5

with an uncertainty of 3.8 arcsec (radius, 90% confidence).  This
position is 19.7 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.156 (+0.011, -2.764).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.09 (+0.20, -0.19). The
best-fitting absorption column is  1.6 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 9.8 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.1 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.6 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 9.8 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.09 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
3.156, the count rate at T+24 hours will be 6.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-13 (2.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/xrt_products/TILED_GRB00056/index_1.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00056.

This circular is an official product of the Swift-XRT team.