GCN Circular 18910

A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R.
Roegiers (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester) and D. M. Palmer report on behalf
of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Swift/BAT-detected burst GRB 160117A (Palmer et al. GCN Circ. 18882
Not found
 
 
).
The observations now extend from T0+68.7 ks to T0+304.5 ks. The source
previously reported by Maselli et al. (GCN Circ. 18888
Not found
 
 
), "Source 1", is
fading with 3-sigma significance, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=20.3683, -0.6541
which is equivalent to:

RA (J2000): 01:21:28.39
Dec(J2000): -00:39:14.7

with an uncertainty of 3.8 arcsec (radius, 90% confidence).  This
position is 65 arcsec from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.1 (+0.5, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 3.7 (+0.9, -0.6). The
best-fitting absorption column is  1.2 (+0.6, -0.4) x 10^22 cm^-2, in
excess of the Galactic value of 4.4 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 2.6 x 10^-11 (3.7 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.2 (+0.6, -0.4) x 10^22 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: 5.2 sigma
Photon index:	     3.7 (+0.9, -0.6)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00670793/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00670793.

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