TITLE: GCN CIRCULAR NUMBER: 18910 SUBJECT: GRB 160117A: Swift-XRT afterglow detection DATE: 16/01/21 07:19:22 GMT FROM: Kim Page at U.of Leicester 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). 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), "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.