TITLE: GCN CIRCULAR NUMBER: 19904 SUBJECT: GRB 160910A: Swift-XRT afterglow detection DATE: 16/09/11 15:04:50 GMT FROM: Antonino D'Ai at IASF-PA A. D'Aì (INAF-IASFPA), V. D'Elia (ISDC), J. Cummings (GSFC/UMBC), 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 P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/GBM (Veres et al., GCN Circ. 19901) and Fermi/LAT (Racusin et al., GCN Circ. 19902) detected burst GRB 160910A in a series of 4 observations tiled on the sky. The total exposure time is 5 ks, distributed over 4 tiles. The data were collected between T0+35 ks and T0+47 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.=221.4423, 39.0668 which is equivalent to: RA (J2000) = 14h45m46.16s Dec (J2000) = +39:04:00.64 with an uncertainty of 4.3 arcsec (radius, 90% confidence). This position is 0.34 deg from the Fermi/LAT position and 0.6 deg from the GBM position. The light curve built with the data collected so far is consistent with a constant source of mean count rate (1.0 +/- 0.1)e-01 ct/sec. More data are required to search for a fading behaviour. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 (+0.6, -0.6). The best-fitting absorption column is 1.3 (+1.7, -1.3) x 10^21 cm^-2, consistent with the Galactic value of 1.0 x 10^20 cm^-2 (Willingale et al. 2013). The [absorbed] unabsorbed 0.3-10 keV flux is 5.1 (+1.6, -1.0)e^-10 erg cm^-2 s^-1 [4.7 (+2.1, -1.3) e-10 erg cm^-2 s^-1]. This circular is an official product of the Swift-XRT team.