TITLE: GCN CIRCULAR NUMBER: 28916 SUBJECT: GRB 201116A: Swift-XRT afterglow detection DATE: 20/11/17 08:48:42 GMT FROM: Phil Evans at U of Leicester A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU) 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 201116A (Axelsson et al. GCN Circ. 28907) in a series of observations tiled on the sky. The total exposure time is 4.8 ks, distributed over 3 tiles; the maximum exposure at a single sky location was 3.1 ks. The data were collected between T0+46.5 ks and T0+62.3 ks, and are entirely in Photon Counting (PC) mode. Four uncatalogued X-ray sources are detected, of which one ("Source 3") is above the RASS 3-sigma upper limit at this position, and shows signes of fading, and is therefore likely the GRB afterglow. Using 2405 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 149.32493, +0.27586 which is equivalent to: RA (J2000): 09h 57m 17.98s Dec(J2000): +00d 16' 33.1" with an uncertainty of 2.5 arcsec (radius, 90% confidence). This position is 2.4 arcmin from the Fermi/LAT position. As already reported by de Ugarte Postigo et al (GCN Circ. 28915), there is an optical afterglow counterpart consistent with this X-ray object. The light curve is consistent with a constant source of mean count rate 5.2e-02 ct/sec, however fading also cannot be ruled out. A power-law fit gives an index of 1.6 (+1.4, -2.0), thus the large uncertainties hinder a confident statement about fading. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.6, -0.5). The best-fitting absorption column is 2.7 (+2.5, -1.9) x 10^21 cm^-2, in excess of the Galactic value of 2.8 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 3.2 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.7 (+2.5, -1.9) x 10^21 cm^-2 Galactic foreground: 2.8 x 10^20 cm^-2 Excess significance: 2.1 sigma Photon index: 1.9 (+0.6, -0.5) The results of the XRT-team automatic analysis of the afterglow are at: https://www.swift.ac.uk/xrt_products/00021040. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00095. This circular is an official product of the Swift-XRT team.