TITLE: GCN CIRCULAR NUMBER: 20112 SUBJECT: GRB 161023A: Swift-XRT afterglow detection DATE: 16/10/24 17:39:57 GMT FROM: Phil Evans at U of Leicester A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the INTEGRAL-detected burst GRB 161023A (Gorbovskoy et al. GCN Circ. 20102), collecting 1.1 ks of Photon Counting (PC) mode data between T0+34.8 ks and T0+53.2 ks. A fading, uncatalogued X-ray source is detected consistent with the MASTER position (Gorbovskoy et al., GCN Circ. 20102) and is therefore believed to be the afterglow. Using 3643 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 = 311.02161, -47.66348 which is equivalent to: RA (J2000): 20h 44m 05.19s Dec(J2000): -47d 39' 48.5" with an uncertainty of 1.5 arcsec (radius, 90% confidence). This position is 0.2 arcsec from the MASTER position. The light curve can be modelled with a power-law decay with a decay index of alpha=2.3 (+0.9, -1.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.09 (+0.30, -0.16). The best-fitting absorption column is consistent with the Galactic value of 3.2 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 (3.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.2 (+/-5.6) x 10^20 cm^-2 Galactic foreground: 3.2 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.09 (+0.30, -0.16) If the light curve continues to decay with a power-law decay index of 2.3, the count rate at T+24 hours will be 9.2 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x 10^-13 (3.1 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020709. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020709. This circular is an official product of the Swift-XRT team.