TITLE: GCN CIRCULAR NUMBER: 12283 SUBJECT: GRB 110818A: Swift-XRT refined Analysis DATE: 11/08/19 07:07:01 GMT FROM: Antonia Rowlinson at U.of Leicester A. Rowlinson (U. Leicester) and C. B. Markwardt (NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 7.8 ks of XRT data for GRB 110818A (Markwardt et al. GCN Circ. 12279), from 384 s to 18.3 ks after the BAT trigger. The data comprise 61 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 317.3378, -63.9813 which is equivalent to: RA (J2000): 21 09 21.07 Dec(J2000): -63 58 52.7 with an uncertainty of 2.0 arcsec (radius, 90% confidence). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=5.8 (+0.6, -0.7). At T+461 s the decay flattens to an alpha of 1.57 (+0.14, -0.11) before breaking again at T+2167 s to a final decay with index alpha=0.95 (+0.10, -0.14). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.12, -0.11). The best-fitting absorption column is 7.8 (+2.7, -2.4) x 10^20 cm^-2, in excess of the Galactic value of 2.6 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.8 (+2.7, -2.4) x 10^20 cm^-2 Galactic foreground: 2.6 x 10^20 cm^-2 Excess significance: 5.3 sigma Photon index: 1.97 (+0.12, -0.11) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00500914. This circular is an official product of the Swift-XRT team.