GCN Circular 11999
K.L. Page (U. Leicester) and M. Stamatikos (OSU/NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 8.2 ks of XRT data for GRB 110503A (Stamatikos et al. GCN Circ. 11991), from 166 s to 28.4 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN. Circ 11992). The light curve can be modelled with a power-law decay with a decay index of alpha=1.059 (+/-0.013). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.86 (+/-0.04). The best-fitting absorption column is 2.2 (+/-0.6) x 10^21 cm^-2, at a redshift of 1.613, in addition to the Galactic value of 2.6 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.01 (+/-0.08) and a best-fitting absorption column of 2.4 (+/-1.0) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground: 2.6 x 10^20 cm^-2 Intrinsic column: 2.4 (+/-1.0) x 10^21 cm^-2 at z=1.613 Photon index: 2.01 (+/-0.08) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00452685. This circular is an official product of the Swift-XRT team.