GCN Circular 11760
A. Rowlinson (U. Leicester) and M. Stamatikos (OSU/NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 5.3 ks of XRT data for GRB 110223B (Stamatikos et al. GCN Circ. 11754), from 56 s to 24.6 ks after the BAT trigger. The data comprise 85 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN. Circ 11758). The light curve can be modelled with an initial power-law decay with an index of alpha=2.8 (+0.9, -0.7), followed by a break at T+99.5 s to an alpha of 0.92 (+/-0.04). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.85 (+/-0.16). The best-fitting absorption column is 1.7 (+0.5, -0.4) x 10^21 cm^-2, consistent with the Galactic value of 1.3 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10^-11 (5.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.7 (+0.5, -0.4) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.85 (+/-0.16) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00446677. This circular is an official product of the Swift-XRT team.