GCN Circular 15963
V. D'Elia (ASDC), L. Izzo (URoma/ICRA) and J. L. Racusin (NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 7.8 ks of XRT data for GRB 140311B (Racusin et al. GCN Circ. 15945), from 3.4 ks to 46.3 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 15958). The light curve can be modelled with a power-law decay with a decay index of alpha=1.66 (+0.18, -0.15). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.68 (+0.25, -0.23). The best-fitting absorption column is 1.6 (+0.9, -0.8) x 10^21 cm^-2, in excess of the Galactic value of 3.6 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 4.1 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.6 (+0.9, -0.8) x 10^21 cm^-2 Galactic foreground: 3.6 x 10^20 cm^-2 Excess significance: 2.7 sigma Photon index: 1.68 (+0.25, -0.23) If the light curve continues to decay with a power-law decay index of 1.66, the count rate at T+24 hours will be 9.6 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.0 x 10^-14 (4.8 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00591392. This circular is an official product of the Swift-XRT team.