GCN Circular 15706
P. D'Avanzo (INAF-OAB), J. L. Racusin (NASA/GSFC), P. A. Evans (U. Leicester) report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 140108A (Racusin et al. GCN Circ. 15699), from 58 s to 41.8 ks after the BAT trigger. The data comprise 2.4 ks in Windowed Timing (WT) mode (the first 8 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 Osborne et al. (GCN. Circ 15705). The light curve shows some initial flaring activity, with the flares peaking at around 90 and 220 s after the trigger. The underlying emission can be modelled with a series of power-law decays. The initial decay index is alpha=2.8 (+/-0.7). At T+184 s the decay flattens to an alpha of 0.52 (+/-0.03) before breaking again at T+7014 s to a final decay with index alpha=1.20 (+0.08, -0.07). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.69 (+0.04, -0.03). The best-fitting absorption column is consistent with the Galactic value of 7.0 x 10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.91 (+0.16, -0.15) and a best-fitting absorption column of 9.3 (+1.3, -1.2) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.7 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 9.3 (+1.3, -1.2) x 10^21 cm^-2 Galactic foreground: 7.0 x 10^21 cm^-2 Excess significance: 3.1 sigma Photon index: 1.91 (+0.16, -0.15) If the light curve continues to decay with a power-law decay index of 1.20, the count rate at T+24 hours will be 0.035 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.0 x 10^-12 (3.8 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00583338. This circular is an official product of the Swift-XRT team.