TITLE: GCN CIRCULAR NUMBER: 13963 SUBJECT: GRB 121108A: Swift-XRT refined Analysis DATE: 12/11/10 17:14:07 GMT FROM: Andrea Melandri at INAF-OAB A. Melandri (INAF-OAB) and T. N. Ukwatta (MSU) report on behalf of the Swift-XRT team: We have analysed 24 ks of XRT data for GRB 121108A (Ukwatta et al. GCN Circ. 13952), from 47 s to 82.6 ks after the BAT trigger. The data comprise 129 s in Windowed Timing (WT) mode (the first 6 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 13959). The light curve (from T0+0.8 ks) can be modelled with a power-law decay with a decay index of alpha=1.0 (+/-0.1). Flare activity is detected from T0+100 and T0+200 s after the burst event. A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.60 (+/-0.09). The best-fitting absorption column is 3.6 (+0.5, -0.4) x 10^21 cm^-2, in excess of the Galactic value of 1.9 x 10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.70 (+0.20, -0.18) and a best-fitting absorption column of 2.3 (+0.7, -0.4) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10^-11 (6.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.3 (+0.7, -0.4) x 10^21 cm^-2 Galactic foreground: 1.9 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.70 (+0.20, -0.18) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00537921. This circular is an official product of the Swift-XRT team.