GCN Circular 12142
V. D'Elia, G. Stratta and J. R. Cummings (NASA/UMBC) report on behalf of the Swift-XRT team: We have analysed 16 ks of XRT data for GRB 110709B (Cummings et al. GCN Circ. 12122), from 70 s to 36.8 ks after the BAT trigger. The data comprise 2.3 ks 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 12136). The light curve shows a prolonged and powerful flaring activity up to T0+2 ks. The late-time light curve (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.08 (+/-0.04). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.550 (+/-0.020). The best-fitting absorption column is 1.79 (+/-0.07) x 10^21 cm^-2, in excess of the Galactic value of 5.6 x 10^20 cm^-2 (Kalberla et al. 2005). We stress that this fit averages over strong spectral variations. In fact, the hard-to-soft band ratio during this time period varies from about 0.5 to about 3. The PC mode spectrum has a photon index of 2.19 (+/-0.08) and a best-fitting absorption column of 2.14 (+0.22, -0.21) 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 (6.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.14 (+0.22, -0.21) x 10^21 cm^-2 Galactic foreground: 5.6 x 10^20 cm^-2 Excess significance: 16.8 sigma Photon index: 2.19 (+/-0.08) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00456967. This circular is an official product of the Swift-XRT team.