GCN Circular 21730
A. D'Ai (IASF-PA), V. D'Elia (ASDC), F. E. Marshall (NASA/GSFC) reports on behalf of the Swift-XRT team: We have analysed 1.1 ks of XRT data for GRB 170827A (Marshall et al. GCN Circ. 21718), from 113 s to 17.3 ks after the BAT trigger. The data comprise 165 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 initial light curve from T0+130s to T0+295s can be modelled with a power-law decay with a decay index of alpha=1.19 (+/-0.08). In the following observation at T0+16.1 ks the afterglow is not detected with a 3-sigma upper limit of 1.6 x 10^-2 count^-1 s^-1. A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.23 (+0.06, -0.05). The best-fitting absorption column is 1.46 (+/-0.15) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.0 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 1.46 (+/-0.15) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^21 cm^-2 Excess significance: 3.0 sigma Photon index: 2.23 (+0.06, -0.05) Given the lack of detection in the second observation, no reliable estimate of the X-ray count rate for the following days is possible. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00769567. This circular is an official product of the Swift-XRT team.