GRB 210820A: Swift-XRT refined Analysis

We have analysed 6.5 ks of XRT data for GRB 210820A (Beardmore et al. GCN Circ. 30664), from 131 s to 18.2 ks after the BAT trigger. The data comprise 339 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30667). The late-time light curve (from T0+5.9 ks) can be modelled with a power-law decay with a decay index of alpha=1.9 (+0.6, -0.5). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.77 (+/-0.03). The best-fitting absorption column is 1.20 (+/-0.11) x 10^21 cm^-2, in excess of the Galactic value of 3.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.28 (+0.25, -0.13) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.9 x 10^-11 (3.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.6 (+4.3, -0.0) x 10^20 cm^-2 Galactic foreground: 3.6 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.28 (+0.25, -0.13) If the light curve continues to decay with a power-law decay index of 1.9, the count rate at T+24 hours will be 2.9 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x 10^-15 (9.6 x 10^-15) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01069551. This circular is an official product of the Swift-XRT team.