GRB 241209A: Swift-XRT refined Analysis

We have analysed 4.9 ks of XRT data for GRB 241209A, from 91 s to 40.8 ks after the BAT trigger. The data comprise 42 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 light curve can be modelled with an initial power-law decay with an index of alpha=1.16 (+0.17, -0.16), followed by a break at T+485 s to an alpha of 8.0 (+0.0, -1.2). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+/-0.3). The best-fitting absorption column is 3.0 (+1.7, -1.4) x 10^21 cm^-2, in excess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 3.0 (+1.7, -1.4) x 10^21 cm^-2 Galactic foreground: 2.1 x 10^20 cm^-2 Excess significance: 3.4 sigma Photon index: 1.8 (+/-0.3) If the light curve continues to decay with a power-law decay index of 8.0, the count rate at T+24 hours will be 1.3 x 10^-18 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x 10^-29 (6.8 x 10^-29) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01273071. This circular is an official product of the Swift-XRT team.