GRB 240805A: Swift-XRT refined Analysis

We have analysed 1.7 ks of XRT data for GRB 240805A, from 89 s to 5.1 ks after the BAT trigger. The data comprise 75 s in Windowed Timing (WT) mode (the first 6 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. Using 1639 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 190.91475, -53.59526 which is equivalent to: RA (J2000): 12h 43m 39.54s Dec(J2000): -53d 35' 42.9" with an uncertainty of 2.1 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=7.82 (+0.18, -2.12), followed by a break at T+97.3 s to an alpha of 1.292 (+0.027, -0.026). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+/-0.08). The best-fitting absorption column is 5.1 (+/-0.5) x 10^21 cm^-2, in excess of the Galactic value of 2.3 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.52 (+0.23, -0.22) and a best-fitting absorption column of 3.8 (+1.5, -1.2) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.3 x 10^-11 (6.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.8 (+1.5, -1.2) x 10^21 cm^-2 Galactic foreground: 2.3 x 10^21 cm^-2 Excess significance: 2.0 sigma Photon index: 1.52 (+0.23, -0.22) If the light curve continues to decay with a power-law decay index of 1.292, the count rate at T+24 hours will be 0.020 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-12 (1.4 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01246907. This circular is an official product of the Swift-XRT team.