GRB 240828A: Swift-XRT afterglow detection

Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 240828A in a series of observations tiled on the sky. The total exposure time is 3.3 ks, distributed over 3 tiles; the maximum exposure at a single sky location was 2.3 ks. The data were collected between T0+42.1 ks and T0+60.2 ks, and are entirely in Photon Counting (PC) mode. Six uncatalogued X-ray sources are detected, of which one ("Source 3") is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=225.5130, +36.5285 which is equivalent to: RA (J2000): 15:02:3.12 Dec(J2000): +36:31:42.5 with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is 6.8 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=3.043 (+0.027, -0.580). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.47 (+0.33, -0.24). The best-fitting absorption column is consistent with the Galactic value of 1.3 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 (3.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.3 (+/-6.6) x 10^20 cm^-2 Galactic foreground: 1.3 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.47 (+0.33, -0.24) If the light curve continues to decay with a power-law decay index of 3.043, the count rate at T+24 hours will be 0.014 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x 10^-13 (5.3 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021707. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00127. This circular is an official product of the Swift-XRT team.