TITLE: GCN CIRCULAR NUMBER: 37149 SUBJECT: GRB 240811A: Swift-XRT refined Analysis DATE: 24/08/12 11:05:46 GMT FROM: Phil Evans at U of Leicester T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), J. D. Gropp (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 6.7 ks of XRT data for GRB 240811A, from 84 s to 39.6 ks after the BAT trigger. The data comprise 41 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=4.2 (+/-0.3), followed by a break at T+240 s to an alpha of 0.42 (+/-0.04). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.19, -0.17). The best-fitting absorption column is 5.1 (+4.6, -3.1) x 10^20 cm^-2, consistent with the Galactic value of 2.0 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 3.4 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.1 (+4.6, -3.1) x 10^20 cm^-2 Galactic foreground: 2.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.97 (+0.19, -0.17) If the light curve continues to decay with a power-law decay index of 0.42, the count rate at T+24 hours will be 0.036 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 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/01248132. This circular is an official product of the Swift-XRT team.