TITLE: GCN CIRCULAR NUMBER: 34676 SUBJECT: GRB 230911C: Swift-XRT refined Analysis DATE: 23/09/12 11:45:52 GMT FROM: Phil Evans at U of Leicester A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 7.2 ks of XRT data for GRB 230911C, from 103 s to 74.1 ks after the BAT trigger. The data comprise 106 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 late-time light curve (from T0+4.2 ks) can be modelled with a power-law decay with a decay index of alpha=0.48 (+0.22, -0.18). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.86 (+0.26, -0.21). The best-fitting absorption column is 1.3 (+0.9, -0.3) x 10^21 cm^-2, consistent with the Galactic value of 9.4 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 3.6 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.3 (+0.9, -0.3) x 10^21 cm^-2 Galactic foreground: 9.4 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.86 (+0.26, -0.21) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01191188. This circular is an official product of the Swift-XRT team.