TITLE: GCN CIRCULAR NUMBER: 23645 SUBJECT: GRB 190109A: Swift-XRT refined Analysis DATE: 19/01/09 11:25:39 GMT FROM: Phil Evans at U of Leicester A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester) and J.D. Gropp report on behalf of the Swift-XRT team: We have analysed 6.1 ks of XRT data for GRB 190109A (Gropp et al. GCN Circ. 23642), from 175 s to 13.0 ks after the BAT trigger. The data comprise 291 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23643). The late-time light curve (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of alpha=0.8 (+/-0.3). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.87 (+/-0.04). The best-fitting absorption column is 9.5 (+1.3, -1.2) x 10^20 cm^-2, in excess of the Galactic value of 5.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.88 (+0.11, -0.09) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 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 (+2.1, -0.0) x 10^20 cm^-2 Galactic foreground: 5.1 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.88 (+0.11, -0.09) If the light curve continues to decay with a power-law decay index of 0.8, the count rate at T+24 hours will be 0.010 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-13 (4.0 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/00882747. This circular is an official product of the Swift-XRT team.