GCN Circular 24749
J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), M.G. Bernadini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , V. D'Elia (ASDC) and K.L. Page report on behalf of the Swift-XRT team: We have analysed 7.7 ks of XRT data for GRB 190604B (Page et al. GCN Circ. 24742), from 120 s to 34.4 ks after the BAT trigger. The data comprise 285 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 24743). The late-time light curve (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of alpha=0.94 (+/-0.12). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.40 (+/-0.03). The best-fitting absorption column is 1.49 (+0.16, -0.13) x 10^21 cm^-2, consistent with the Galactic value of 1.4 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.02 (+/-0.12) and a best-fitting absorption column of 3.0 (+/-0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (5.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.0 (+/-0.5) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^21 cm^-2 Excess significance: 5.7 sigma Photon index: 2.02 (+/-0.12) If the light curve continues to decay with a power-law decay index of 0.94, the count rate at T+24 hours will be 0.057 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x 10^-12 (3.1 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/00906654. This circular is an official product of the Swift-XRT team.