TITLE: GCN CIRCULAR NUMBER: 21198 SUBJECT: GRB 170604A: Swift-XRT refined Analysis DATE: 17/06/05 08:21:54 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Cholden-Brown (PSU), B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB) and K.L. Page report on behalf of the Swift-XRT team: We have analysed 7.6 ks of XRT data for GRB 170604A (Page et al. GCN Circ. 21191), from 110 s to 39.1 ks after the BAT trigger. The data comprise 308 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. Using 4539 s of PC mode data and 10 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 342.65623, -15.41233 which is equivalent to: RA (J2000): 22h 50m 37.49s Dec(J2000): -15d 24' 44.4" with an uncertainty of 1.4 arcsec (radius, 90% confidence). The late-time light curve (from T0+3.8 ks) can be modelled with a power-law decay with a decay index of alpha=0.98 (+/-0.05). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.27 (+/-0.04). The best-fitting absorption column is 1.95 (+/-0.11) x 10^21 cm^-2, in excess of the Galactic value of 3.8 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.85 (+/-0.09) and a best-fitting absorption column of 6.9 (+2.3, -2.2) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.9 (+2.3, -2.2) x 10^20 cm^-2 Galactic foreground: 3.8 x 10^20 cm^-2 Excess significance: 2.3 sigma Photon index: 1.85 (+/-0.09) If the light curve continues to decay with a power-law decay index of 0.98, the count rate at T+24 hours will be 0.046 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x 10^-12 (1.9 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/00755867. This circular is an official product of the Swift-XRT team.