TITLE: GCN CIRCULAR NUMBER: 22756 SUBJECT: GRB 180602A: Swift-XRT refined Analysis DATE: 18/06/03 02:52:27 GMT FROM: Phil Evans at U of Leicester J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU) and S.B. Cenko report on behalf of the Swift-XRT team: We have analysed 8.4 ks of XRT data for GRB 180602A (Cenko et al. GCN Circ. 22752), from 130 s to 59.7 ks after the BAT trigger. The data comprise 85 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. 22753). The late-time light curve (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of alpha=1.38 (+0.16, -0.13). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.36 (+0.28, -0.25). The best-fitting absorption column is 4.0 (+1.3, -1.1) x 10^21 cm^-2, in excess of the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.16, -0.15) and a best-fitting absorption column of 4.2 (+0.8, -0.7) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (6.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.2 (+0.8, -0.7) x 10^21 cm^-2 Galactic foreground: 1.6 x 10^20 cm^-2 Excess significance: 9.1 sigma Photon index: 2.04 (+0.16, -0.15) If the light curve continues to decay with a power-law decay index of 1.38, the count rate at T+24 hours will be 7.3 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x 10^-13 (4.5 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/00835200. This circular is an official product of the Swift-XRT team.