TITLE: GCN CIRCULAR NUMBER: 22817 SUBJECT: GRB 180620B: Swift-XRT refined Analysis DATE: 18/06/21 10:04:12 GMT FROM: Phil Evans at U of Leicester A. Melandri (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 6.6 ks of XRT data for GRB 180620B (Evans et al. GCN Circ. 22807), from 89 s to 56.0 ks after the BAT trigger. The data comprise 237 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 22814). The late-time light curve (from T0+4.3 ks) can be modelled with a power-law decay with a decay index of alpha=0.61 (+/-0.08). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.34 (+/-0.05). The best-fitting absorption column is 2.30 (+/-0.15) x 10^21 cm^-2, in excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.99 (+0.11, -0.10) and a best-fitting absorption column of 1.6 (+/-0.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.6 (+/-0.3) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^20 cm^-2 Excess significance: 7.9 sigma Photon index: 1.99 (+0.11, -0.10) If the light curve continues to decay with a power-law decay index of 0.61, the count rate at T+24 hours will be 0.12 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.1 x 10^-12 (5.5 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/00843211. This circular is an official product of the Swift-XRT team.