TITLE: GCN CIRCULAR NUMBER: 27852 SUBJECT: GRB 200528A: Swift-XRT refined Analysis DATE: 20/05/29 10:39:04 GMT FROM: Phil Evans at U of Leicester M. Perri (ASDC), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and E. Ambrosi report on behalf of the Swift-XRT team: We have analysed 5.7 ks of XRT data for GRB 200528A (Ambrosi et al. GCN Circ. 27832), from 73 s to 50.9 ks after the BAT trigger. The data comprise 226 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. 27836). The late-time light curve (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of alpha=0.97 (+/-0.07). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.30 (+/-0.05). The best-fitting absorption column is 3.63 (+0.19, -0.18) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.24 (+0.16, -0.15) and a best-fitting absorption column of 3.8 (+0.7, -0.6) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.8 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^20 cm^-2 Excess significance: 9.8 sigma Photon index: 2.24 (+0.16, -0.15) If the light curve continues to decay with a power-law decay index of 0.97, the count rate at T+24 hours will be 0.041 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x 10^-12 (2.5 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/00974827. This circular is an official product of the Swift-XRT team.