TITLE: GCN CIRCULAR NUMBER: 22837 SUBJECT: GRB 180624A: Swift-XRT refined Analysis DATE: 18/06/25 00:10:26 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (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), A. Tohuvavohu (PSU) and S.L. Gibson report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 180624A (Gibson et al. GCN Circ. 22832), from 118 s to 23.6 ks after the BAT trigger. The data comprise 538 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22835). The late-time light curve (from T0+3.9 ks) can be modelled with a power-law decay with a decay index of alpha=0.84 (+0.12, -0.11). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.472 (+/-0.025). The best-fitting absorption column is 1.53 (+/-0.11) x 10^21 cm^-2, in excess of the Galactic value of 4.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.08 (+0.19, -0.18) and a best-fitting absorption column of 1.3 (+/-0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.3 (+/-0.5) x 10^21 cm^-2 Galactic foreground: 4.6 x 10^20 cm^-2 Excess significance: 3.0 sigma Photon index: 2.08 (+0.19, -0.18) If the light curve continues to decay with a power-law decay index of 0.84, the count rate at T+24 hours will be 0.013 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x 10^-13 (5.5 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/00844192. This circular is an official product of the Swift-XRT team.