TITLE: GCN CIRCULAR NUMBER: 22540 SUBJECT: GRB 180325A: Swift-XRT refined Analysis DATE: 18/03/25 12:09:56 GMT FROM: Phil Evans at U of Leicester K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester) and E. Troja report on behalf of the Swift-XRT team: We have analysed 6.0 ks of XRT data for GRB 180325A (Troja et al. GCN Circ. 22532), from 61 s to 29.3 ks after the BAT trigger. The data comprise 329 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22539). The late-time light curve (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of alpha=2.02 (+0.09, -0.08). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.60 (+/-0.09). The best-fitting absorption column is 1.7 (+0.6, -0.5) x 10^22 cm^-2, at a redshift of 2.25, in addition to the Galactic value of 1.3 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 1.3 (+/-0.4) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground: 1.3 x 10^20 cm^-2 Intrinsic column: 1.3 (+/-0.4) x 10^22 cm^-2 at z=2.25 Photon index: 1.85 (+/-0.09) If the light curve continues to decay with a power-law decay index of 2.02, the count rate at T+24 hours will be 7.0 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.5 x 10^-13 (2.9 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/00817564. This circular is an official product of the Swift-XRT team.