TITLE: GCN CIRCULAR NUMBER: 30515 SUBJECT: GRB 210724A: Swift-XRT refined Analysis DATE: 21/07/25 21:43:29 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P. D'Avanzo report on behalf of the Swift-XRT team: We have analysed 2.5 ks of XRT data for GRB 210724A (D'Avanzo et al. GCN Circ. 30497), from 123 s to 84.6 ks after the BAT trigger. The data comprise 263 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 331 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 227.41131, -6.29009 which is equivalent to: RA (J2000): 15h 09m 38.72s Dec(J2000): -06d 17' 24.3" with an uncertainty of 2.0 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=1.17 (+0.10, -0.09). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.48 (+/-0.08). The best-fitting absorption column is 2.08 (+0.23, -0.22) x 10^21 cm^-2, in excess of the Galactic value of 8.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.8 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 2.08 (+0.23, -0.22) x 10^21 cm^-2 Galactic foreground: 8.2 x 10^20 cm^-2 Excess significance: 9.5 sigma Photon index: 2.48 (+/-0.08) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01061482. This circular is an official product of the Swift-XRT team.