TITLE: GCN CIRCULAR NUMBER: 29625 SUBJECT: GRB 210305A: Swift-XRT refined Analysis DATE: 21/03/08 16:48:20 GMT FROM: Antonino D'Ai at IASF-PA M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and A. D'Ai (INAF-IASFPA) report on behalf of the Swift-XRT team: We have analysed 11 ks of XRT data for GRB 210305A (D'Ai et al. GCN Circ. 29595), from 67 s to 178.1 ks after the BAT trigger. The data comprise 184 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. Using 2502 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 319.81054, +34.55403 which is equivalent to: RA (J2000): 21h 19m 14.53s Dec(J2000): +34d 33' 14.5" with an uncertainty of 1.4 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.3 ks) can be modelled with a power-law decay with a decay index of alpha=0.91 (+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 1.44 (+/-0.08). The best-fitting absorption column is 3.2 (+0.5, -0.4) x 10^21 cm^-2, in excess of the Galactic value of 2.0 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.84 (+/-0.16) and a best-fitting absorption column of 4.0 (+0.9, -0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.0 (+0.9, -0.8) x 10^21 cm^-2 Galactic foreground: 2.0 x 10^21 cm^-2 Excess significance: 4.1 sigma Photon index: 1.84 (+/-0.16) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01035922. This circular is an official product of the Swift-XRT team.