TITLE: GCN CIRCULAR NUMBER: 21904 SUBJECT: GRB 170921A: Swift-XRT refined Analysis DATE: 17/09/21 22:29:22 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (INAF-OAB/PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), V. D'Elia (ASDC), A. Melandri (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU) and A. D'Ai report on behalf of the Swift-XRT team: We have analysed 6.5 ks of XRT data for GRB 170921A (D'Ai et al. GCN Circ. 21901), from 82 s to 16.5 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 26.8487, 30.9067 which is equivalent to: RA (J2000): 01 47 23.70 Dec(J2000): +30 54 24.3 with an uncertainty of 2.1 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=3.3 (+0.5, -0.4), followed by a break at T+238 s to an alpha of 0.85 (+0.10, -0.09). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.7 (+0.4, -0.3). The best-fitting absorption column is 1.00 (+0.27, -0.23) x 10^22 cm^-2, in excess of the Galactic value of 5.5 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 3.5 x 10^-11 (1.2 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.00 (+0.27, -0.23) x 10^22 cm^-2 Galactic foreground: 5.5 x 10^20 cm^-2 Excess significance: 6.7 sigma Photon index: 2.7 (+0.4, -0.3) If the light curve continues to decay with a power-law decay index of 0.85, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x 10^-14 (2.5 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/00773509. This circular is an official product of the Swift-XRT team.