TITLE: GCN CIRCULAR NUMBER: 22219 SUBJECT: GRB 171209A: Swift-XRT refined Analysis DATE: 17/12/09 22:25:41 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Tohuvavohu (PSU) and S.J. LaPorte report on behalf of the Swift-XRT team: We have analysed 6.3 ks of XRT data for GRB 171209A (LaPorte et al. GCN Circ. 22213), from 92 s to 17.9 ks after the BAT trigger. The data comprise 349 s in Windowed Timing (WT) mode (the first 5 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. Using 2454 s of PC mode data and 6 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 = 139.40280, -30.52016 which is equivalent to: RA (J2000): 09h 17m 36.67s Dec(J2000): -30d 31' 12.6" with an uncertainty of 1.7 arcsec (radius, 90% confidence). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=1.87 (+/-0.08). At T+231 s the decay steepens to an alpha of 3.42 (+0.26, -0.22) before breaking again at T+629 s to a final decay with index alpha=0.74 (+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 1.37 (+/-0.04). The best-fitting absorption column is 2.54 (+0.27, -0.25) x 10^21 cm^-2, in excess of the Galactic value of 2.1 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.06 (+0.08, -0.07) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.11 (+0.10, -0.00) x 10^21 cm^-2 Galactic foreground: 2.1 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 2.06 (+0.08, -0.07) If the light curve continues to decay with a power-law decay index of 0.74, the count rate at T+24 hours will be 0.036 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-12 (1.6 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00796100. This circular is an official product of the Swift-XRT team.