TITLE: GCN CIRCULAR NUMBER: 13999 SUBJECT: GRB 121125A: Swift-XRT refined Analysis DATE: 12/11/25 23:43:03 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (INAF-OAB/PSU), G. Stratta (ASDC), M.C. Stroh (PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester), J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB) and B.N. Barlow report on behalf of the Swift-XRT team: We have analysed 9.6 ks of XRT data for GRB 121125A (Barlow et al. GCN Circ. 13993), from 73 s to 23.5 ks after the BAT trigger. The data comprise 196 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN. Circ 13998). The late-time light curve (from T0+4.0 ks) can be modelled with a power-law decay with a decay index of alpha=1.28 (+/-0.08). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.02 (+0.10, -0.09). The best-fitting absorption column is 1.49 (+0.23, -0.22) x 10^21 cm^-2, in excess of the Galactic value of 1.3 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.14 (+0.13, -0.12) and a best-fitting absorption column of 1.02 (+0.28, -0.26) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.02 (+0.28, -0.26) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^20 cm^-2 Excess significance: 5.6 sigma Photon index: 2.14 (+0.13, -0.12) If the light curve continues to decay with a power-law decay index of 1.28, the count rate at T+24 hours will be 0.013 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x 10^-13 (5.7 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/00539563. This circular is an official product of the Swift-XRT team.