TITLE: GCN CIRCULAR NUMBER: 13989 SUBJECT: GRB 121123A: Swift-XRT refined Analysis DATE: 12/11/24 02:17:12 GMT FROM: Phil Evans at U of Leicester A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), G. Stratta (ASDC), J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester) and E.A. Helder report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 121123A (Helder et al. GCN Circ. 13982), from 129 s to 23.8 ks after the BAT trigger. The data comprise 669 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 307.3183, -11.85898 which is equivalent to: RA (J2000): 20 29 16.39 Dec(J2000): -11 51 32.3 with an uncertainty of 3.5 arcsec (radius, 90% confidence). The late-time light curve (from T0+4.7 ks) can be modelled with a power-law decay with a decay index of alpha=0.50 (+/-0.09). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.448 (+0.023, -0.022). The best-fitting absorption column is 8.9 (+/-0.6) x 10^20 cm^-2, in excess of the Galactic value of 4.0 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.97 (+/-0.11) and a best-fitting absorption column of 5.8 (+2.3, -1.8) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.8 (+2.3, -1.8) x 10^20 cm^-2 Galactic foreground: 4.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.97 (+/-0.11) If the light curve continues to decay with a power-law decay index of 0.50, the count rate at T+24 hours will be 0.051 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x 10^-12 (2.1 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/00539358. This circular is an official product of the Swift-XRT team.