High-redshift Faranoff-Riley type II radio galaxies: X-ray properties of the cores

We present an extensive X-ray spectral analysis of the cores of 19 FRII sources in the redshift range 0.5 < z < 1.0 which were selected to be matched in isotropic radio power. The sample consists of 10 radio galaxies and 9 quasars. We compare our results with the expectations from a unification model that ascribes the difference betwee n these two types of sources to the viewing angle to the line of sight, beaming and the presence of a dust and gas torus. We find that the spectrum of all the quasars can be fitted with a sin gle power law, and that the spectral index flattens with decreasing angle to the line of s ight. We interpret this as the effect of increasingly dominant inverse Compton X-ray emission, beamed such that the jet emission outshines other core components. For up to 70 per cent of the radio galaxies we detect intrinsic absorption; their core spectra are best fitted with an unabso rbed steep power law of average spectral index = 2 .1 and an absorbed power law of spectral index = 1 .6, which is flatter than that observed for radio-quiet quasars. We furth er conclude that the presence of a jet affects the spectral properties of absorbed nuclear em ission in AGN. In radio galaxies, any steep-spectrum component of nuclear X-ray emission, similar to that seen in radio-quiet quasars, must be masked by a jet or by jet-related emission.