ROSAT observations of distant 3CR radio galaxies .2.

The X-ray emission from high-redshift radio galaxies reveals the properties of their environment, as well as the degree of obscuration of the nuclear component. In this paper we compile and compare all archival ROSAT PSPC observations, pointed and serendipitous, of 3CR radio galaxies above a redshift z of 0.4. We present new detections of four galaxies in the redshift range 1 < z < 1.8, one of which is an 'N galaxy'. Of the three radio galaxies where no optical nucleus is seen, the X-ray emission of one (3C 194) appears to be dominated by an absorbed power-law component; the others (3C 294 and 324) are fitted equally well by either an absorbed power law or a thermal model. The inferred thermal X-ray luminosity is consistent with the radio galaxy being in a moderately rich X-ray cluster of galaxies. If our interpretation is correct, the cluster around 3C 294 at a redshift of 1.786 is the most distant yet detected in X-rays. Our results support a simple model where the X-ray emission from distant objects classified as radio galaxies originates in a hot surrounding intracluster medium, sometimes with the addition of an absorbed power-law component from the central nucleus.