SIMBAD references

We present Rosat X-ray observations (with PSPC and HRI instruments) of a sample of radio galaxies belonging to the B2 catalog (3C31, 0206+35, 1113+29 and 1553+24). The source 3C31 (B2 0104+32) belongs to the group Arp 331 which shows emission from a diffuse hot intra-group gas and from the brightest members of the group. We discuss both the morphological and spectral features of this extended region deriving, in the framework of an isothermal beta model, the core radius, the central density, the mass of the emitting material and gravitational mass, obtaining indications on the presence of dark matter. A comparison of X-ray and radio data shows that the thermal pressure of the external gas exceeds the minimum pressure of the radio components, apart from the region close to the nucleus where the jet is over-pressured. The X-ray emission from the single members of the group and from the other galaxies of the sample are consistent with a thermal spectrum originating from hot halos, which is a common feature in early type galaxies. We discuss the main morphological and physical properties of these coronae. Furthermore, using also the data of previous works, we confirm the strong correlation between the X-ray and optical luminosities of all these galaxies.