SIMBAD references

The cross-correlation of the source list from the ROSAT All-Sky Survey with the 5 GHz Green Bank survey yields a list of 2127 objects. About two thirds of them are optically unidentified. The majority of the objects with known optical counterparts are quasars and radio galaxies, most of them detected in X-rays for the first time. In this paper we present a list of the previously optically identified objects with their main characteristics and discuss their general (bulk) properties. We find strong correlations between luminosities in the radio, optical, and X-ray bands which differ for quasars and radio galaxies. The fraction of flat radio spectrum sources increases with redshift and ∼ 95% of all sources above z = 2 are of that type. In the relatively large group of blazars, radio selected BL Lacs and highly polarized quasars show very similar properties while X-ray selected BL Lacs have markedly different X-ray to radio flux ratios. The sample is large enough and of sufficiently high quality to reveal clearly various detection biases and selection effects which strongly influence the physical interpretation of the correlations. Radio follow - up observations for the determination of the morphological and spectral properties as well as environmental conditions are needed for a statistically reliable study of the sample. The very large group of sources with low radio fluxes remains to be optically identified.