EF B0155+5816 , the SIMBAD biblio

We report on results of multifrequency radio continuum observations with the Effelsberg 100-m telescope of 234 radio sources which have counterparts in the ROSAT all-sky survey. Observations have been made at 21 cm, 11 cm, 6 cm and 2.8 cm wavelength in the flux density range above 20 mJy. We have determined the spectrum, size, linear polarization and improved positions of these sources. We give the statistical properties of the ROSAT selected radio sources and compare them with results from unbiased radio source surveys so far available. In general the differences are small. We find a weak excess of flat spectrum sources and a higher fraction of unresolved sources. Cumulative counts of the radio sources become incomplete already at a relatively high flux density level. At 11 cm wavelength we have about three times less sources at 100 mJy than expected and this deficit increases towards lower flux densities. The reason is the limited ROSAT all-sky survey sensitivity making cumulative counts of X-ray sources incomplete below 10^–12 erg/cm²/s. No global correlation is found between the integrated radio flux densities and the X-ray flux densities for the entire sample. Our sample is not large enough and we do not have enough optical identifications for a general study of correlations for distinct groups of radio sources. Differences are seen, however, between very steep (α←0.7 (S_ν∝ν^+α)) and very flat spectrum radio sources (α> -0.3). About 1/3 of the very flat spectrum radio sources have an unusually small ratio of X-ray to radio emission. Most of them are quasars. The remaining flat spectrum sources show a significant correlation between their X-ray and radio flux densities. Nevertheless, the differences in the properties of radio sources, which have strong enough X-ray emission to be seen in the ROSAT survey, and those which are not seen remain unclear.