MULTIFREQUENCY OBSERVATIONS OF ROSAT SELECTED RADIO-SOURCES

We report on results of multifrequency radio continuum observations wi th the Effelsberg 100-m telescope of 234 radio sources which have coun terparts in the ROSAT all-sky survey. Observations have been made at 2 1 cm, 11 cm, 6 cm and 2.8 cm wavelength in the flux density range abov e 20 mJy. We have determined the spectrum, size, linear polarization a nd improved positions of these sources. We give the statistical proper ties of the ROSAT selected radio sources and compare them with results from unbiased radio source surveys so far available. In general the d ifferences are small. We find a weak excess of flat spectrum sources a nd a higher fraction of unresolved sources. Cumulative counts of the r adio sources become incomplete already at a relatively high flux densi ty level. At 11 cm wavelength we have about three times less sources a t 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 cm- 2 s-1. No global correlation is found between the integrated radi o flux densities and the X-ray flux densities for the entire sample. O ur sample is not large enough and we do not have enough optical identi fications for a general study of correlations for distinct groups of r adio sources. Differences are seen, however, between very steep (alpha < -0.7 (S(nu) is-proportional-to nu(+alpha))) and very flat spectrum radio sources (alpha > -0.3). About 1/3 of the very flat spectrum radi o sources have an unusually small ratio of X-ray to radio emission. Mo st of them are quasars. The remaining flat spectrum sources show a sig nificant correlation between their X-ray and radio flux densities. Nev ertheless, the differences in the properties of radio sources, which h ave strong enough X-ray emission to be seen in the ROSAT survey, and t hose which are not seen remain unclear.