Comparison of VLBI radio-core and X-ray flux densities of extragalactic radio sources

Abstract

We have selected a sample of 56 extragalactic sources that were observed both at X-ray energies with the Einstein Observatory and at radio frequencies with VLBI. From the VLBI observations found in the literature, we extract 10.7 GHz core flux densities, in some cases extrapolating by assuming an inverted core spectrum of index -0.5. If the X-ray emission mechanism is synchrotron self-Compton (SSC), we expect the core radio and X-ray flux densities to be closely related. We test this hypothesis by analyzing the correlation between the radio-core and X-ray flux densities and luminosities. The radio-core-X-ray spectral indices are narrowly distributed about a mean of 0.85, with a dispersion of 0.10. The detailed correlation between the radio-core and X-ray flux densities is poor, however. The correlation between the luminosities, on the other hand, is quite good. The strong luminosity correlation is due mainly to the common distance factor. The slope of the luminosity correlation deviates significantly from unity. The probable cause of this is an excess of X-ray flux from low-redshift sources. The narrow dispersion of spectral index and the poor one-to-one correspondence of radio-core and X-ray flux density are consistent with the SSC process. The basic level of SSC emission requires a radio brightness temperature of order 2 x 10^11^ K, as occurs in most compact radio sources, while the exact X-ray flux density is highly sensitive to several radio source parameters.