Recent studies of the cosmic X-ray background (XRB) have suggested the
possible existence of a population of relatively faint sources with hard X-ray
spectra; however, the emission mechanism remains unclear. If the hard X-ray
emission is from the radiatively inefficient, advection dominated accretion
flows (ADAFs) around massive black holes in galactic nuclei, X-ray luminosity
and radio luminosity satisfy the approximate relation LR∼7×1035(ν/15GHz)7/5(M/107M⊙)(Lx/1040ergs−1)1/10ergs−1 where LR=νLν is the radio luminosity at frequency ν, M
is the mass of the accreting black hole, and 10^{40} \simle L_x\simle 10^{42}
erg s^{-1} is the 2-10 keV X-ray luminosity. These sources are characterized
by inverted radio spectra Iν∝ν2/5. For example, an ADAF X-ray
source with luminosity Lx∼1041ergs−1 has a nuclear radio
luminosity of ∼4×1036(M/3×107M⊙)ergs−1 at
∼20 GHz and if at a distance of ∼10(M/3×107M⊙)1/2Mpc would be detected as a ∼1mJy point radio source. High frequency
(∼20GHz), high angular resolution radio observations provide an
important test of the ADAF emission mechanism. Since LR depends strongly on
black hole mass and only weakly on X-ray luminosity, the successful measurement
of nuclear radio emission could provide an estimate of black hole mass. Because
the X-ray spectra produced by ADAFs are relatively hard, sources of this
emission are natural candidates for contributing to the hard, >2 keV,
background.Comment: 16 pages, 2 figures, Ap