Hard X-rays from Emission Line Galaxies and the X-ray Background: A Test for Advection Dominated Accretion with Radio Sources
Abstract
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
L
R
∼7×
10
35
(ν/15GHz
)
7/5
(M/
10
7
M
⊙
)(
L
x
/
10
40
erg
s
−1
)
1/10
erg
s
−1
where
L
R
=ν
L
ν
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
I
ν
∝
ν
2/5
. For example, an ADAF X-ray source with luminosity
L
x
∼
10
41
erg
s
−1
has a nuclear radio luminosity of
∼4×
10
36
(M/3×
10
7
M
⊙
)erg
s
−1
at
∼20
GHz and if at a distance of
∼10(M/3×
10
7
M
⊙
)
1/2
Mpc
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
L
R
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.