Thermal Comptonization in compact sources and the cosmic X-ray background

Abstract

We model thermal Compionization and e+e − pair production in high viscosity black hole accretion flows. We assume that magnetic fields are in equipartition with the energy density of the flow and that cyclotron radiation is the major source of seed soft photons. The electrons are assumed to receive a constant fraction of the gravitational energy. We find that the resulting spectra have a constant X-ray energy spectral index, α x ≅ 0, for accretion rates 0.5 ≳ \(\dot m\) ≳ 20. Here \(\dot m\) ≡ \(\dot M\)c22/L E , and LE is the Eddington luminosity. The spectra are cut off above ∼ 150 keV. Such spectra closely resemble that of the residual cosmic X-ray background (after subtraction of the contributions of the known classes of extragalactic sources if the background originated at a redshift z ∼ 4–5. Thus, the residual cosmic background can be explained by an early population of active galaxies emitting thermal Comptonized spectra, as originally suggested by Leiter and Boldt (1982).