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Paper: Growth of Massive Black Holes during Radiatively Inefficient Accretion Phases
Volume: 373, The Central Engine of Active Galactic Nuclei
Page: 53
Authors: Cao, X.-W.
Abstract: The massive black holes in most faint active galactic nuclei (AGNs) and even normal galaxies are still accreting gases, though their accretion rates are very low. Radiatively inefficient accretion flows (RIAFs) are supposed in these faint sources, and they radiate mostly in the hard X-ray band. We calculate the contribution to the X-ray background from both the bright AGNs and the RIAFs in faint AGNs/normal galaxies. Our calculations show that both the observed intensity and spectral shape of the XRB with an energy peak at ~ 30 keV can be well reproduced without including the emission of Compton-thick AGNs, if the massive black holes in faint AGNs/normal galaxies are spinning rapidly with a ~ 0.9 and accreting at rates m ~ 2.5 10−4. It indicates that less than ~5 per cent of local massive black hole mass density was accreted during radiatively inefficient accretion phases. If Compton-thick AGNs are considered, the fraction of local black hole mass density accumulated during inefficient accretion phases should be lower than ~2 percent.
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