Constraints on radiatively inefficient accretion flow models from Eddington ratio distribution of active galactic nuclei

The radiative efficiencies of accretion flows become significantly lower than that of standard thin disks, if they are accreting at rates lower than a critical accretion rate ṁcrit. The transition of a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, while ṁ ∼ ṁcrit (ṁ = Ṁ/ṀEdd). We simulate the distributions of Eddington ratio λL B λ/LEdd for active galactic nuclei (AGNs) with such accretion mode transitions in their evolutionary history. It is found that bimodal Eddington ratio distributions appear for most cases. The sources with higher Eddington ratios have standard disks, while the lower component in the Eddington ratio distributions is attributed to the presence of RIAFs in AGNs. Many simulated distributions exhibit a doublepeaked feature. The lower peak appears at λLλ/LEdd ∼ 10 −6 − 10 in the Eddington ratio distributions, only if the RIAFs without winds, or the adiabatic inflow and outflow solution (ADIOS) flows with weak or moderate winds match to standard thin disks at fixed radii larger than one hundred Schwarzschild radii. It is believed that the initial transition radius is small just after the accretion mode transition, and then the transition radius increases with decreasing accretion rate. In such variable transition radius models, a low peak appears in the simulated distributions for almost all cases except the ADIOS flows with strong winds, if