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

Abstract

The transition of a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, when the accretion rate is close to the critical rate. The radiative efficiencies of accretion flows accreting at rates lower than the critical accretion rate become significantly lower than that of standard thin disks. 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, as suggested by some theoretical models and observations. Based on such variable transition radius models, we derive how the dimensionless accretion rate evolves with time from the observed Eddington ratio distribution for a sample of low-luminosity active galactic nuclei in the local universe. The derived time-dependent accretion rates show a rapid decrease after the transition of the standard thin accretion disk to a RIAF, which is consistent with that derived from the hard X-ray background.