On the Mean Radiative Efficiency of Accreting Massive Black Holes in AGNs And QSOs

Abstract

Radiative efficiency is an important physical parameter that describes the fraction of accretion material converted to radiative energy for accretion onto massive black holes (MBHs). With the simplest So{\l}tan argument, the radiative efficiency of MBHs can be estimated by matching the mass density of MBHs in the local universe to the accreted mass density by MBHs during AGN/QSO phases. In this paper, we estimate the local MBH mass density through a combination of various determinations of the correlations between the masses of MBHs and the properties of MBH host galaxies, with the distribution functions of those galaxy properties. We also estimate the total energy density radiated by AGNs and QSOs by using various AGN/QSO X-ray luminosity functions in the literature. We then obtain several hundred estimates of the mean radiative efficiency of AGNs/QSOs. Under the assumption that those estimates are independent of each other and free of systematic effects, we apply the median statistics as described by Gott et al.\cite{got01} and find the mean radiative efficiency of AGNs/QSOs is $\epsilon=0.105^{+0.006}_{-0.008}$, which is consistent with the canonical value $\sim0.1$. Considering that about $20\%$ Compton-thick objects may be missed from current available X-ray surveys, the true mean radiative efficiency may be actually $\sim 0.12$.