On the Mean Radiative Efficiency of Accreting Massive Black Holes in
  AGNs And QSOs
  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 $\sim 0.12$.
