Title: Kinetic and radiative power from optically thin accretion flows,
Abstract: We perform a set of general relativistic, radiative, magneto-hydrodynamical
simulations (GR-RMHD) to study the transition from radiatively inefficient to
efficient state of accretion on a non-rotating black hole. We study ion to
electron temperature ratios ranging from $T_{\rm i}/T_{\rm e}=10$ to $100$, and
simulate flows corresponding to accretion rates as low as $10^{-6}\dot M_{\rm
Edd}$, and as high as $10^{-2}\dot M_{\rm Edd}$. We have found that the
radiative output of accretion flows increases with accretion rate, and that the
transition occurs earlier for hotter electrons (lower $T_{\rm i}/T_{\rm e}$
ratio). At the same time, the mechanical efficiency hardly changes and accounts
to ${\approx}\,3\%$ of the accreted rest mass energy flux, even at the highest
simulated accretion rates. This is particularly important for the mechanical
AGN feedback regulating massive galaxies, groups, and clusters. Comparison with
recent observations of radiative and mechanical AGN luminosities suggests that
the ion to electron temperature ratio in the inner, collisionless accretion
flow should fall within $10<T_{\rm i}/T_{\rm e}<30$, i.e., the electron
temperature should be several percent of the ion temperature.