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.