We present axisymmetric numerical simulations of radiatively inefficient accretion flows onto black holes combining general relativity, magnetohydrodynamics, self-consistent electron thermodynamics, and frequency-dependent radiation transport. We investigate a range of accretion rates up to $10^{ -5} \dot{ M}_{ \mathrm{ Edd}}$ onto a $10^8 M_{ \odot}$ black hole with spin $a_{ \star} = 0.5$. We report on averaged flow thermodynamics as a function of accretion rate. We present the spectra of outgoing radiation and find that it varies strongly with accretion rate, from synchrotron-dominated in the radio at low $\dot{ M}$ to inverse Compton-dominated at our highest $\dot{ M}$. In contrast to canonical analytic models, we find that by $\dot{ M} \approx 10^{ -5} \dot{ M}_{ \mathrm{ Edd}}$, the flow approaches $\sim 1\%$ radiative efficiency, with much of the radiation due to inverse Compton scattering off Coulomb-heated electrons far from the black hole. These results have broad implications for modeling of accreting black holes across a large fraction of the accretion rates realized in observed systems.
Categories: astro-ph.HE