The Radiative Efficiency and Spectra of Slowly Accreting Black Holes from Two-Temperature GRRMHD Simulations

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 ${10}^{-5}{\dot{M}}_{\mathrm{E}\mathrm{d}\mathrm{d}}$ 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{E}\mathrm{d}\mathrm{d}$}$,theflowapproaches$\sim 1%$ 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.