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

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

Ryan, Benjamin R.; Ressler, Sean M.; Dolence, Joshua C.; Tchekhovskoy, Alexander; Gammie, Charles; Quataert, Eliot

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}}{Edd} onto a {10}8 {M}⊙ 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}≈ {10}-5 {\dot{M}}{Edd}, the flow approaches ˜ 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.