The Radiative Efficiency and Spectra of Slowly Accreting Black Holes from Twoerature GRRMHD Simulations

We present axisymmetric numerical simulations of radiatively inefficient accretion flows onto black holes combining general relativity, magnetohydrodynamics, self-consistent electron thermodynamics, and frequencydependent radiation transport. We investigate a range of accretion rates up to 10^-5 M_Edd onto a 108 M_black hole with spin a = 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 M to inverse-Compton-dominated at our highest M. In contrast to canonical analytic models, we find that by M M 10-5 Edd, the flow approaches ~1% radiative efficiency, with much of the radiation due to inverseCompton 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.