  We present global radiation GRMHD simulations of strongly magnetized
accretion onto a spinning, stellar mass black hole at sub-Eddington rates.
Using a frequency-dependent Monte Carlo procedure for Compton scattering, we
self-consistently evolve a two-temperature description of the ion-electron
fluid and its radiation field. For an Eddington ratio $L/L_{\rm Edd} \gtrsim
10^{-3}$, the emergent spectrum forms an apparent power law shape from thermal
Comptonization up to a cutoff at $\simeq 100$ keV, characteristic of that seen
in the hard spectral states of black hole X-ray binary systems. At these
luminosities, the radiative efficiency is high ($\approx 24\%$) and results in
a denser midplane region where magnetic fields are dynamically important. For
$L/L_{\rm Edd} \sim 10^{-2}$, our hot accretion flow appears to undergo thermal
runaway and collapse. Our simulations demonstrate that hot accretion flows can
be radiatively efficient and provide an estimate of their maximum luminosity.
