We present a model of optically thin , two-temperature , accretion flows using an exact Monte Carlo treatment of global Comptonization , with seed photons from synchrotron and bremsstrahlung emission , as well as with a fully general relativistic description of both the radiative and hydrodynamic processes .
We consider accretion rates for which the luminosities of the flows are between \sim 10 ^ { -3 } and 10 ^ { -2 } of the Eddington luminosity .
The black hole spin parameter strongly affects the flow structure within the innermost \simeq 10 gravitational radii .
The resulting large difference between the Coulomb heating in models with a non-rotating and a rapidly rotating black hole is , however , outweighed by a strong contribution of compression work , much less dependent on spin .
The consequent reduction of effects related to the value of the black spin is more significant at smaller accretion rates .
For a non-rotating black hole , the compressive heating of electrons dominates over their Coulomb heating , and results in an approximately constant radiative efficiency of \approx 0.4 per cent in the considered range of luminosities .
For a rapidly rotating black hole , the Coulomb heating dominates , the radiative efficiency is \simeq 1 per cent and it slightly increases ( but less significantly than estimated in some previous works ) with increasing accretion rate .
Our study neglects the direct heating of electrons , which effect can lead to larger differences between the radiative properties of models with a non-rotating and a rapidly rotating black hole than estimated here .
Flows with the considered parameters produce rather hard spectra , with the photon spectral index \Gamma \sim 1.6 , and with high energy cut-offs at several hundred keV .
We find an agreement between our model , in which the synchrotron emission is the main source of seed photons , and observations of black-hole binaries in their hard states and AGNs at low luminosities .
In particular , our model predicts a hardening of the X-ray spectrum with increasing luminosity , as indeed observed below \sim 0.01 L _ { { E } } or so in both black-hole binaries and AGNs .
Also , our model approximately reproduces the luminosity and the slope of the X-ray emission in Cen A .