  We study radiative properties of hot accretion flows in a general
relativistic model with an exact treatment of global Comptonization, developed
in our recent works. We note a strong dependence of electron temperature on the
strength of magnetic field and we clarify that the underlying mechanism
involves the change of the flow structure, with more strongly magnetised flows
approaching the slab geometry more closely. We find that the model with thermal
synchrotron radiation being the main source of seed photons agrees with the
spectral index vs Eddington ratio relation observed in black hole transients
below 1 per cent of the Eddington luminosity, LEdd, and models with a weak
direct heating of electrons (small delta) are more consistent with
observations. Models with large delta predict slightly too soft spectra,
furthermore, they strongly overpredict electron temperatures at ~0.01 LEdd. The
low-luminosity spectra, at <0.001 LEdd, deviate from a power-law shape in the
soft X-ray range and we note that the first-scattering bump often resembles a
thermal like component, with the temperature of a few hundred eV, superimposed
on a power-law spectrum. The model with thermal Comptonization of thermal
synchrotron radiation does not agree with well studied AGNs observed below
~0.01 LEdd, for which there is a substantial evidence for the lack of an inner
cold disc. This indicates that the model of hot flows powering AGNs should be
revised, possibly by taking into account an additional (but internal to the
flow) source of seed photons.
