  Magnetic field behaviour in a spherically-symmetric accretion flow for
parameters typical of single black holes in the Galaxy is discussed. It is
shown that in the majority of Galaxy volume, accretion onto single stellar-mass
black holes will be spherical and have a low accretion rate ($10^{-6} -
10^{-9}$ of the Eddington rate). An analysis of plasma internal energy growth
during the infall is performed. Adiabatic heating of collisionless accretion
flow due to magnetic adiabatic invariant conservation is 25% more efficient
than in the standard non-magnetized gas case. It is shown that magnetic field
line reconnections in discrete current sheets lead to significant nonthermal
electron component formation. In a framework of quasi-diffusion acceleration,
the "energy-radius" electron distribution is computed and the function
describing the shape of synchrotron radiation spectrum is constructed. It is
shown that nonthermal electron emission leads to formation of a hard (UV,
X-ray, up to gamma), highly variable spectral component in addition to the
standard synchrotron optical component first derived by Shvartsman generated by
thermal electrons in the magnetic field of accretion flow. For typical
interstellar medium parameters, a black hole at 100 pc distance will be a
16-25$^{\rm m}$ optical source coinciding with the highly variable bright X-ray
counterpart, while the variable component of optical emission will be about
18-27$^{\rm m}$. The typical time scale of the variability is $10^{-4}$ sec,
with relative flare amplitudes of 0.2-6% in various spectral bands. Possible
applications of these results to the problem of search for single black holes
are discussed.
