  We model non-thermal emission spectrum of the extremely sub-Eddington X-ray
binary system A0620-00. It is believed that this non-thermal emission is
produced by a radiatively inefficient "quiescent" accretion onto a stellar-mass
black hole present in the system. We post-process GRMHD simulations with
multiwavelength, fully polarized, relativistic radiative transfer calculations
to predict broadband spectra and emission polarization levels for a range of
electron models and accretion rates. We find that a model with strong coupling
of electrons and ions in the accretion disk and accretion rate of only
$\dot{M}=3\times10^{-13} [M_\odot yr^{-1}]$ is able to recover the observed
X-ray spectral slope as well as the excess of linear polarization detected in
the source in NIR/optical bands. Our models constrain the spectral properties
of a putative relativistic jet produced in this system. In addition we show
that the magnetized winds from our hot accretion flow carry away a small
fraction of the orbital angular momentum of the binary unable to explain the
observed rapid orbital decay of the system. Similar to the present GRMHD
simulations are often used to explain emission from sub-Eddington supermassvie
black holes in Sgr A* or M87; the present simulations allow us to test whether
some aspects of the quiescent black hole accretion are scale invariant.
