  The observed correlation between the radio and X-ray fluxes in the hard state
of black-hole X-ray binaries (BHXRBs) has been around for more than two
decades. It is currently accepted that the hard X-rays in BHXRBs come from
Comptonization in the corona and the radio emission from the relativistic jet
(Lorentz $\gamma >> 1$), which is a narrow structure of a few $R_g=GM/c^2$ at
its base. The relativistic jet and the corona, however, are separate entities
with hardly any communication between them, apart from the fact that both are
fed from the accreting matter. It is also widely accepted that the accretion
flow around black holes in BHXRBs consists of an outer thin disk and an inner
hot flow. From this hot inner flow, an outflow emanates in the hard and
hard-intermediate states of the source. By considering Compton up-scattering of
soft disk photons in the outflow (i.e., in the outflowing corona, which is a
wider structure, tens to hundreds of $R_g$ at its base, with low Lorentz gamma)
as the mechanism that produces the hard X-ray spectrum, we have been able to
explain quantitatively a number of observed correlations. Here, we demonstrate
that this outflowing corona can also explain quantitatively the observed radio
- X-ray correlation. In addition, we make the following theoretical predictions
for GX 339-4: 1) the radio flux in the hard and hard-intermediate states should
be a bell-shaped curve as a function of the photon-number spectral index Gamma,
2) the radio - X-ray correlation should break down when the source moves from
the hard to the hard-intermediate state and instead the radio flux should first
increase sharply in the hard-intermediate state and then decrease also sharply,
in a very narrow range of the X-ray flux, and 3) the X-ray polarization will be
parallel to the outflow in the hard state and perpendicular to it in the
hard-intermediate one.
