  The X-ray spectra of black hole binaries in the low/hard state first harden
as the flux decreases, then soften. This change in behaviour has been variously
attributed to either the X-rays switching from being produced in the flow to
being dominated by the jet, or to the flow switching seed photons from the disc
to self generated seed photons from cyclo-synchrotron. Here we build a simple
truncated disc, hot inner flow, plus standard conical synchrotron jet model to
explore what this predicts for the X-ray emission mechanism as a function of
mass accretion rate.
  We find that the change in X-ray spectral index can be quantitatively (not
just qualitatively) explained by the seed photon switch in the hot flow i.e.
this supports models where the X-rays are always produced by the hot flow. By
contrast, standard conical jet models are as radiatively inefficient as the hot
flow so there is no transition in X-ray production mechanism with $\dot{m}$.
Including the effects of electron cooling allows the jet X-rays to drop more
slowly with accretion rate and hence overtake the X-rays from the hot flow,
however this produces a corresponding change in the radio-X-ray correlation,
which is not observed. We argue that the unbroken radio-X-ray correlation down
to quiescence rules out the jet transition model as an explanation for the
trend in X-ray spectral index.
  Our favoured model is then a truncated disc with an inner, hot, radiatively
inefficient flow which always dominates the hard X-rays, coupled to a conical
synchrotron jet which produces the radio emission. However, even this has
issues at low $\dot{m}$ as the low optical depth and high temperature of the
flow means that the Compton spectrum is not well approximated by a power law.
This shows the need for a more sophisticated model for the electron
distribution in the hot flow.
