  X-ray irradiation heating of accretion discs in black hole X-ray binaries
(BHXBs) plays a key role in regulating their outburst cycles. However, despite
decades of theoretical and observational efforts, the physical mechanism(s)
responsible for irradiating these discs remains largely unknown. We have built
an observationally-based methodology to estimate the strength of irradiation of
BHXB discs by combining multiwavelength X-ray and optical/IR data throughout
transient outbursts. We apply this to $\sim15$ yrs of activity in the Galactic
BHXB GX339$-$4. Our findings suggest that the irradiation heating required by
the optical data is large in this system. Direct illumination of the outer disc
does not produce sufficient irradiation, but this should also produce a
thermal-radiative wind which adds to the irradiation heating by scattering flux
down onto the disc. However, analytic estimates of X-ray illumination from
scattering in the wind is still not sufficient to produce the observed heating,
even in combination with direct illumination. Either the analytic
thermal-radiative wind models are underestimating the effect of the wind, or
there are additional scattering mechanisms at work, such as magnetically-driven
outflows, acting to increase the optical/IR flux. While wind-driven irradiation
is likely a common feature among long-period BHXBs, fully understanding the
driving mechanism(s) behind such a wind will require radiation-hydrodynamic
simulations.
