  Outbursts of the black hole (BH) X-ray binaries are dramatic events occurring
in our Galaxy approximately once a year. They are detected by the X-ray
telescopes and often monitored at longer wavelengths. We analyse the X-ray and
optical/infrared (OIR) light curves of the BH binary XTE J1550-564 during the
2000 outburst. By using the observed extreme colours as well as the
characteristic decay time-scales of the OIR and X-ray light curves, we put
strong constraints on the extinction towards the source. We accurately separate
the contributions to the OIR flux of the irradiated accretion disc and a
non-thermal component. We show that the OIR non-thermal component appears
during the X-ray state transitions both during the rising and the decaying part
of the outburst at nearly the same X-ray hardness but at luminosities differing
by a factor of 3. The line marking the quenching/recovery of the non-thermal
component at the X-ray hardness-flux diagram seems to coincide with the "jet
line" that marks the presence of the compact radio jet. The inferred spectral
shape and the evolution of the non-thermal component during the outburst,
however, are not consistent with the jet origin, but are naturally explained in
terms of the hybrid hot flow scenario, where non-thermal electrons emit
synchrotron radiation in the OIR band. This implies a close, possibly causal
connection between the presence of the hot flow and the compact jet. We find
that the non-thermal component is hardening during the hard state at the
decaying stage of the outburst, which indicates that the acceleration
efficiency is a steep function of radius at low accretion rate.
