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Outbursts of the black hole (BH) X-ray binaries are dramatic events occurringin our Galaxy approximately once a year. They are detected by the X-raytelescopes and often monitored at longer wavelengths. We analyse the X-ray andoptical/infrared (OIR) light curves of the BH binary XTE J1550-564 during the2000 outburst. By using the observed extreme colours as well as thecharacteristic decay time-scales of the OIR and X-ray light curves, we putstrong constraints on the extinction towards the source. We accurately separatethe contributions to the OIR flux of the irradiated accretion disc and anon-thermal component. We show that the OIR non-thermal component appearsduring the X-ray state transitions both during the rising and the decaying partof the outburst at nearly the same X-ray hardness but at luminosities differingby a factor of 3. The line marking the quenching/recovery of the non-thermalcomponent at the X-ray hardness-flux diagram seems to coincide with the "jetline" that marks the presence of the compact radio jet. The inferred spectralshape and the evolution of the non-thermal component during the outburst,however, are not consistent with the jet origin, but are naturally explained interms of the hybrid hot flow scenario, where non-thermal electrons emitsynchrotron radiation in the OIR band. This implies a close, possibly causalconnection between the presence of the hot flow and the compact jet. We findthat the non-thermal component is hardening during the hard state at thedecaying stage of the outburst, which indicates that the accelerationefficiency is a steep function of radius at low accretion rate.

Colours of black holes: infrared flares from the hot accretion disc in XTE J1550-564