Fast multi-wavelength variability from a black hole

Correlated fast multi-wavelength variability is quickly becoming a key tool for studying the physics of accretion and ejection of relativistic jets. Using simultaneous X-ray and near-infrared data we have recently discovered vibrations from a relativistic jet down to 62.5 ms time scale, which allowed us to estimate the speed and the size of the jet. We have also shown that the properties of the jet depend on the spectral state of the black hole. We propose to monitor the outburst evolution of a black hole transient with XMM, simultaneously with ground-based optical and infrared facilities, in order to apply the same technique to different spectral states. We propose to perform 10 short XMM observations (7 ks each) of an active black hole, in order to cover the different stages of the outburst.