Fast multiwavelength variability from jets in X-ray Binaries

While jets appear as a fundamental result of the accretion process onto compact objects in X-ray binaries, there is as yet no standard model for their underlying physics. The origin of the observed accretion disk-jet coupling also remain largely unknown. X-ray variability studies have revealed complex variability in the accretion flow onto stellar-mass black holes and neutron stars, on timescales as short as milliseconds. The detection of correlated broad-band rapid time variability in the jet emission would provide valuable information on how the variability is transferred along the jet and on the timescales of physical processes operating in these jets, ultimately helping to constrain internal jet physics, probe disk-jet coupling and infer accretion geometry. In recent years there have been indirect evidences for optical fast variability arising from a powerful jet. However, in optical and ultraviolet light the emission from the outer accretion disk can strongly contaminate the jet signal, which results in an ambiguous identification of the source of the observed variability. On the other hand, at much longer wavelengths the variability will be smeared out in time due as it comes from far out in the jet. Infrared variability studies are thus ideal for looking at jet variability on the fastest possible timescales. Thanks to newly available detectors and fast-readout modes, fast infrared and mid-infrared photometry is now possible. This is opening a new exciting window to study the geometry and the Physics of relativistic jets and their connection with the accretion flow. Here we present the first results from a large ongoing fast-timing multi-wavelength project, showing the first unambiguous evidence for sub-second jet variable emission. We show how this type of data already allows us to put quantitative constraints to the jet speed, geometry and physics, and discuss the great potential of new observations in the near future.