Unveiling the physical conditions around black holes is one of the major astrophysical challenges of the 21st century. Indeed we are still at the dawn of black hole astrophysics and very little about these objects is understood: what are the geometry, dynamics and energetics of the accretion flow onto the black hole? How can accretion of material onto the hole be simultaneously intimately related to violent relativistic ejecta and/or powerful self-confined persistent jets? What are the processes at the origin of the fascinating hysteresis cycles observed in some X ray binaries?

These unsolved and puzzling issues are at the heart of this CHAOS project, funded by the french Research National Agency for 4 years (2013-2016), and shared by three different french institutes (IPAG, IRAP and AIM). It is focused on Black-Hole X-ray Binaries as they are actually the best candidates for such studies, their complex multi-wavelength behavior occurring on timescales that can be handled very well (i.e. minutes to months).