Non-Equilibrium Thermodynamics of Black-Hole Coronae: QPOs, Turbulence, and Jets

The variability of X-rays observed from accreting black hole systems, including quasi-periodic oscillations (QPOs), suggests a complex nonlinear dynamics in the corona. Here, we propose a new theoretical framework for this variability, based on non-equilibrium thermodynamics. In this model, coronal variability arises from feedback between a macroscopic oscillation of the plasma and the rate at which it is cooled by the inverse Compton scattering of soft photons from the disc. The "pair thermostat'' mechanism then allows the corona to act as a heat engine that extracts work cyclically from the underlying thermal disequilibrium between the low-entropy heating from the black hole and the high-entropy cooling by soft photons from the disk, in close analogy to the well-known $\kappa$-mechanism for pulsating stars. This coronal self-oscillation may explain QPOs without invoking an external periodic driving. Moreover, we argue that this mechanism can generate coronal turbulence and jets.