Two-Temperature Models of AGN Coronae: Structure, Outflows, and Inefficient Heating

Short Compton cooling times in the coronae of radiatively efficient active galactic nuclei (AGN) accretion flows have traditionally been used as justification for isothermal, force-free models. However, these assumptions only apply to coronal electrons and positrons. The thermodynamics of the mass-carrying ions is determined not by Compton cooling, but by inefficient Coulomb coupling in the tenuous surface layers of AGN disks. We elucidate the consequences of this weak coupling: the inexorable formation of a two-temperature, ion-dominated corona. Using a suite of local stratified shearing box magnetohydrodynamics (MHD) simulations, we explore how the strength of Coulomb coupling and the initial magnetic field configuration (both strength and existence of net magnetic flux through the disk) affect the density, ion temperature, and magnetic field structure of two-temperature coronae. We focus only on the ion dynamics, treating ions as a single ideal MHD fluid subject to cooling by a simple, parameterized, optically-thin cooling function motivated by Coulomb collisions.

Efficient cooling in our thin accretion disks combined with inefficient cooling in their surface layers inevitably forms inverted temperature profiles, with a hot corona surrounding a cold, dense disk. Unlike in isothermal simulations, the strength of outflows is significantly enhanced in these two-temperature models, leading to rapid depletion of the disk. Despite the large initial magnetization of coronae with net magnetic flux, strong disk winds violate the force free assumption. Heating, evaluated by solving the entropy equation at every time-step of the simulation, is inefficient in these models, with <10% of the injected turbulent energy dissipated in the corona. We examine the source of this inefficiency by studying the energy transport and dissipation mechanisms in these local models. Finally, we conclude with a discussion of the coupling between coronae and disks via field-aligned anisotropic conduction, which has been invoked as a means of mediating state transitions in X-ray binary and AGN disks. Our work provides an in-depth study of the effect of thermodynamics on the formation of AGN coronae which can constrain and inform global models.