On particle energization in accretion flows

Two-temperature advection-dominated accretion flow (ADAF) or hot ion tori (HIT) models help to explain low-luminosity stellar and galactic accreting sources and may complement observational support for black holes in nature. However, low radiative efficiencies demand that ions receive a fraction η ≥ 99 per cent of energy dissipated in the turbulent accretion. The η value depends on the ratio of particle to magnetic pressure. If modes of dissipation involving compressions at least perpendicular to the magnetic field (like magnetic mirroring) dominate, then even when the pressure ratio is O(1), the required large η can be attained. However, the relative importance of compressible versus incompressible modes is hard to estimate. The plasma is more compressible on larger scales and the relevant length-scale for particle energization can be estimated by equating the longest eddy turnover time (which corresponds to the energy-dominating scale) to the time for which an energy equal to that in the turbulence can be drained into particles. Based on the large scales resulting from this estimate, it is suggested that magnetic mirroring may be important. Also, regardless of the precise η or dissipation mechanism, non-thermal protons seem natural in two-temperature discs because all dissipation mechanisms, and the use of an isotropic pressure, require wave-particle resonances that operate only on a subset of the particles. Finally, it is briefly mentioned how mirroring may help to generate an ADAF or HIT in the first place.