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 s ources and may complement observational support for black holes in nature. However, low radiative efficiencies demand that ions receive a fraction eta greater than or similar to 99 per cent of energy dissipated in the turbule nt accretion. The YI value depends on the ratio of particle to magnetic pre ssure. If modes of dissipation involving compressions at least perpendicula r to the magnetic field (like magnetic mirroring) dominate, then even when the pressure ratio is O(1), the required large eta can be attained. However , the relative importance of compressible versus incompressible modes is ha rd to estimate. The plasma is more compressible on larger scales and the re levant length-scare 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 es timate, it is suggested that magnetic mirroring may be important. Also, reg ardless of the precise eta or dissipation mechanism, non-thermal protons se em natural in two-temperature discs because all dissipation mechanisms, and the use of an isotropic pressure, require wave-particle resonances that op erate only on a subset of the particles. Finally, it is briefly mentioned h ow mirroring may help to generate an ADAF or HIT in the first place.