Electron Heating in Hot Accretion Flows
Abstract
Local (shearing box) simulations of the nonlinear evolution of the magnetorotational instability in a collisionless plasma show that angular momentum transport by pressure anisotropy (p⊥≠p∥, where the directions are defined with respect to the local magnetic field) is comparable to that due to the Maxwell and Reynolds stresses. Pressure anisotropy, which is effectively a large-scale viscosity, arises because of adiabatic invariants related to p⊥ and p∥ in a fluctuating magnetic field. In a collisionless plasma, the magnitude of the pressure anisotropy, and thus the viscosity, is determined by kinetic instabilities at the cyclotron frequency. Our simulations show that ~50% of the gravitational potential energy is directly converted into heat at large scales by the viscous stress (the remaining energy is lost to grid-scale numerical dissipation of kinetic and magnetic energy). We show that electrons receive a significant fraction [~(Te/Ti)1/2] of this dissipated energy. Employing this heating by an anisotropic viscous stress in one-dimensional models of radiatively inefficient accretion flows, we find that the radiative efficiency of the flow is greater than 0.5% for M˙>~10-4M˙Edd. Thus, a low accretion rate, rather than just a low radiative efficiency, is necessary to explain the low luminosity of many accreting black holes. For Sgr A* in the Galactic center, our predicted radiative efficiencies imply an accretion rate of ~3×10-8 Msolar yr-1 and an electron temperature of ~3×1010 K at ~10 Schwarzschild radii; the latter is consistent with the brightness temperature inferred from VLBI observations.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2007
- DOI:
- 10.1086/520800
- arXiv:
- arXiv:astro-ph/0703572
- Bibcode:
- 2007ApJ...667..714S
- Keywords:
-
- Accretion;
- Accretion Disks;
- Galaxy: Center;
- Magnetohydrodynamics: MHD;
- Plasmas;
- Astrophysics;
- Physics - Plasma Physics;
- Physics - Space Physics
- E-Print:
- to appear in Oct 1 2007 issue of ApJ