Year: 2007
Type: article
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 [~( T e / T i ) 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 ≳ 10 -4 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 M ☉ yr -1 and an electron temperature of ≈3 × 10 10 K at ≈10 Schwarzschild radii; the latter is consistent with the brightness temperature inferred from VLBI observations.
Source: The Astrophysical Journal
DOI: 10.1086/520800
Authors: Prateek Sharma, Eliot Quataert, Gregory W. Hammett, James M. Stone
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2054286147