Electron Heating in Hot Accretion Flows

By P. Sharma, E. Quataert, Gregory W. Hammett, James M. Stone. 2007

178 citations recorded by the source corpus.

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 ≳ 10-4Edd. 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 × 1010 K at ≈10 Schwarzschild radii; the latter is consistent with the brightness temperature inferred from VLBI observations.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/dd0f352c2bc371b1d7171de10cc98f71f4833c69

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Citations in the corpus

A prescription for the turbulent heating of astrophysical plasmas (2010, methodology): “Following Sharma et al. (2007), we take the plasma βi ∼ 10 and Ti/Te ∼ 10, so for a value of k0ρi ∼ 10 −7, we see from Fig.”
RADIATIVE MODELS OF SGR A* FROM GRMHD SIMULATIONS (2009, background): “…are assumed to have a thermal distribution function, but with a temperature ratio Ti/Te that may be different from one (see Section 2.2.1 in Sharma et al. 2007, for a discussion of temperature ratios in a collisionless accretion flow model; their work suggests that Ti/Te ∼ 10 may be a…”
The Fermi blazar sequence (2017, background): “Narayan et al. 1997, Sharma et al. 2007), but the existence of a transition between a radiatively efficient and a radiatively inefficient regime is rather certain.”
ASTROPHYSICAL GYROKINETICS: KINETIC AND FLUID TURBULENT CASCADES IN MAGNETIZED WEAKLY COLLISIONAL PLASMAS (2007, methodology): “…is, therefore, crucial for understanding accretion flows—and the answer to this question depends on the detailed properties of the small-scale kinetic turbulence (e.g., Quataert & Gruzinov 1999; Sharma et al. 2007), as well as on the linear properties of the collisionless MRI (Quataert etal.”
Electron Thermodynamics in GRMHD Simulations of Low-Luminosity Black Hole Accretion (2015, background): “Sharma et al.(2007) found that the typical pressure anisotropy satisfied∆P/P > 0 in simulations that explicitly evolved a pressure tensor.”
THE SUBMILLIMETER BUMP IN Sgr A* FROM RELATIVISTIC MHD SIMULATIONS (2010, result): “This electron temperature is consistent with electron temperatures found in previous work (Yuan et al. 2003; Goldston et al. 2005; Sharma et al. 2007; Mościbrodzka et al. 2009).”
Hot Accretion Flows Around Black Holes (2014, background): “…(Bisnovatyi-Kogan & Lovelace 1997; Quataert & Gruzinov 1999; Ding et al. 2010; Hoshino 2012, 2013), or MHD turbulence (Quataert 1998, Quataert & Gruzinov 1999, Blackman 1999, Medvedev 2000, Lehe et al. 2009), or dissipation of pressure anisotropy in a collisionless plasma (Sharma et al. 2007a).”
EVIDENCE FOR LOW BLACK HOLE SPIN AND PHYSICALLY MOTIVATED ACCRETION MODELS FROM MILLIMETER-VLBI OBSERVATIONS OF SAGITTARIUS A* (2010, background): “The applicability of the MHD prescription to Sgr A* is still unclear, where the accretion rate is sufficiently low that non-MHD effects may become important (Sharma et al. 2006, 2007).”
Radiative efficiency of hot accretion flows (2012, background): “However, a detailed analysis of the microphysics in the accretion flow has indicated that the electrons can receive a comparable fraction of viscous heating to that of the ions (Bisnovatyi-Kogan & Lovelace 1997; Blackman 1999; Quataert & Gruzinov 1999; Sharma et al. 2007).”