Microphysical dissipation, turbulence and magnetic fields in hyper-accreting discs

Abstract

Hyper-accreting discs occur in compact-object mergers and collapsars, and may power gamma-ray bursts (GRBs). We calculate the microscopic viscosity and resistivity of plasma in these discs, and discuss the implications for their global structure and evolution. In the neutrino-cooled innermost regions, the viscosity is provided mainly by mildly degenerate electrons, while the resistivity is modified from the Spitzer value due to the effects of both relativity and degeneracy. The plasma behaves as an almost ideal MHD fluid. Among the non-ideal MHD effects the Hall term is relatively the most important, while the magnetic Prandtl number, Pr (the ratio of viscosity to resistivity), is typically larger than unity: 10 < Pr < 6000. The outer radiatively inefficient regions also display high Pr. Numerical simulations of the magneto-rotational instability indicate that the saturation level and angular momentum transport efficiency may be greatly enhanced at high Pr. If this behaviour persists in the presence of a strong Hall effect we would expect that hyper-accreting discs should be strongly magnetised and highly variable. The expulsion of magnetic field that cannot be dissipated at small scales may also favour a magnetic outflow. We note the similaries between the Prandtl number in hyper-accreting discs and X-ray binary discs, which suggests that a comparison between late-time activity in GRBs and X-ray binary accretion states may be fruitful. Our results imply that the behavior of high Prandtl number MHD flows needs to be considered in studies of hyper-accreting discs.

14 pages, 10 figures. MNRAS, accepted. Minor revisions. Comments added in discussion section

References in corpus (23)

• Hyper-Accreting Black Holes and Gamma-Ray Bursts
• The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
• General Relativistic Magnetohydrodynamic Simulations of Jet Formation and Large-Scale Propagation from Black Hole Accretion Systems
• Bright X-ray Flares in Gamma-Ray Burst Afterglows
• Simulations of small-scale turbulent dynamo
• Accretion Models of Gamma-Ray Bursts
• Neutrino trapping and accretion models for Gamma-Ray Bursts
• Neutrino-Cooled Accretion Disks around Spinning Black Hole
• Quenching of the radio jet during the X-ray high state of GX 339-4
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
• Impact of dimensionless numbers on the efficiency of MRI-induced turbulent transport
• Can neutrino-cooled accretion disk be an origin of gamma-ray bursts?
• Nuclear Composition of Gamma-Ray Burst Fireballs
• The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: I. Local Axisymmetric Simulations
• Variability in black hole accretion discs
• Axisymmetric MHD simulations of the collapsar model for gamma-ray bursts
• Are Particles in Advection-Dominated Accretion Flows Thermal?
• On the Magnetic Prandtl Number Behavior of Accretion Disks
• Hot Accretion With Conduction: Spontaneous Thermal Outflows
• The Dusty Starburst Nucleus of M33
• A weakly nonlinear analysis of the magnetorotational instability in a model channel flow
• Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar Disks : A Possible Origin of Short-Term Variability in the Prompt Emission of Gamma-Ray Bursts
• On the nonlinear saturation of the magnetorotational instability near threshold in a thin-gap Taylor-Couette setup

Cited by in corpus (13)

• A Reconnection Switch to Trigger Gamma-Ray Burst Jet Dissipation
• A prescription for the turbulent heating of astrophysical plasmas
• Neutrino-dominated accretion flows as the central engine of gamma-ray bursts
• Development of General Relativistic Magnetohydrodynamic Code and its Application to Central Engine of Long Gamma-Ray Bursts
• Chiral magnetohydrodynamic turbulence in core-collapse supernovae
• MRI-driven dynamo at very high magnetic Prandtl numbers
• MRI turbulence in accretion discs at large magnetic Prandtl numbers
• Characterizing the time variability in magnetized neutrino--cooled accretion disks: signatures of the gamma-ray burst central engine
• MRI turbulence in vertically stratified accretion discs at large magnetic Prandtl numbers
• MHD waves and instabilities in flowing solar structures in the framework of Hall magnetohydrodynamics
• Neutrino-Dominated Accretion Flows with Magnetic Prandtl Number-Dependent MRI-driven Turbulence
• Side-entrainment in a jet embedded in a sidewind
• MRI-driven $\alpha \Omega$ dynamo at high Pm numbers