Influence of the Magnetic Coupling Process on the Advection Dominated Accretion Flows around Black Holes

TL;DR

This paper studies how magnetic coupling between a rotating black hole and its accretion flow influences the flow's dynamics and radiative efficiency, especially in hot, advection-dominated accretion flows, with effects depending on the viscosity parameter.

Contribution

It introduces a detailed analysis of magnetic coupling effects on ADAF dynamics using an equivalent circuit approach, highlighting the impact of black hole spin and viscosity parameter.

Findings

Radial velocity and electron temperature decrease due to magnetic coupling.

Ion temperature and surface density increase with magnetic coupling.

Radiative efficiency can be enhanced by about 30% for certain parameters.

Abstract

A large-scale closed magnetic field can transfer angular momentum and energy between a black hole (BH) and its surrounding accretion flow. We investigate the effects of this magnetic coupling (MC) process on the dynamics of a hot accretion flow (e.g., an advection dominated accretion flow, hereafter ADAF). The energy and angular momentum fluxes transported by the magnetic field are derived by an equivalent circuit approach. For a rapidly rotating BH, it is found that the radial velocity and the electron temperature of the accretion flow decrease, whereas the ion temperature and the surface density increase. The significance of the MC effects depends on the value of the viscous parameter \alpha. The effects are obvious for \alpha=0.3 but nearly ignorable for \alpha=0.1. For a BH with specific angular momentum, a_*=0.9, and \alpha=0.3, we find that for reasonable parameters the radiative efficiency of a hot accretion flow can be increased by about 30%.