Magnetized Accretion Flows: Effects of Gas Pressure

TL;DR

This study investigates how the adiabatic index gamma influences the structure and outflows of magnetized accretion flows, revealing that torus thickness affects outflow strength and has implications for different accretion states in astrophysical systems.

Contribution

The paper extends previous hydrodynamical models to include magnetic fields, demonstrating how gamma-dependent torus geometries influence outflow behavior in MHD accretion flows.

Findings

Thick tori form for gamma >~ 4/3, producing strong bipolar outflows.

Thin tori at low gamma generate weak, unsteady outflows.

Magnetic fields suppress oscillations seen in hydrodynamical models.

Abstract

We study how axisymmetric magnetohydrodynamical (MHD) accretion flows depend on gamma adiabatic index in the polytropic equation of state. This work is an extension of Moscibrodzka & Proga (2008), where we investigated the gamma dependence of 2-D Bondi-like accretion flows in the hydrodynamical (HD) limit. Our main goal is to study if simulations for various gamma can give us insights into to the problem of various modes of accretion observed in several types of accretion systems such as black hole binaries (BHB), active galactic nuclei (AGN), and gamma-ray bursts (GRBs). We find that for gamma >~ 4/3, the fast rotating flow forms a thick torus that is supported by rotation and gas pressure. As shown before for gamma=5/3, such a torus produces a strong, persistent bipolar outflow that can significantly reduce the polar funnel accretion of a slowly rotating flow. For low gamma, close to 1, the torus is thin and is supported by rotation. The thin torus produces an unsteady outflow which is too weak to propagate throughout the polar funnel inflow. Compared to their HD counterparts, the MHD simulations show that the magnetized torus can produce an outflow and does not exhibit regular oscillations. Generally, our simulations demonstrate how the torus thickness affects the outflow production. They also support the notion that the geometrical thickness of the torus correlates with the power of the torus outflow. Our results, applied to observations, suggest that the torus ability to radiatively cool and become thin can correspond to a suppression of a jet as observed in the BHB during a transition from a hard/low to soft/high spectral state and a transition from a quiescent to hard/low state in AGN.