Study of general relativistic magnetohydrodynamic accretion flow around black holes

TL;DR

This paper develops a comprehensive general relativistic magnetohydrodynamic model for thin accretion disks around black holes, analyzing magnetic field roles, pressure dominance, and angular momentum transport.

Contribution

It introduces a novel GRMHD framework for steady, axisymmetric accretion flows, including detailed magnetic and thermodynamic property analysis and scaling relations.

Findings

Toroidal magnetic field dominates over radial component at the disk equator.

Disks are primarily gas pressure dominated except near the horizon.

Maxwell stress facilitates angular momentum transport, with a radially varying viscosity parameter.

Abstract

We present a novel approach to study the global structure of steady, axisymmetric, advective, geometrically thin, magnetohydrodynamic (MHD) accretion flow around black holes in full general relativity (GR). Considering ideal MHD conditions and relativistic equation of state (REoS), we solve the governing equations to obtain all possible smooth global accretion solutions. We examine the dynamical and thermodynamical properties of accreting matter in terms of the flow parameters, namely energy (${\cal E}$), angular momentum (${\cal L}$), and local magnetic fields. For a thin GRMHD flow, we observe that toroidal component ($b^\phi$) of the magnetic fields generally dominates over radial component ($b^r$) at the disk equatorial plane. This evidently suggests that toroidal magnetic field indeed plays important role in regulating the disk dynamics. We further notice that the disk remains mostly gas pressure ($p_{\rm gas}$) dominated ($\beta = p_{\rm gas}/p_{\rm mag} > 1$, $p_{\rm mag}$ refers magnetic pressure) except at the near horizon region, where magnetic fields become dynamically important ($\beta \sim 1$). We observe that Maxwell stress is developed that eventually yields angular momentum transport inside the disk. Towards this, we calculate the viscosity parameter ($\alpha$) that appears to be radially varying. In addition, we examine the underlying scaling relation between $\alpha$ and $\beta$, which clearly distinguishes two domains coexisted along the radial extent of the disk. Finally, we discuss the utility of the present formalism in the realm of GRMHD simulation studies.