The three-dimensional structure of black hole accretion flows within the plunging region

TL;DR

This study combines analytical models and GRMHD simulations to explore the 3D structure of black hole accretion flows inside the plunging region, revealing spiral-mediated accretion and two-dimensional characteristics with potential observational impacts.

Contribution

It provides the first detailed analysis of thick disc accretion flows within the plunging region using combined analytical and numerical approaches.

Findings

Analytical expressions from thin disc theory match numerical results well.

Accretion is mediated by spiral arms within the plunging region.

The flow exhibits fundamentally two-dimensional properties.

Abstract

We analyse, using new analytical models and numerical general relativistic magnetohydrodynamic simulations, the three-dimensional properties of accretion flows inside the plunging region of black hole spacetimes (i.e., at radii smaller than the innermost stable circular orbit). These simulations are of thick discs, with aspect ratios of order unity $h/r \sim 1$, and with a magnetic field geometry given by the standard low-magnetization "SANE" configuration. This work represents the first step in a wider analysis of this highly relativistic region. We show that analytical expressions derived in the "thin disc" limit describe the numerical results remarkably well, despite the large aspect ratio of the flow. We further demonstrate that accretion within this region is typically mediated via spiral arms, and that the geometric properties of these spiral structures can be understood with a simple analytical model. These results highlight how accretion within the plunging region is fundamentally two dimensional in character, which may have a number of observational implications. We derive a modified theoretical description of the pressure within the plunging region which accounts for turbulent heating and may be of use to black hole image modelling.