[Paper Review] Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

Nicolas Scepi, Mitchell C. Begelman+1 more|arXiv (Cornell University)|Feb 20, 2023

Astrophysical Phenomena and Observations4 citations

TL;DR

This study investigates magnetically arrested discs (MADs) with varying thermal thicknesses using a cooling function to regulate temperature, revealing that thin MADs are nearly Keplerian radially but vertically supported by turbulent magnetic pressure. Wind-driven angular momentum transport accelerates accretion, enhancing dissipation in low-density winds and explaining fast outflows and coronal heating in XRBs and AGN.

ABSTRACT

Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, $h_\mathrm{th}/r=0.3, 0.1$ and $0.03$. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with $h_\mathrm{th}/r=0.03$. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of $z/r = \pm 0.03$ and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGN). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass loss rates in good agreement with the properties of ultra-fast outflows observed in AGN.

Motivation & Objective

• To understand how magnetic pressure deviations from standard accretion disc theory affect disc structure and dynamics in magnetically arrested discs (MADs).
• To investigate the role of wind-driven angular momentum transport in accelerating accretion and reducing disc density in thin MADs.
• To explore how enhanced energy dissipation in low-density regions, particularly in winds, could power coronal emission in X-ray binaries and active galactic nuclei.
• To determine whether MAD winds can reproduce the terminal velocities and mass loss rates observed in ultra-fast outflows (UFOs) in AGN.
• To examine the interplay between magnetic flux saturation, wind pressure, and jet pressure in regulating magnetic field strength at the black hole.

Proposed method

• Simulates magnetically arrested discs (MADs) with three thermal thicknesses: $h_{\mathrm{th}}/r = 0.3$, $0.1$, and $0.03$ using a numerical MHD code.
• Applies an artificial cooling function to regulate gas temperature and maintain specified thermal scale heights.
• Analyzes radial and vertical structure, angular momentum transport, and stress components ($r\phi$ and $\theta\phi$) to isolate dominant accretion mechanisms.
• Quantifies energy dissipation rates across spatial regions, particularly outside the thermal core ($|z/r| > 0.03$) and within outflows.
• Measures terminal velocities and mass loss rates of winds to compare with observations of ultra-fast outflows in AGN.
• Evaluates magnetic flux saturation by comparing wind+accretion disc pressure to jet pressure as regulating mechanisms.

Experimental results

Research questions

• RQ1 How does magnetic pressure influence radial and vertical disc structure in thin magnetically arrested discs compared to standard theory?
• RQ2 What is the relative contribution of MRI-driven stress ($r\phi$) versus wind-driven stress ($\theta\phi$) to angular momentum transport in MADs?
• RQ3 Where and how is energy dissipated in thin MADs, and can this explain coronal heating in XRBs and AGN?
• RQ4 Can the properties of winds in thin MADs reproduce the terminal velocities and mass loss rates of observed ultra-fast outflows in AGN?
• RQ5 What physical mechanisms regulate magnetic flux saturation on the black hole in MADs, and how do wind and jet pressures compete?

Key findings

• Thin MADs with $h_{\mathrm{th}}/r = 0.03$ are nearly Keplerian radially but vertically extended due to turbulent magnetic pressure support, deviating significantly from standard disc theory.
• Wind-driven $\theta\phi$ stress dominates angular momentum transport, leading to faster accretion and lower disc densities than predicted by standard theory.
• Up to 30% of local energy dissipation occurs in the outflowing wind, particularly in low-density regions outside $|z/r| = 0.03$, promoting coronal heating.
• The disc in the thinnest model is more radiatively efficient than Novikov & Thorne (1973) predicts due to additional stress inside the ISCO, but less efficient than Agol & Krolik (2000) due to wind-driven accretion.
• MAD winds achieve terminal velocities of $0.2c$ to $0.5c$ and mass loss rates of $0.1$ to $1\,\dot{M}$ per log radius, matching observed ultra-fast outflows in AGN.
• Magnetic flux saturation on the black hole results from a competition between wind+accretion disc pressure and jet pressure, with stronger saturation in hotter, windier discs.