Characterizing the Scale Height and Filamentary Structure of Radiatively Cooled MADs

TL;DR

This study uses GR-MHD simulations to explore how radiative cooling affects the structure and dynamics of magnetically arrested disks around black holes, revealing a transition in disk morphology at a critical accretion rate.

Contribution

It introduces a new method to characterize disk scale height based on density maxima, accounting for magnetic and cooling effects in MADs.

Findings

Radiative cooling significantly thins and densifies accretion filaments.

A critical accretion rate marks the transition where cooling dominates heating.

The new density-based scale height measure better captures filamentary structures.

Abstract

Radiative cooling can strongly influence the structure and dynamics of black hole accretion disks. Here, we perform general relativistic magnetohydrodynamic (GR-MHD) simulations of magnetically arrested disks (MADs) around a non-spinning black hole. Radiative cooling is consistently included in the simulations and its intensity is scaled by the mass accretion rate ranging from $10^{-7}$ to $10^{-4} \dot{M}_{\mathrm{Edd}}$. Considering synchrotron and bremsstrahlung emission, we quantify how radiative losses modify the disk structure and the accretion dynamics. In the inner MAD disk regions, accumulation of magnetic field regulates gas accretion, enforcing the gas into a discrete interchange-driven filamentary structure. We identify, both analytically and numerically, a transition mass accretion rate above which radiative cooling becomes faster than the heating, which is assumed to occur via local coupling to the magnetic field. Above this mass accretion rate, cooling substantially reduces the gas thermal pressure, leading to considerably thinner and denser accretion filaments, and a substantial increase in radiative efficiency, relative to lower accretion rates. We show that under these conditions, conventional measures of the disk scale height become misleading in MAD flows. We therefore introduce an alternative definition based on the polar position of the density maximum, which more robustly characterizes the filamentary structure of the disks in the presence of strong magnetic fields and cooling.