Radiative cooling changes the dynamics of magnetically arrested disks

TL;DR

This paper investigates how radiative cooling influences the behavior of magnetically arrested disks around rotating black holes, identifying a critical accretion rate where cooling significantly impacts disk dynamics.

Contribution

The study introduces a critical mass accretion rate for radiative cooling effects in MADs and verifies it through GRMHD simulations across various black hole spins.

Findings

Critical accretion rate $ ightarrow 10^{-5.5} ext{ M}_ ext{Edd}$

MAD parameter and jet efficiency vary with accretion rate

Magnetic forces increase as thermal forces decrease

Abstract

We studied magnetically arrested disks (MAD) around rotating black holes (BH), under the influence of radiative cooling. We introduce a critical value of the mass accretion rate $\dot M_{\rm crit}$ for which the cooling by the synchrotron process efficiently radiates the thermal energy of the disk. We find $\dot M_{\rm crit} \approx 10^{-5.5} \dot M_{\rm Edd}$, where $\dot M_{\rm Edd}$ is the Eddington mass accretion rate. The normalization constant depends on the saturated magnetic flux and on the ratio of electron to proton temperatures, but not on the BH mass. We verify our analytical estimate using a suite of general relativistic magnetohydrodynamic (GRMHD) simulations for a range of black hole spin parameters $a \in \{ -0.94, -0.5, 0, 0.5, 0.94 \}$ and mass accretion rates ranging from $10^{-7}\dot M_{\rm Edd}$ to $10^{-4}\dot M_{\rm Edd}$. We numerically observe that the MAD parameter and the jet efficiency vary by a factor of $\approx 2$ as the mass accretion rate increases above $\dot M_{\rm crit}$, which confirms our analytical result. We further detail how the forces satisfying the quasi-equilibrium of the disk change, with the magnetic contribution increasing as the thermal contribution decreases.