The impact of resistivity on the variability of black hole accretion flows

TL;DR

This paper investigates how resistivity influences the variability of black hole accretion flows using 3D resistive GRMHD simulations, revealing that resistivity impacts magnetic flux accumulation and flow dynamics differently depending on the accretion regime.

Contribution

It introduces the first detailed analysis of resistivity effects on black hole accretion variability through comprehensive 3D simulations, highlighting the role of resistivity in magnetic flux behavior.

Findings

High resistivity prevents MAD state formation.

Resistivity influences magnetic flux diffusion and accumulation.

Flow variability is reduced at high resistivity but increases with low resistivity in multi-loop setups.

Abstract

Context. The accretion of magnetized plasma onto black holes is a complex and dynamic process, where the magnetic field plays a crucial role. The amount of magnetic flux accumulated near the event horizon significantly impacts the accretion flow behavior. Resistivity, a measure of how easily magnetic fields can dissipate, is thought to be a key factor influencing this process. This work explores the influence of resistivity on accretion flow variability. We investigate simulations reaching the magnetically arrested disk (MAD) limit and those with an initial multi-loop magnetic field configuration. Methods. We employ 3D resistive general relativistic magnetohydrodynamic (GRMHD) simulations to model the accretion process under various regimes, where resistivity has a global uniform value. Results. Our findings reveal distinct flow behaviors depending on resistivity. High resistivity simulations never achieve the MAD state, indicating a disturbed magnetic flux accumulation process. Conversely, low resistivity simulations converge towards the ideal MHD limit. The key results are: i) For the standard MAD model, resistivity plays a minimal role in flow variability, suggesting that flux eruption events dominate the dynamics. ii) High resistivity simulations exhibit strong magnetic field diffusion into the disk, rearranging efficient magnetic flux accumulation from the accretion flow. iii) In multi-loop simulations, resistivity significantly reduces flow variability, which was not expected. However, magnetic flux accumulation becomes more variable due to frequent reconnection events at very low resistivity values. Conclusions. This study shows that resistivity affects how much the flow is distorted due to magnetic field dissipation. Our findings provide new insights into the interplay between magnetic field accumulation, resistivity, variability and the dynamics of black hole accretion.