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

¹ Research Center for Astronomy and Applied Mathematics, Academy of Athens, Athens 11527, Greece
² Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany
³ Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 201210, China
⁴ School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
⁵ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Strasse 31, 97074 Würzburg, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁷ Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264, 04510 Ciudad de México, Mexico
⁸ Instituto de Astrofísica de Andalucía, Gta. de la Astronomía, s/n, Genil, 18008 Granada, Spain
⁹ School of Mathematics, Trinity College, Dublin 2, Ireland
¹⁰ Frankfurt Institute for Advanced Studies, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main, Germany

^⋆ Corresponding author; anathanail@academyofathens.gr

Received: 8 August 2024
Accepted: 19 November 2024

Abstract

Context. The accretion of magnetized plasma onto black holes is a complex and dynamic process in which the magnetic field plays a crucial role. The amount of magnetic flux that is accumulated near the event horizon significantly impacts the accretion flow behavior. Resistivity, which is a measure of how easily magnetic fields can dissipate, is thought to be a key factor influencing this process.

Aims. This work explores the influence of resistivity on the accretion flow variability. We investigated simulations that reached the limit of the magnetically arrested disk (MAD) and simulations with an initial multi-loop magnetic field configuration.

Methods. We employed 3D resistive general relativistic magnetohydrodynamic (MHD) simulations to model the accretion process under various regimes, where resistivity is globally constant (uniform resistivity).

Results. Our findings reveal distinct flow behaviors depending on resistivity. High-resistivity simulations never achieved the MAD state, which indicates a disturbed magnetic-flux accumulation process. Conversely, low-resistivity simulations converged toward the ideal MHD limit. The key results are that i) for the standard MAD model, resistivity plays a minimum role in flow variability, suggesting that flux eruption events dominate the dynamics. ii) High-resistivity simulations exhibit strong magnetic field diffusion into the disk that rearranges the efficient magnetic flux accumulation from the accretion flow. iii) In multi-loop simulations, resistivity significantly reduces the flow variability, which was not expected. However, magnetic flux accumulation becomes more variable as a result of frequent reconnection events at very low resistivity values.

Conclusions. This study shows that resistivity affects how much the flow is distorted as a result of the magnetic field dissipation. Our findings provide new insights into the interplay between magnetic field accumulation, resistivity, variability, and the dynamics of black hole accretion.