Stable Collisionless Tori Around Kerr Black Holes

TL;DR

This paper introduces a new class of analytic collisionless tori around Kerr black holes, enabling stable initial conditions for kinetic simulations of accretion flows in low-luminosity active galactic nuclei.

Contribution

It presents the first analytic kinetic equilibria of collisionless tori around Kerr black holes and demonstrates their stability in simulations, facilitating future collisionless accretion studies.

Findings

Successfully implemented collisionless tori in GRPIC simulations.

Found the tori to be stable over hundreds to thousands of dynamical times.

Provided a new initial condition for kinetic modeling of accretion flows.

Abstract

In low-luminosity active galactic nuclei like M87$^\ast$ and Sgr A$^\ast$, the accretion flow in the vicinity of the black hole is in the collisionless regime, meaning that the collisional mean free path of charged particles is much larger than the dynamical length scales. To properly model the particle energization and emission from the collisionless accretion flow, a promising approach is to employ the global general-relativistic particle-in-cell simulations$\unicode{x2014}$a newly developed, fully kinetic, first-principles method. However, it has been challenging to set up an initial condition that involves collisionless gas with finite angular momentum. We present, for the first time, a class of analytic kinetic equilibria of collisionless tori around a Kerr black hole. We have successfully implemented the collisionless tori in our GPU-based GRPIC code framework Aperture, and found them to be stable for hundreds to thousands of dynamical times in 2D axisymmetric simulations when there is no initial seed magnetic field. These kinetic equilibria serve as ideal starting points for future studies of the physics of collisionless accretion and jet launching.