Collisionless Accretion of Finite-Angular-Momentum Plasma onto a Spinning Black Hole

Abstract

In low-luminosity active galactic nuclei like M87* and Sgr A*, the accretion disk around the central supermassive black hole is tenuous and collisionless. As a result, the usual ideal magnetohydrodynamics (MHD) approximation may not be applicable. In this Letter, we report on the first fully kinetic simulations of the accretion process where the plasma initially has finite angular momentum. The simulated accretion flow behaves remarkably similarly to the magnetically arrested disk (MAD) regime of ideal MHD, reproducing episodes of magnetic flux saturation and eruption typical of MADs. The resemblance to fluid models owes largely to kinetic instabilities, which regulate pressure anisotropy in the disk, allowing fluid terms to dominate the angular momentum transfer. In addition, by handling vacuum regions effectively, our kinetic approach probes the matter supply to the jet funnel. We observe no efficient penetration of the accreting material into this region, which suggests that a pair discharge may be required to sustain the Blandford-Znajek process.

Figures (4)

• Figure 1: Snapshots at $t=1476 r_g/c$ of the simulations with and without pair production. Panels display the auxiliary magnetic field component $H_\varphi$, and the FIDO-measured total plasma density $n_{\rm tot}$. As shown in the End Matter, in a steady state, $H_\varphi(r,\theta)$ indicates the electric current through a spherical cap of radius $r$ and half-opening angle $\theta$ measured from the $\theta=0$ axis.
• Figure 2: Horizon-penetrating magnetic flux, $\Phi_{\rm H}$, mass accretion rate, $\dot{M}_{\rm H}$, and normalized flux, $\phi = \Phi_{\rm H} (4\pi / (\dot{M}_{\rm H} r_g^2 c))^{1/2}$. To limit noise when calculating $\phi$, we exclude times when $\dot{M}_{\rm H}$ is less than the threshold $\dot{M}_{\rm thr} = 0.02 (4 \pi r_g^2 n_0 c)$.
• Figure 3: Panel a): Spacetime diagrams from both simulations (with and without pair production) of the decomposed angular momentum fluxes, $\dot{L}_s$. Panel b): Spatial maps of $\beta_{\rm pl}$ and the perpendicular-to-parallel temperature ratio, $T_\perp/T_\parallel = p_\perp / p_\parallel$, both pictured at $t = 1476r_g/c$ in the simulation with pair production. Panel c): Joint probability density of $\beta_{\parallel}=2p_\parallel/b^2$ and $T_\perp / T_\parallel$ generated from the same simulation snapshot as panel b). Dashed lines indicate where the mirror (top) and firehose (bottom) instability growth rates exceed $10$ per cent of the ion cyclotron frequency. Growth rates are calculated semi-analytically via a method detailed in the supplemental material.
• Figure 4: Top: Maps of the local FIDO-measured average Lorentz factor, $\langle \Gamma \rangle$, and spatial decomposition of the domain into disk, current sheet, and jet regions. Bottom: Contributions to the total particle energy distribution from the different spatial regions. FIDO-measured Lorentz factors $\Gamma$ and four-velocities $u$ are related through $u=\sqrt{\Gamma^2-1}$.