Emergence of cyclic flux eruptions in kinetic simulations of magnetized spherical accretion onto a Schwarzschild black hole

Abstract

The dynamics of black hole magnetospheres critically depend on the black hole spin and on the structure of the accretion flow. In the limit of a Schwarzschild black hole immersed in a zero-net angular momentum flow, accretion is spherical. However, in the presence of a large-scale vertical magnetic field, the classical Bondi accretion model is significantly altered. The frozen-in field is stretched radially as the plasma is pulled inward by gravity. This continues until the restoring force from the magnetic tension suddenly expels the material and resets the field, allowing a new cycle to begin. Although this scenario has been well depicted in previous studies, it remains incomplete as the issues of dissipation and particle acceleration are not yet fully resolved. In this work, we aim to revisit these issues with a first-principles kinetic plasma model. We perform two-dimensional global general relativistic particle-in-cell simulations of magnetized spherical accretion onto a Schwarzschild black hole, for both pair and electron-ion plasmas. The simulations are evolved over long timescales to capture multiple flux eruption events and establish a quasi-steady state. For each accretion cycle, we find that the system goes through three main stages: (i) an ideal advection phase where magnetic flux through the horizon increases quasi-linearly with time; (ii) a reconnection-regulated phase where the net increase of the flux is slowed down by intermittent reconnection events near the horizon; and (iii) a flaring phase when a major, large-scale reconnection event expels the flux, leading to efficient particle acceleration. The emergence of large-amplitude quasi-periodic flux eruptions and concomitant particle acceleration is reminiscent of Sgr A* flaring activity. This phenomenon could also be applicable to quiescent black holes, especially isolated black holes accreting the interstellar medium.

Figures (9)

• Figure 1: Sketch of the initial GRPIC setup representing zero-angular-momentum accretion onto a central black hole (labeled BH) in the center. On the left, we show in cyan the initial cloud of plasma, at a density $n_0$ and temperature $T_0$, and in pink the injection area of fresh plasma. On the right, the initial uniform magnetic field lines are shown in blue, and the limit of the outer matching layer in red.
• Figure 2: Top half: Time evolution of: the horizon magnetic flux, $\Phi_{\rm H}$; the horizon accretion rate, $\dot{M}$, normalized by $\dot{M}_{0}=4\pi m_i n_0 r_{\rm inj}^2 v_{\rm th}$; the normalized horizon magnetic flux, $\Phi_{\rm H}/\sqrt{\dot{M}}$; and the total dissipation rate, $\dot{\mathcal{E}}$, normalized by $\dot{M}_{0}$. Bottom half: Simulation snapshots at $t/t_g=2689, 3286, 3596,\,\rm and\, 3648$. Each snapshot shows the number density $n$ (top left), the electromagnetic energy dissipation $\mathbf{J}\cdot\mathbf{E}$ (top right), the magnetization $\sigma$ (bottom left) and the local average particle Lorentz factor $\langle\gamma-1\rangle$ (bottom right). Black lines show magnetic field lines.
• Figure 3: Top panel: Particle energy distributions at different stages of the eruption cycle. Transparent lines show instantaneous distributions; opaque lines represent distributions time-averaged over each phase. Phases one, two, and three are color-coded, respectively, as green, blue, and red. Bottom panel: Time evolution of $\Phi_{\rm H}$ over one eruption cycle. Shaded regions (green, blue, red) indicate the time intervals during which particle distributions are presented in the top panel.
• Figure 4: Top: map of the azimuthal electric current carried by the electrons ($J^-_\phi$). Center: map of the azimuthal electric current carried by the ions ($J^+_\phi$). Bottom: map of the auxiliary azimuthal magnetic field $H_\phi$. Magnetic field lines are represented in solid black lines for all panels. The mass ratio is $m_i/m_e = 256$ in the depicted simulation.
• Figure 5: Map of the local average particle energy for electrons (left) and ions (right) for the simulation with $m_i/m_e=256$. The energies are normalized to $m_e c^2$ and grey lines represent magnetic field lines.