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

TL;DR

This study uses kinetic plasma simulations to reveal cyclic magnetic flux eruptions and associated particle acceleration in spherical black hole accretion, providing insights into black hole magnetosphere dynamics and flaring activity.

Contribution

First-principles kinetic simulations uncover the cyclic flux eruption mechanism and particle acceleration in spherical accretion onto Schwarzschild black holes.

Findings

Identification of three main accretion stages: advection, reconnection, and flaring.

Discovery of quasi-periodic flux eruptions linked to large-scale reconnection events.

Simulation results resemble observed flaring activity in Sgr A*.

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.