One-dimensional force-free numerical simulations of Alfven waves around a spinning black string

TL;DR

This study uses one-dimensional force-free simulations to explore Alfven wave propagation around a spinning black string, revealing energy amplification and wave interactions relevant to black hole physics.

Contribution

It introduces a numerical approach to analyze Alfven wave dynamics around a black string, highlighting wave-induced energy transfer and wave interactions in relativistic magnetospheres.

Findings

Alfven wave energy increases as it propagates outward.

Energy conservation appears violated without considering additional waves.

Fast magnetosonic waves are induced, restoring energy conservation.

Abstract

We performed one-dimensional force-free magnetodynamic numerical simulations of the propagation of Alfven waves along magnetic field lines around a spinning black-hole-like object, the Banados--Teitelboim--Zanelli black string, to investigate the dynamic process of wave propagation and energy transport with Alfven waves. We considered axisymmetric and stationary magnetosphere and perturbed the background magnetosphere to obtain the linear wave equation for the Alfven wave mode. The numerical results show that the energy of Alfven waves monotonically increases as the waves propagate outwardly along the rotating curved magnetic field line around the ergosphere, where energy seems not to be conserved, in the case of energy extraction from the black string by the Blandford--Znajek mechanism. The apparent breakdown of energy conservation suggests the existence of an additional wave induced by the Alfven wave. Considering the additional fast magnetosonic wave induced by the Alfven wave, the energy conservation is recovered. Similar relativistic phenomena, such as the amplification of Alfven waves and induction of fast magnetosonic waves, are expected around a spinning black hole.