Current sheet alignment in oblique black hole magnetospheres -- a black hole pulsar?

TL;DR

This study uses 3D general relativistic magnetohydrodynamics simulations to show that the magnetic current sheet around a spinning black hole aligns with its equatorial plane over time, affecting electromagnetic emissions.

Contribution

It demonstrates the exponential alignment of the current sheet with the black hole's equatorial plane and relates the alignment timescale to the black hole's spin, providing new insights into black hole magnetosphere evolution.

Findings

Inclined split monopole current sheet aligns with the equatorial plane over time.

Alignment timescale inversely proportional to the square of black hole spin.

Magnetic flux decays exponentially due to reconnection, with a consistent timescale across spins.

Abstract

We study the magnetospheric evolution of a non-accreting spinning black hole (BH) with an initially inclined split monopole magnetic field by means of three-dimensional general relativistic magnetohydrodynamics simulations. This serves as a model for a neutron star (NS) collapse or a BH-NS merger remnant after the inherited magnetosphere has settled into a split monopole field creating a striped wind. We show that the initially inclined split monopolar current sheet aligns over time with the BH equatorial plane. The inclination angle evolves exponentially towards alignment, with an alignment timescale that is inversely proportional to the square of the BH angular velocity, where higher spin results in faster alignment. Furthermore, magnetic reconnection in the current sheet leads to exponential decay of event horizon penetrating magnetic flux with nearly the same timescale for all considered BH spins. In addition, we present relations for the BH mass and spin in terms of the period and alignment timescale of the striped wind. The explored scenario of a rotating, aligning and reconnecting current sheet can potentially lead to multimessenger electromagnetic counterparts to a gravitational wave event due to the acceleration of particles powering high-energy radiation, plasmoid mergers resulting in coherent radio signals, and pulsating emission due to the initial misalignment of the BH magnetosphere.