Interaction of Black Hole Magnetospheres with Inclined Ambient Fields

TL;DR

This paper investigates how external inclined magnetic fields influence black hole magnetospheres, affecting magnetic flux, null points, and jet formation, with implications for astrophysical phenomena like jet quenching and the absence of large-scale jets.

Contribution

It introduces a model combining external inclined magnetic fields with the Blandford-Znajek monopole field, revealing effects on flux suppression, null points, and particle acceleration around Schwarzschild black holes.

Findings

External fields can significantly suppress horizon flux.

Complete flux cancellation occurs only in axisymmetric aligned cases.

Jet acceleration efficiency depends critically on external field orientation.

Abstract

Magnetic fields play a central role in black hole astrophysics, powering relativistic jets and other energetic phenomena. While near-horizon magnetic field is usually assumed to originate from the accretion flow, additional large-scale magnetic fields - such as those supplied by a companion neutron star in stellar-mass binaries or by galactic fields around supermassive black holes - may also affect the horizon-threading flux. In this work, we study the superposition of a weak arbitrarily inclined external uniform magnetic field with the internal Blandford-Znajek split-monopole field around a Schwarzschild black hole. This setup generically gives rise to magnetic null points, where the total field vanishes. We compute the magnetic flux through an arbitrarily tilted hemisphere of the event horizon and show that the flux can be substantially suppressed by the external field. In the axisymmetric case, the flux can even vanish completely. However, with nonzero inclination, complete cancellation becomes impossible, despite significant reduction. We further explore the ionization and subsequent particle acceleration from a Keplerian accretion disk, finding that efficient collimated outflows persist even under significant field inclination. We show that the acceleration is critically dependent on the external field orientation, with the escape fraction maximized at non-zero inclinations due to the destabilization of trapping zones and minimized in the anti-aligned configuration, where closed magnetic loops effectively suppress the outflow. We discuss the astrophysical implications of these findings, proposing that geometric flux cancellation can serve as a mechanism for jet quenching in compact binaries and offering an explanation for the lack of a prominent large-scale jet in Sgr A*.