# Oblique magnetic fields and the role of frame dragging

## TL;DR

This paper explores how magnetic null points form near rotating black holes due to frame dragging and external magnetic fields, with implications for particle acceleration in low-accretion galactic nuclei.

## Contribution

It analyzes the formation of magnetic null points near black holes caused by external magnetic fields and frame dragging, comparing different magnetic field configurations and their effects.

## Key findings

- Magnetic null points can develop near the ergosphere boundary of rotating black holes.
- Both dipole and uniform magnetic fields can create null points near the event horizon.
- Null points are sites for potential efficient particle acceleration.

## Abstract

Magnetic null points can develop near the ergosphere boundary of a rotating black hole by the combined effects of strong gravitational field and the frame-dragging mechanism. The electric component does not vanish in the magnetic null and an efficient process or particle acceleration can occur. The situation is relevant for starving (low-accretion-rate, such as the Milky Way's supermassive black hole) nuclei of some galaxies which exhibit only episodic accretion events. The presence of the magnetic field field of an external origin is an important aspect. We propose that such conditions can develop when a magnetized neutron star approaches the supermassive black hole during late stages of its inspiral motion. The field lines of the neutron star dipole thread the black hole's event horizon and change rapidly their connectivity. We put in comparison the case of a dipole-type magnetic field of a sinking and orbiting star near a non-rotating black hole, and the near-horizon structure of an asymptotically uniform magnetic field of a distant source near a fast-rotating black hole. Although the two cases are qualitatively different from each other, they both develop magnetically neutral null points near the event horizon.