Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots

TL;DR

This study investigates how external magnetic fields influence the light curves of hot spots orbiting Kerr black holes and naked singularities, revealing observable effects especially with dipolar fields, which could help test cosmic censorship.

Contribution

It introduces a detailed ray-tracing analysis of hot spot emissions in magnetic fields around Kerr spacetimes, highlighting potential observable signatures of magnetic field configurations.

Findings

Dipolar magnetic fields significantly alter hot spot light curves.

Uniform magnetic fields have negligible effects on observed signals.

Emission near naked singularities can become notably harder due to magnetic fields.

Abstract

Context. Quasi-periodic variability has been observed in a number of X-ray binaries harboring black hole candidates. In general relativity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularities always have to be clothed by an event horizon. Aims. In this paper, we study the effect of an external magnetic field on the observed light curves of orbiting hot spots in thin accretion discs around Kerr black holes and naked singularities. Methods. We employ a ray-tracing algorithm to calculate the light curves and power spectra of such hot spots as seen by a distant observer for uniform and dipolar magnetic field configurations assuming a weak coupling between the magnetic field and the disc matter. Results. We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of these signals for both Kerr black holes and naked singularities, while an external uniform magnetic field has practically no effect. In particular, we demonstrate that the emission from a hot spot orbiting near the innermost stable circular orbit of a naked singularity in a dipolar magnetic field can be significantly harder than the emission of the same hot spot in the absence of such a magnetic field. Conclusions. The comparison of our model with observational data may allow us study the geometry of magnetic fields around compact objects and to test the cosmic censorship conjecture in conjunction with other observables such as thermal continuum spectra and iron line profiles.