Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots

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.

7 pages, 3 figures, submitted to Astronomy and Astrophysics

References in corpus (8)

• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• Can accretion disk properties observationally distinguish black holes from naked singularities?
• Probing the space-time geometry around black hole candidates with the resonance models for high-frequency QPOs and comparison with the continuum-fitting method
• Distinguishing black holes and wormholes with orbiting hot spots
• Instability of hyper-compact Kerr-like objects
• Gravitational instabilities in Kerr space-times
• Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*
• Magnetised accretion discs in Kerr spacetimes

Cited by in corpus (5)

• Sgr A* and General Relativity
• Testing the No-Hair Theorem with Observations of Black Holes in the Electromagnetic Spectrum
• General purpose ray-tracing and polarized radiative transfer in General Relativity
• Distinguishing Brans-Dicke-Kerr type naked singularities and black holes with their thin disk electromagnetic radiation properties
• Observational signatures from higher-order images of moving hotspots in accretion disks