Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots

FEDERICO GARCÍA; IGNACIO F. RANEA-SANDOVAL; TIM JOHANNSEN

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016 vol. 587 p. 1 - 7

0004-6361

Context. Quasi-periodic variability has been observed in a number of X-ray binaries that harbor black hole candidates. In general rela-tivity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularitiesalways have to be clothed by an event horizon.Aims. In this paper, we study the observed light curves that arise from orbiting hotspots in thin accretion discs around Kerr blackholes and naked singularities, and the effect introduced by the presence of an external magnetic field.Methods. We employ a ray-tracing algorithm to calculate the light curves and power spectra of these hot spots as seen by a distantobserver for uniform and dipolar magnetic field configurations, assuming a weak coupling between the magnetic field and the discmatter.Results. We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of theselight curves 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 hotspot, which is orbiting near the innermost stable circular orbit of a nakedsingularity in a dipolar magnetic field, can be significantly harder than the emission of the same hotspot in the absence of this type ofmagnetic field.Conclusions. The comparison of our model with observational data may allow us to study the geometry of magnetic fields aroundcompact objects and to test the cosmic censorship conjecture in conjunction with other observables, such as thermal continuum spectraand iron line profiles.