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.
Highlights
Understanding the fate of an initial mass distribution after its gravitational collapse is still one of the most active fields of research within the general relativistic community
Even though there are studies that predict the linear instability of completely collapsed objects without event horizons, such as naked singularities (NS), through this work, we study potentially observable differences in the light curves that are produced by a hotspot orbiting at the innermost regions of an accretion disc that is formed around both black holes (BH) and NSs, with the idea of developing a tool to probe the nature of the spacetime in the strong gravitational regime close to fully-collapsed objects
Implementing the numerical algorithm described in the previous section, we have been able to generate a family of theoretical light curves that have been emitted by corotating hotspots formed in magnetised accretion discs around both BHs and NSs
Summary
Understanding the fate of an initial mass distribution after its gravitational collapse is still one of the most active fields of research within the general relativistic community. Even though there are studies that predict the linear instability of completely collapsed objects without event horizons, such as NSs (see, e.g., Dotti et al 2008, 2012, for studies related to the Kerr solution and Cardoso et al 2008, and Pani et al 2010, for superspinars), through this work, we study potentially observable differences in the light curves that are produced by a hotspot orbiting at the innermost regions of an accretion disc that is formed around both BHs and NSs, with the idea of developing a tool to probe the nature of the spacetime in the strong gravitational regime close to fully-collapsed objects.
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