Appearance of Keplerian discs orbiting Kerr superspinars

Abstract

We study optical phenomena related to appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by the string theory. The superspinar exterior is described by the standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by the string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with growing spin, while the frequency shift grows with descending spin. This behavior differs substantially from appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and causes self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending of the superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a ∼ 1.

Perfect fluid tori orbiting Kehagias–Sfetsos naked singularities

“…For all of the considered phenomena related to the K-S naked singularities, an "antigravity" effect [31,35] plays a fundamental role; a similar situation has also been discovered in braneworld naked singularity spacetimes [36][37][38][39][40]. However, it seems to be different in comparison to the rotating Kerr naked singularity spacetimes [41][42][43][44][45][46][47][48]; for a detailed analysis of the magnetized thick discs see [49].…”

Shadow of a Kaluza-Klein rotating dilaton black hole

“…The shadows (or apparent shapes) of nonrotating black holes are circular, but rotating ones present a deformation caused by the spin [12,13]. This topic has been recently considered by several authors [10,[14][15][16][17][18][19][20][21][22][23][24][25], both in Einstein gravity and in modified theories, since it is expected that direct observation of black holes will be possible in the near future [25][26][27]. In that case, the analysis of the shadows will be a useful tool for obtaining properties of astrophysical black holes and comparing different gravitational theories.…”

Disconnected Regions of Stable Circular Orbits in Presence of Massive Scalar Field

“…It was shown by Chowdhury et al (2012) that, in presence of the linear massless scalar field, there can exist disconnected regions of SCO separated by a ring of unstable orbits. Examples of gaps in the SCO distribution have been found by Stuchlík & Schee (2010); Pugliese et al (2013Pugliese et al ( , 2017; Vieira et al (2014); Boshkayev et al (2016) in presence of naked singularities (NS). We wonder, is NS always necessary for the existence of such a discontinuous SCO structure?…”