Observational Features of Black Holes

TL;DR

This paper explores the formation of gravitational mirages near Kerr black holes, proposing methods to measure black hole parameters through observed mirage shapes, including shadows, using future radio interferometry.

Contribution

It introduces a procedure to analyze black hole glories and shadows to determine mass and spin parameters, extending gravitational lensing studies in strong fields.

Findings

Calculated shadow boundaries numerically for Kerr black holes.

Proposed using RADIOASTRON to measure mirage shapes and black hole spin.

Discussed the formation of glories near rotating black holes.

Abstract

Recently considered a very attracting possibility to detect retro-MACHOs, i.e. retro-images of the Sun by a Schwarzschild black hole. In this paper we discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. In some sense that is a manifestation of gravitational lens effect in the strong gravitational field near black hole horizon and a generalization of the retro-gravitational lens phenomenon. We analyze the case of a Kerr black hole rotating at arbitrary speed for some selected positions of a distant observer with respect to the equatorial plane of a Kerr black hole. We discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. Some time ago suggested to search shadows at the Galactic Center. In this paper we present the boundaries for shadows calculated numerically. We also propose to use future radio interferometer RADIOASTRON facilities to measure shapes of mirages (glories) and to evaluate the black hole spin as a function of the position angle of a distant observer.