Mirages around Kerr black holes and retro-gravitational lenses

TL;DR

This paper explores the formation of gravitational mirages near Kerr black holes, proposing methods to measure black hole parameters by analyzing these mirages with future radio interferometry.

Contribution

It extends the concept of retro-gravitational lensing to rotating Kerr black holes and introduces a procedure to determine black hole mass and spin from mirage shapes.

Findings

Calculated shadow boundaries numerically for Kerr black holes.

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

Analyzed mirage formation for various observer positions around Kerr black holes.

Abstract

Recently Holz & Wheeler (2002) 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 Falcke et al. (2000) 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.