Eur. Phys. J. C (2018) 78: 69
https://doi.org/10.1140/epjc/s10052-018-5569-7

Regular Article - Theoretical Physics

Observers in Kerr spacetimes: the ergoregion on the equatorial plane

¹ Faculty of Philosophy and Science, Institute of Physics, Silesian University in Opava, Bezručovo náměstí 13, 74601, Opava, Czech Republic
² Dipartimento di Fisica, Università di Roma “La Sapienza”, 00185, Rome, Italy
³ Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, AP 70543, 04510, Mexico, DF, Mexico
⁴ Department of Theoretical and Nuclear Physics, Kazakh National University, Almaty, 050040, Kazakhstan

^* e-mail: d.pugliese.physics@gmail.com

Received: 11 May 2017
Accepted: 17 January 2018
Published online: 25 January 2018

Abstract

We perform a detailed analysis of the properties of stationary observers located on the equatorial plane of the ergosphere in a Kerr spacetime, including light-surfaces. This study highlights crucial differences between black hole and the super-spinner sources. In the case of Kerr naked singularities, the results allow us to distinguish between “weak” and “strong ” singularities, corresponding to spin values close to or distant from the limiting case of extreme black holes, respectively. We derive important limiting angular frequencies for naked singularities. We especially study very weak singularities as resulting from the spin variation of black holes. We also explore the main properties of zero angular momentum observers for different classes of black hole and naked singularity spacetimes.