Effects of Homogeneous Plasma on Strong Gravitational Lensing of Kerr Black Holes

TL;DR

This study investigates how a homogeneous plasma medium influences strong gravitational lensing around Kerr black holes, revealing increased photon-sphere radius and deflection angles, with effects diminishing at higher black hole spins.

Contribution

It provides a detailed analysis of plasma effects on gravitational lensing parameters around Kerr black holes, highlighting the dependence on black hole spin and plasma presence.

Findings

Plasma increases photon-sphere radius and deflection angles.

Effects diminish as black hole spin increases in prograde orbits.

In extreme black holes, plasma effects become negligible.

Abstract

Considering a Kerr black hole surrounded by the homogenous unmagnetised plasma medium, we study the strong gravitational lensing on the equatorial plane of the Kerr black hole. We find that the presence of the uniform plasma increases the photon-sphere radius $r_{ps}$, the coefficient $\bar{a},\bar{b}$, the angular position of the relativistic images $\theta_{\infty}$, the deflection angle $\alpha(\theta)$ and the angular separation $s$. However the relative magnitudes $r_m$ decrease in presence of the uniform plasma medium. It is also shown that the impact of the uniform plasma on the effect of strong gravitational become smaller as the spin of the Kerr black increace in prograde orbit($a>0$). Especially, for the extreme black hole(a=0.5), the effect of strong gravitational lensing in homogenous plasma medium is the same as the case in vacuum for the prograde orbit.