Effects of Homogeneous Plasma on Strong Gravitational Lensing of Kerr Black Holes**Supported by the National Natural Science Foundation of China under Grant Nos 11447168 and 11247013, and the Hunan Provincial Natural Science Foundation under Grant Nos 12JJ4007 and 2015JJ2085.

Abstract

Considering the Kerr black hole surrounded by a homogeneous unmagnetized plasma medium, we study the strong gravitational lensing on the equatorial plane of the Kerr black hole. It is found that the presence of the uniform plasma can increase the photon-sphere radius , the coefficients and , the angular position of the relativistic images (), the deflection angle and the angular separation s. However, the relative magnitude decreases in the presence of the uniform plasma medium. It is also shown that the impact of the uniform plasma on the effect of strong gravitational lensing becomes smaller as the spin of the Kerr black hole increases in the prograde orbit (). In particular, for the extreme black hole (a = 0.5), the effect of strong gravitational lensing in the homogeneous plasma medium is the same as the case in vacuum for the prograde orbit.