Strong gravitational lensing by a rotating non-Kerr compact object

We study the strong gravitational lensing in the background of a rotating non-Kerr compact object with a deformed parameter $ϵ$ and an unbound rotation parameter $a$. We find that the marginally circular stable orbit radius and the deflection angle depend sharply on the parameters $ϵ$ and $a$. For the case in which the black hole is more prolate than a Kerr black hole, the marginally circular photon orbit exists only in the regime $ϵ\ensuremath{\le}{ϵ}_{\mathrm{max}}$ for a prograde photon. The upper limit ${ϵ}_{\mathrm{max}}$ is a function of the rotation parameter $a$. As $ϵ>{ϵ}_{\mathrm{max}}$, the deflection angle of the light ray very close to the naked singularity is a positive finite value, which is different from that in the rotating naked singularity described by Janis-Newman-Winicour metric. For the oblate black hole and the retrograde photon, there does not exist such a threshold value. Modeling the supermassive central object of the Galaxy as a rotating non-Kerr compact object, we estimated the numerical values of the coefficients and observables for gravitational lensing in the strong field limit.

Hali tarjima qilinmagan