Strong gravitational lensing by a rotating non-Kerr compact object

Understand

We study the strong gravitational lensing in the background of a rotating non-Kerr compact object with a deformed parameter $\epsilon$ and an unbound rotation parameter $a$.

• We find that the photon sphere radius and the deflection angle depend sharply on the parameters $\epsilon$ and $a$.
• For the case in which the black hole is more prolate than a Kerr black hole, the photon sphere exists only in the regime $\epsilon\leq\epsilon_{max}$ for prograde photon.
• The upper limit $\epsilon_{max}$ is a function of the rotation parameter $a$.