Magnification relations for Kerr lensing and testing Cosmic Censorship

Understand

A Kerr black hole with mass parameter m and angular momentum parameter a acting as a gravitational lens gives rise to two images in the weak field limit.

• We study the corresponding magnification relations, namely the signed and absolute magnification sums and the centroid up to post-Newtonian order.
• We show that there are post-Newtonian corrections to the total absolute magnification and centroid proportional to a/m, which is in contrast to the spherically symmetric case where such corrections vanish.
• Hence we also propose a new set of lensing observables for the two images involving these corrections, which should allow measuring a/m with gravitational lensing.