Lensing by Kerr black holes. II: Analytical study of quasi-equatorial lensing observables

In this second paper, we develop an analytical theory of quasi-equatorial lensing by Kerr black holes. In this setting we solve perturbatively our general lens equation with displacement given in Paper I, going beyond weak-deflection Kerr lensing to third order in our expansion parameter ε, which is the ratio of the angular gravitational radius to the angular Einstein radius. We obtain new formulas and results for the bending angle, image positions, image magnifications, total unsigned magnification, and centroid, all to third order in ε and including the displacement. New results on the time delay between images are also given to second order in ε, again including displacement. For all lensing observables we show that the displacement begins to appear only at second order in ε. When there is no spin, we obtain new results on the lensing observables for Schwarzschild lensing with displacement.