How to Measure Black Hole's Mass, Spin and Direction of Spin Axis in Kerr Lens Effect 1: test case with simple source emission near BH

Understand

We propose a theoretical principle to measure the mass, spin and direction of spin axis of Kerr black holes (BHs) through observing 2 quantities of the spinning strong gravitational lens effect of BHs.

• Those observable quantities are generated by 2 light rays emitted at the same time by a source near the BH: the primary and secondary rays that reach a distant observer, respectively, the earliest and secondary temporally.
• The time delay between detection times and the ratio of observed specific fluxes of those rays are the observable quantities.
• Rigorously, our proposal is applicable to a single burst-like (short duration) isotropic emission by the source.