Universal Properties of Light Rings for Stationary Axisymmetric Spacetimes.

Light rings (LRs) play an important role in gravitational wave observations and black hole photographs. In this letter, we investigate general features of LRs in stationary, axisymmetric, asymptotically flat spacetimes with or without horizons. For a nonextremal black hole, we show explicitly that there always exist at least two radially unstable LRs propagating in opposite directions on the equatorial plane. For an extremal black hole, we show that there exists at least one retrograde LR. Our method also applies to horizonless spacetimes and we prove that there always exist even number of LRs on the equatorial plane, where the outermost one is unstable in the radial direction. Only some natural and generic assumptions are used in our proof. The results are applicable to general relativity as well as most modified theories of gravity. In contrast to previous works on this issue, we obtain stronger and clearer results with a much more straightforward approach.