Photon Rings and Shadow Size for General Integrable Spacetimes

Understand

There are now multiple direct probes of the region near black hole horizons, including direct imaging with the Event Horizon Telescope (EHT).

• As a result, it is now of considerable interest to identify what aspects of the underlying spacetime are constrained by these observations.
• For this purpose, we present a new formulation of an existing broad class of integrable, axisymmetric, stationary spinning black hole spacetimes, specified by four free radial functions, that makes manifest which functions are responsible for setting the location and morphology of the event horizon and ergosphere.
• We explore the size of the black hole shadow and high-order photon rings for polar observers, approximately appropriate for the EHT observations of M87*, finding analogous expressions to those for general spherical spacetimes.