Photon spheres in dynamical spacetimes

The characterization of the photon region -- i.e. the region of space-time filled with unstable bound null geodesics -- is critical to understand the behavior of radiation near a compact-enough object, such as black holes. However, its study has been typically focused on stationary space-times that leave outside interesting theoretical and phenomenological scenarios such as collapsing, accreting, or evaporating black holes, besides time-dependent configurations such as those found in some boson star models. In this work, we present a novel covariant approach to describe radiation in dynamical spherical space-times. This allows a general description of photon surfaces and their dynamics in non-static space-times, recovering well known expressions in the static limit and clarifying their meanings. Furthermore, we illustrate our results via several examples of dynamical scenarios in stellar collapse and accreting/evaporating models, and discuss the open caveats regarding such scenarios.