Black hole photon ring autocorrelations and where to find them

The properties of light emitted by hot gas surrounding black holes are determined by the properties of the plasma creating it and by the spacetime geometry. It should be possible in principle to separate the characteristics that are influenced by the geometry from the characteristics that depend on the plasma, given that one has fewer degrees of freedom to describe the spacetime geometry (the black hole's mass, spin, and the observer's inclination), and that general relativity predicts light echoes, resulting from photons emitted from the same region but taking different paths to reach the observer, whose time delay is almost independent of the spin of the black hole. These echoes, however, are not detected when analyzing light curves from astrophysical sources and from simulations. In this talk, we will present a simple analytical model that shows how, while these echoes and time correlations exist in the data, they remain hidden within the overall signal as long as the time correlations of the emitting plasma exceed those dictated by the spacetime geometry. This phenomenon is illustrated through simulations of light curves combining stochastic accretion models with a general relativistic ray-tracing code. Our findings provide an explanation for the absence of time correlations predicted by the geometry of the black hole.