The EHT Collaboration has recently published images of the supermassive black hole in M87. The images are dominated by a bright ring-like structure with an angular brightness asymmetry. While the diameter of this ring is resolved by the EHT, its thickness and detailed substructure are not. General relativity predicts that within this image lies a thin photon ring, composed of an infinite sequence of bright self-similar subrings. I will discuss the theoretical aspects of these general features of astrophysical black hole images, as well as the potential observability of the photon ring at extensions of the EHT.
Reading material:
arXiv:1907.04329, arXiv:1906.11242, arXiv:1905.11406