Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics

Under very general assumptions on the accretion flow geometry, images of a black hole illuminated by electromagnetic radiation display sequence photon rings (demagnified and rotated copies direct image) which asymptotically approach purely theoretical critical curve---the outline shell. To distant observer, these appear dominated emission, forms ring whose diameter is primarily determined effective radius emitting region. For that reason, connecting image seen observer to properties underlying spacetime crucially relies calibration necessarily depends assumed astrophysical source model. On other hand, more detailed geometry than structure. As such, detection would allow for metric be probed in less model-dependent way, enabling robust tests relativity Kerr hypothesis. Here we present structure several spherically symmetric spacetimes perform comparisons with Schwarzschild/Kerr case. We offer our perspective future rings, discussing challenges opportunities involved.