Black hole shadows, photon rings, and lensing rings

The presence of a bright ``photon ring'' surrounding a dark ``black hole shadow'' has been discussed as an important feature of the observational appearance of emission originating near a black hole. We clarify the meaning and relevance of these heuristics with analytic calculations and numerical toy models. The standard usage of the term ``shadow'' describes the appearance of a black hole illuminated from all directions, including from behind the observer. A backlit black hole casts a somewhat larger shadow. Neither ``shadow'' heuristic is particularly relevant to understanding the appearance of emission originating near the black hole, where the emission profile and gravitational redshift play the dominant…

• NS
  National Science Foundation

  Awards: PHY-1752809, PHY-1708081, PHY-1804216, PHY-1505124
• KF
  Kavli Foundation