arXiv2020
In general relativity, isolated black holes are invisible due to an infinitely large redshift of photons propagating from the event horizon to the remote observer. However, the dark shadow (silhouette) of a black hole can be visible on the background of matter radiation lensed by the gravitational field of black holes. The black hole shadow is the celestial sphere projection of the cross section of photon capture by the black hole. If the illuminating background is far behind the black hole (at a distance much greater than the event horizon radius), a classic black hole shadow of a maximal size can also be observed. A minimal-size shadow can be observed if the same black hole is illuminated by the inner part of the accretion disk adjacent to the event horizon. In this case, the shadow of an accreting black hole is a lensed image of the northern or southern hemisphere of the event horizon, depending on the orientation of the black hole spin axis. A dark silhouette of the southern hemisphere of the event horizon is seen in the first image of the supermassive black hole M87* presented by the Event Horizon Telescope. The brightness of accretion matter is much higher than the corresponding one of the usual astrophysical stationary background in the form of numerous stars or extensive hot gas clouds. For this reason, it is improbable that a black hole shadow can be observed in the presence of very luminous accretion matter.