Silhouettes of invisible black holes

Abstract

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.

21 pages, 20 figures, published version

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Cited by in corpus (21)

• Calculating black hole shadows: Review of analytical studies
• Black holes with scalar hair in light of the Event Horizon Telescope
• Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon
• QED Effect on Black Hole Shadow
• Analytical study of higher-order ring images of accretion disk around black hole
• Shape of higher-order images of equatorial emission rings around a Schwarzschild black hole: Analytical description with polar curves
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• Acoustic black hole in Schwarzschild spacetime: quasi-normal modes, analogous Hawking radiation and shadows
• Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime
• Neutrino scattering off a black hole surrounded by a magnetized accretion disk
• Modeling the motion of a bright spot in jets from black holes M87* and SgrA*
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• Decoding a black hole metric from the interferometric pattern of the relativistic images of a compact source
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