Silhouettes of invisible black holes

Abstract

In general relativity, isolated black holes are invisible due to the infinitely large redshift of photons propagating from the event horizon to a 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 the black hole. 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.

Quasinormal modes, stability and shadows of a black hole in the 4D Einstein–Gauss–Bonnet gravity

“…Theoretical analysis of shapes of the black hole shadows have been recently considered in a great number of papers (see, for example, [76][77][78][79][80][81][82][83] and references therein). The radius of the photon sphere r ph of a spherically symmetric black hole is determined by means of the following function: (see, for example, [84,85] and references therein)…”

Visible Shapes of Black Holes M87* and SgrA*

“…Parameters of these two photon trajectories are λ 1 = −0.047 and q 1 = 2.19 and, respectively, λ 2 = −0.029 and q 2 = 1.52. Note, that the dark event horizon silhouettes, similar to ones in Figures 7-10, were reproduced during many years in numerical modeling of accretion disks with the inner edge at the black hole event horizon (see, e.g., [167,[274][275][276][277][278][279][280][281][282][283][284][285][286][287]). Figure 11 demonstrates a numerical model for the gravitational lensing of a compact star, falling into the fast rotating black hole SgrA* (a = 0.9982) and observed in discrete time intervals by a distant static observer, placed a little bit above the equatorial plane.…”

“…Figure 7 from [274] shows a visible dark silhouette of the northern hemisphere of the black hole event horizon illuminated by a thin accretion disk in the equatorial plane of the black hole with the spin a = 0.9982, corresponding to the orientation of the supermassive black hole SgrA*. The typical photon trajectory (multicolored 3D curve) is shown, with parameters λ = 0.063 and q = 0.121, emitted by the hot accreting matter in the black hole equatorial plane at the radius r = 1.01 r h and reaching a distant observer near the external boundary (contour) of the dark silhouette of the northern hemisphere of the event horizon globe.…”

“…The brightness of the lensed star is exponentially faded in time during successive windings (see animation in [288]). The closed blue curves are meridians and parallels on the reconstructed image of the lensed event horizon globe (for details see [241,242,274]). Finally, Figure 12 shows the superposition of the image of M87*, obtained by the EHT with both the contours of classical black hole shadows (purple closed curves) and the event horizon silhouettes (dark regions) for different values of the black hole spin, a = 0.998 (left panel), a = 0.75 (central panel) and a = 0 (right panel).…”

How to form a wormhole

“…1a). They are increasingly attracting attention of physicists for many different reasons [1][2][3][4][5][6][7][8][9][10][11][12][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. However, so far, there is no realistic physical model of a wormhole formation.…”