{"id":2259,"date":"2019-06-12T13:41:59","date_gmt":"2019-06-12T13:41:59","guid":{"rendered":"https:\/\/blogs.futura-sciences.com\/e-luminet\/?p=2259"},"modified":"2019-11-13T10:40:13","modified_gmt":"2019-11-13T10:40:13","slug":"40-years-of-black-hole-imaging-3-from-kerr-black-holes-to-eht","status":"publish","type":"post","link":"https:\/\/blogs.futura-sciences.com\/e-luminet\/2019\/06\/12\/40-years-of-black-hole-imaging-3-from-kerr-black-holes-to-eht\/","title":{"rendered":"40 Years of Black Hole Imaging (3): from Kerr black holes to EHT"},"content":{"rendered":"

Sequel of the previous post 40 Years of Black Hole Imaging (2) : Colors and Movies 1989-1993<\/a><\/strong><\/p>\n

Generalizations to Kerr Black Holes<\/strong><\/h5>\n

Unfortunately Marck\u2019s simulations of black hole accretion disks remained mostly ignored from the professional community, due to the fact that they were not published in peer-reviewed journals and, after their author prematurely died in May 2000, nobody could find the trace of his computer program\u2026<\/p>\n

Then, unaware of Marck\u2019s results, several researchers of the 1990\u2019s were involved in the program of calculating black hole gravitational lensing effects in various situations. Stuckey (1993) studied photon trajectories which circle a static black hole one or two times and terminate at their emission points (\u00ab boomerang photons \u00bb), producing a sequence of ring-shaped mirror images. Nemiroff (1993) described the visual distortion effects to an observer traveling around and descending to the surface of a neutron star and a black hole, discussing multiple imaging, red- and blue-shifting, the photon sphere and multiple Einstein rings. He displayed computer-generated illustrations highlighting the distortion effects on a background stellar field but no accretion disk, and made a short movie now available on the internet (Nemiroff 2018), two snapshots of which are shown in figure 1.<\/p>\n

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Figure 1. Trip to a black hole by Robert Nemiroff, 1993.<\/figcaption><\/figure>\n

The first simulations of the shape of accretion disks around Kerr black holes were performed by Viergutz (1993). He treated slightly thick disks and produced colored contours, including the disk’s secondary image which wraps under the black hole (figure 2). The result is a colored generalization of the picture by Cunningham and Bardeen (1973) shown in 40 Years of Black Hole Imaging (1)<\/strong><\/a>.<\/p>\n

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Figure 2. Primary and secondary images of a simple accretion disk model around a Kerr black hole, seen by a faraway observer. Colors indicate combined gravitational and Doppler shifts (from Viergutz 1993).<\/figcaption><\/figure>\n

More elaborate views of a geometrically thin and optically thick accretion disk around a Kerr black hole were obtained by Fanton et al. (1997). They developed a new program of ray tracing in Kerr metric, and added false colors to encode the degree of spectral shift and temperature maps (figure 3). Zhang et al. (2002) used the same code to produce black-and-white images of standard thin accretion disks around black holes with different spins, viewing angles and energy bands (figure 4).<\/p>\n

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Figure 3. False color contour maps showing how the monochromatic radiation emitted by a Keplerian accretion disk would be seen at infinity for various values of the inclination angle to the plane of the disk (top to bottom : 5\u00b0, 45\u00b0, 85\u00b0). The left column refers to a non-rotating black hole, the right one to a rapidly rotating black hole with a=0.998 M. The white zones stand for the regions with zero redshift. Left-hand side of the disk is approaching the observer and blueshifted (from Fanton et al. 1997).<\/figcaption><\/figure>\n
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Figure 4. Disk images of accretion disks extending up to 20 Schwarzschild radii for different spins of Kerr black holes, viewed in different energy ranges and inclination angles (from Zhang et al. 2002).<\/figcaption><\/figure>\n

Ben Bromley et al. (1997) calculated integrated line profiles from a geometrically thin disk about a Schwarzschild and an extreme Kerr black hole, in order to get an observational signature of the frame-dragging effect (Figure 5).<\/p>\n

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Figure 5. Image of a geometrically thin disk around an extreme Kerr (maximally rotating) black hole seen at an inclination of 75\u00b0. The inner and outer radii of the Keplerian (circularly rotating) disk are at 1.24 M and 6 M. The colors encode the apparent light frequency, the white strip divides redshifted and blueshifted regions. The asymmetric appearance of the inner disk edge results from the frame-dragging effect of black hole rotation (from Bromley et al. 1997).<\/figcaption><\/figure>\n

In 1998 Andrew Hamilton started to develop for a student project at the University of Colorado a \u201cBlack Hole Flight Simulator<\/a><\/strong>\u201d, with film clips that have been shown at planetariums, also available on the Internet. The first depictions were very schematic, but the website was constantly implemented. It now offers journeys into a Schwarzschild or a Reissner-Nordstr\u00f6m (i.e. electrically charged) black hole with effects of gravitational lensing on a stellar background field, as well as animated visualizations of magneto-hydrodynamic simulations of a disk and jet around a non-rotating black hole (Hamilton 2018).<\/p>\n

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