Explanation of image of black hole M87*

Using an array of 8 radio telescopes at 6 different sites, astrophysicists of Event horizon collaboration turned 4 petabytes of data obtained from observations of black hole M87* into the first image of a black hole event horizon. In mid-April 2019, this image of black hole appeared on the front pages of newspapers and magazines worldwide.


To explain the image of the black hole, the superposition of the image of black hole M87* and the modeled thin accretion disc of specific angular momentum (a = J/M), a= 0.75 is used.


At the outermost part you can see a purple color curve which represents the contour of the classical black hole shadow, which is invisible at this image due to too much brightness of the accretion disc.

There are basically two types of black hole shadow. One is formed by the distant luminous background, while the other forms due to the accretion matter falling into the black hole. Classical black hole shadow forms when the light comes from the distant luminous background behind the black hole, and this shadow is of maximum size.


The brightness of accreting matter is much higher than the brightness of distant luminous background, which is in the form of hot gas clouds and bright stars. This is the reason why it is very difficult to observe the classical black hole shadow in the presence of the accreting matter. You can clearly see there is no such shadow visible in the image of black hole M87* shown above. Even if there was no accreting matter around the black hole, the brightness of the distant luminous background would have been insufficient for the telescopes we currently have, to observe it!


And if the shadow is formed by the luminous matter falling into the black hole then the black hole shadow of minimal size will form.


Inside the purple curve you can see a green color curve. This green closed curve is a lensed image of a circle with the radius r = rISCO. What is this rISCO? Basically in the accretion disk, there is an internal boundary in the accretion disk known as the “Innermost Stable Circular Orbit” and the radius of this orbit is represented as rISCO.

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In the inner region of thin accretion disk, outside the event horizon and inside the Innermost Stable Circular Orbit”, stable circular orbits for accreting matter are absent. This part of the accretion disk is a region of non-stationary motion of accreting matter and the separate fragments of the accretion disk fall into the black hole independently of one another along the contracting spiral path. The brightest point in the accretion disk is also found on the “Innermost Stable Circular Orbit”. This brightest point is marked by the red star in the image shown above.

Inside the green color curve you can see a black spot in the central part. This black spot is the observed event horizon silhouette as predicted by general relativity. This central black spot contains another dotted curve which represents the black hole event horizon image in the imaginary Euclidean space. The magenta arrow that you can see in the central part of the image is the black hole rotation axis. 

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