Black Hole Accretion Disk

A Black Hole Accretion Disk is a massive disk composed of superheated gas, dust, and plasma that rotates around a black hole in an orbital pathway just beyond the event horizon — the spherical region around a black hole singularity inside of which the gravitational force exerted by the black hole is so great that even light cannot escape the inexorable gravitational pull exerted by this extreme curvature of space-time.

Accretion disks around a black hole are composed mostly of the remnants of stars, exoplanets, and other matter consumed by the black hole. Beyond a certain radius gravity functions like normal, meaning that objects orbiting would be of no greater risk of being sucked into the black hole than the Earth is of being consumed by the Sun. However, on occasion depending on a range of variables, stars, exoplanets and other matter — even other black holes — are pulled into the event horizon and torn apart by the extreme forces, resulting in the creation of these accretion disks.

Accretion disks rotate, causing the atoms of the various types of matter to emit energy across a wide range of the electromagnetic spectrum. Accretion disks emit in the X-Ray, IR (infrared), radio, and visible light electromagnetic spectral ranges. These emissions are essentially the only signature of a black hole by which we can detect them.

Black hole accretion disks are most visible in the infrared range of the electromagnetic spectrum, where they emit most strongly, with a definitive spectral signature. They emit a pair of energy spikes in the infrared range of the spectrum, both from hydrogen. One wavelength of energy as hydrogen falls away from the observer towards the event horizon, and the other as hydrogen spirals away from the event horizon towards the observer. Researchers call this a “double peak”, the electromagnetic signature of a black hole accretion disk, and of the black hole themselves. The double-peak originates from the edge of the accretion disk closest to the event horizon, which can therefore tell us both where the accretion disk begins (though not how far away from the black hole the accretion disk extends into space) along with an indication of the radius of the black hole at its event horizon, helping us to determine the size of the object.

Scientists have just recently measured the exact size of the accretion disk of a black hole due to an accidental discovery of the two hydrogen peaks around the black hole III Zw 002, accidentally, while attempting only to determine that the accretion disk was in fact there, after its discovery in 2003. Black hole III Zw 002 is located 22 million light-years from Earth (well outside of the Milky Way) and is estimated to be some 400 million times the mass of the Sun. [1] The radius of the accretion disk was estimated at about 52.4 light-days (about 1.35 trillion km) giving it a diameter of some 2.7 trillion km (more than 18,000 times the Earth-Sun distance).

Further Reading

1. For the 1st time, scientists accidentally measure the swirling ring around a black hole. Harry Baker. 7 Sept 2023. Live Science. <https://www.livescience.com/space/black-holes/for-the-1st-time-scientists-accidentally-measure-the-swirling-ring-around-a-black-hole>. Accessed 13 Sept 2023.
2. First Observation of a Double-peaked O i Emission in the Near-infrared Spectrum of an Active Galaxy. Denimara Dias dos Santos et al. 8 Aug 2023. <https://iopscience.iop.org/article/10.3847/2041-8213/ace974>. Accessed 13 Sept 2023.