If you were lucky enough to witness a total eclipse, you surely remember the bright halo of light around the Moon during totality. It helps us understand the shape of a black hole’s corona.
This halo, known as the corona, represents the diffuse outer atmosphere of the Sun. Although it is so thin that on Earth it would be considered a vacuum, its temperature reaches millions of degrees, which is why it becomes visible during a total eclipse. According to the current understanding of the dynamics of black holes, they should also have a corona. Like that of the Sun, it is difficult to observe normally.
However, a study published in The Astrophysical Journal was able to make observations about this elusive region.
What is the shape of the corona of a black hole?
In the case of an active black hole, it is assumed that there is a torus (a “doughnut”) of gas and dust around the black hole, in which lies an accretion disk of heated material aligned with the plane of rotation of the black hole. Jets of ionized gas traveling at nearly the speed of light are emitted from the black hole’s polar regions. This model explains the different types of active galactic nuclei (AGN) we observe because the orientation of the black hole relative to us changes the appearance of these nuclei.
According to this model, the region closest to the accretion disk should be a superheated one with a near-vacuum density that flows towards the black hole. It is a corona similar to that of the Sun, but with a temperature of billions of degrees. However, due to its diffusion, its light is overwhelmed by the glow of the accretion disc, writes Science Alert.
In this new study, the researchers used a technique similar to observing the Sun’s corona during a total eclipse. The orientation of a black hole towards us means that in some cases the gas and dust torus blocks the view of the accretion disk region, while in other cases the disk is visible. These black holes are classified as obscure or non-obscure. Dusky black holes are similar to an eclipsed Sun because the light from the accretion disk is blocked.
Unfortunately, the black hole’s corona is also blocked in such cases. However, the corona is so hot that it emits extremely high-energy X-rays, which can be scattered by material in the torus and reflected into our line of sight.
A better understanding of black holes
Using data provided by NASA’s Imaging X-ray Polarimetry Explorer (IPXE), the research team analyzed a dozen obscure black holes, including the Milky Way’s Cygnus X-1 and X-3, as well as the LMG X- 1 and X-3, from the Large Magellanic Cloud, to find out what the shape of a black hole’s corona is.
Scientists have not only been able to observe X-rays scattered from the coronas of these black holes, they have also identified a pattern. According to the data, the corona surrounds the black hole in the form of a disk, similar to the accretion disk, and not in the form of a sphere, like the Sun’s corona.
These findings will help astronomers improve models of black holes and better understand how they consume matter and power AGNs observed in distant galaxies.
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Source: www.descopera.ro
