How light echoes can help to better understand black holes

– January 21, 2020 – Science

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A team of scientists used the phenomenon of light echoes to study a supermassive black hole. The technique allowed us to learn more about the crown of the object.

Thanks to the light echoes emitted by a black hole, scientists have been able to get a more accurate representation of the inner regions of this celestial object. On January 20, 2020, they explained in the journal Nature Astronomy how they were able to study the crown of a black hole and its dynamics.

“Observations of active galaxies tell us that there must be a hot ‘corona’ of electrons near the black hole, which constitutes a substantial fraction of their total brightness,” the authors write. They wanted to learn more about the environment of the black hole in the active galaxy IRAS 13224-3809, located a billion light years from Earth.

  • Read: The first black hole in the photo emits a jet that almost defies the speed of light.

How light echoes can help to better understand black holes

A representation of a black hole // Source: Flickr/CC/Roxanne Ready (cropped photo)

The black hole (a region of space whose gravitational field prevents any radiation from exiting) in this galaxy is supermassive, meaning that its mass is at least a million times that of the Sun. It is described as “one of the most variable X-ray objects in the sky” by scientists.

What is a light echo?

When matter falls into a black hole, it causes X-rays to be emitted. As with the phenomenon of sound echo (the reflection of sound), a light echo can occur when the light beam interacts with gas in the vicinity of the black hole. To the observer, the phenomenon may give the impression that this area of space is distorted, due to the gravitational attraction exerted by the black hole. It is these light echoes that the researchers wanted to exploit in order to obtain a more complete picture of the environment of the black hole in the galaxy IRAS 13224-3809.

  • Read: Mysterious objects discovered in orbit around the Milky Way black hole

The technique provides a dynamic representation of “the immediate area around the black hole, which is otherwise inaccessible,” the authors note. Thanks to the light echoes, it becomes possible to better understand the innermost regions of the black hole. For example, the scientists found that the crown had changed in size very quickly, in just a few days (because the light echo also changed). They were also able to determine that the black hole is two million times more massive than the Sun.

How light echoes can help to better understand black holes

The light echo // Source: Wikimedia/CC/Arkyan (annotations Numerama)

A promising technique for the study of distant black holes

To observe the light echoes around this black hole, scientists used XMM-Newton, a European Space Agency space observatory. Observations were conducted between 2011 and 2016.

The observation of black holes and their close environment using light echoes is a promising technique. A project like the Horizon Telescope Event, which immortalized the first image of a black hole in 2019, can only work with the closest supermassive black holes on Earth (the M87 and Milky Way black holes). Therefore, other methods, such as the one presented in this study, should be considered for more distant black holes.

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