Last year in April, the community of scientists gathered around the Event Horizon Telescope (EHT) – a network of several radio telescopes around the world – presented the very first image of the event horizon. of a black hole. Today, a team of researchers from the Harvard-Smithsonian Center for Astrophysics says they can get much sharper, more detailed images of these fascinating objects.
The EHT project, made up of eight interconnected terrestrial telescopes, makes it possible to obtain a giant “virtual” telescope, the size of the Earth, with high sensitivity and high resolving power. It was notably designed to study Sagittarius A *, the supermassive black hole which is at the center of our galaxy and M87 *, located in the heart of the Messier 87 galaxy. Almost a year ago, the astrophysicists involved in the project unveiled to the world the very first “snapshot” of M87 *.
Image of M87 * obtained by the Event Horizon Telescope in April 2019. The brilliant ring that we observe here delimits the horizon of the events of the black hole, its “border” in a way. Credits: EHT
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A series of light rings
The image obtained reveals a beautiful luminous ring. However, it does not give information on its thickness or its detailed structure. However, researchers from the Harvard-Smithsonian Center for Astrophysics have shown that general relativity makes it possible to predict the presence of a complex structure within this ring, displaying distinct signatures during interferometric measurements (performed by the EHT network ). These signatures would open the way to the determination of physical parameters essential to the understanding of black holes.
Michael Johnson, who heads this new study, points out that the image obtained thanks to the EHT made it possible to observe the structure of a black hole in its entirety. The telescope network has indeed succeeded in capturing the “ring of photons” surrounding it, an area where the force of gravity is so powerful that the photons move in orbit.
The black holes cast a shadow over the image of the surrounding shiny material, as their strong gravitational field captures light. The shadow is delimited by a luminous ring, formed by matter and gas which pass near the black hole before escaping from it. The animation above shows how the image of a black hole is formed from photon substructures and trajectories (credits: Center for astrophysics | Harvard & Smithsonian).
According to Johnson and his team, this ring of photons would itself be composed of several rings, closer and closer to the black hole. “ The image of a black hole consists of a series of nested rings “, explains Mr. Johnson. ” These rings have almost the same diameter, but they become successively more and more clear, because the light made several turns around the black hole before reaching the observer “
A track towards essential data
Focusing the next analyzes on this internal structure could greatly improve future images, which would be much sharper than those obtained so far. Experts describe this ring of photons as the “fingerprint” of the black hole: its size and shape make it possible in particular to estimate the mass and define the rotation of the black hole, essential parameters for the scientific community.
The image of a black hole consists of a luminous ring surrounding a “shadow zone” projected by the black hole. This ring is made up of a stack of increasingly clear substructures, which correspond to the number of orbits that photons have made around the black hole before reaching the observer. Credits: George Wong (UIUC) and Michael Johnson (CfA)
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Invisible to the naked eye, this set of nested rings nevertheless emits remarkable signals over the network of interferometers that make up the EHT. Johnson adds that a particularly promising study of these sub-elements would require a relatively inconsistent hardware deployment: ” While capturing a whole black hole requires several networked telescopes, these substructures can be studied with only two telescopes very far apart. ” The specialist specifies that it would suffice to add a space telescope to the EHT.
He is not the only one to suggest space installations to learn even more about black holes. Recently, a team of Dutch astronomers proposed sending satellites equipped with radio imagery to allow much sharper observations of these intriguing celestial objects. Freek Roelofs, who heads this team at Radboud University, explains: ” In space, you can make observations at higher radio frequencies; frequencies on Earth are attenuated by the atmosphere. “
Sources: Science Advances, M. D. Johnson and Harvard-Smithsonian Center for Astrophysics
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