A computer simulation of the black hole at the center of the galaxy M87 shows two rings: a thick orange line and a pixel-thin bright band at the inner edge of that thick line. New telescopes could help identify the presence of such rings, which were hidden in the previous black hole image.
M.D. JOHNSON ET AL/SCIENCE ADVANCES 2020
Faint rings of light surrounding enormous black holes could be spotted with the help of a future generation of telescopes in space.
The doughnut-shaped glow spotted in the first image of a black hole, released in April 2019 by the Event Horizon Telescope collaboration (SN: 4/10/19), is more complex than the worldwide network of radio telescopes could discern. The black hole’s gravity is so intense that some particles of light, called photons, can circle the black hole partway — or once, twice or multiple times — before escaping to be picked up by telescopes. Those orbiting photons produce a “photon ring,” made up of a series of subrings — circles of light that appear successively thinner and harder for telescopes to pick out.
“It’s sort of like a hall of mirrors, where we’re getting an infinite series of images,” says astrophysicist Michael Johnson of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Now, Johnson and colleagues calculate that, with the help of new telescopes in space, the photon subrings theoretically could be observed around the supermassive black hole at the center of the galaxy M87, the subject of that first black hole snapshot.
The Event Horizon Telescope, or EHT, combines the powers of telescopes across the world, via a technique called very long baseline interferometry, so that they operate like one, larger telescope (SN: 4/10/19). But to tease out more details, such as black hole subrings, researchers would need to add telescopes separated by even larger distances.
A radio telescope orbiting Earth could capture the first subring, the team reports March 18 in Science Advances. But observing the second subring would require an even more distant telescope — on the moon. The third subring could be detected with a telescope even farther out, 150 million kilometers from Earth.
Scientists previously have proposed such telescopes, but the plans haven’t yet gotten off the ground. Johnson says that the new study provides new motivation for adding a space-based telescope to the EHT’s network.
Although the EHT wouldn’t directly photograph the subrings, it could detect their existence. That detection would reaffirm Einstein’s theory of gravity, the general theory of relativity, which predicts the rings’ existence. It also could allow for better measurements of the black hole’s mass and how fast it is spinning.
The idea “will be challenging, but it’s something to look forward to,” says astrophysicist Avi Loeb at Harvard University, who was not involved with the research. “It is an exciting goal for the next generation.”
Research Team Discovers Path to Razor-Sharp Black Hole Images
The image of a black hole has a bright ring of emission surrounding a "shadow" cast by the black hole. This ring is composed of a stack of increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer.
Last April, the Event Horizon Telescope (EHT) sparked international excitement when it unveiled the first image of a black hole. Today, a team of researchers have published new calculations that predict a striking and intricate substructure within black hole images from extreme gravitational light bending.
"The image of a black hole actually contains a nested series of rings," explains Michael Johnson of the Center for Astrophysics | Harvard and Smithsonian (CfA). "Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
Because black holes trap any photons that cross their event horizon, they cast a shadow on their bright surrounding emission from hot infalling gas. A "photon ring" encircles this shadow, produced from light that is concentrated by the strong gravity near the black hole. This photon ring carries the fingerprint of the black hole—its size and shape encode the mass and rotation or "spin" of the black hole. With the EHT images, black hole researchers have a new tool to study these extraordinary objects.
"This is an extremely exciting time to be thinking about the physics of black holes," says Daniel Kapec, Member in the School of Natural Sciences at the Institute for Advanced Study. "Einstein’s theory of general relativity makes a number of striking predictions for the types of observations that are finally coming within reach, and I think we can look forward to lots of advances in the coming years. As a theorist, I find the rapid convergence between theory and experiment especially rewarding, and I hope we can continue to isolate and observe more universal predictions of general relativity as these experiments become more sensitive."
The research team included observational astronomers, theoretical physicists, and astrophysicists.
"Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation," notes George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign. Wong developed software to produce simulated black hole images at higher resolutions than had previously been computed and to decompose these into the predicted series of sub-images. "What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits."
The researchers also found that the black hole's image substructure creates new possibilities to observe black holes. "What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images—even perfect images—they are strong and clear signals for arrays of telescopes called interferometers," says Johnson. "While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough."
"Black hole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science," says Alex Lupsasca from the Harvard Society of Fellows. "As a theorist, I am delighted to finally glean real data about these objects that we've been abstractly thinking about for so long."
The results were published in Science Advances and are available here.
This research was supported by grants from the National Science Foundation, the United States Department of Energy, the Gordon and Betty Moore Foundation, the John Templeton Foundation, the Jacob Goldfield Foundation, and NASA.
A release from the Center for Astrophysics | Harvard & Smithsonian is available here.
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