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Black hole team discovers path to razor-sharp black hole images

(18 March 2020 - Center for Astrophysics | Harvard and Smithsonian) 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.

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(couresy: Center for Astrophysics | Harvard and Smithsonian)

"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.

"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 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."

The results were published in Science Advances.

About Center for Astrophysics | Harvard & Smithsonian

Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard & Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.