New telescopes could help spot ‘photon ring’ of the first black hole ever imaged

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, 1.5 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.”