How does the Event Horizon Telescope work? How we finally saw the Milky Way’s central black hole

In the past weekthe planet-wide Event Horizon Telescope revealed a new view of the supermassive black hole at the center of our galaxy. The historic first image of Sagittarius A* (Sgr A*) showed its shape and activity at submillimeter waves, based on 3.5 petabytes of data from multiple telescopes.

Obtaining images of a black hole like this is extremely challenging, requiring astronomers to identify a small target in the sky while handling amounts of data so large that observatory staff need to send hard drives to other facilities for analysis. So how did the EHT get the job done?

Technical details of the Event Horizon telescope

In 2022, EHT will scatter from 11 radio telescope facilities located around the world using a technique called very long baseline interferometry. The goal is for these various observatories to work together to create a single virtual mirror powerful enough to generate images of a distant black hole.

The EHT Collaboration undertook the newly launched Sgr A* campaign in 2017, with fewer observatories, but it took a while to process the data. A March 2022 campaign using all 11 telescopes observed a variety of targets, including Sgr A*, but the results are still being processed.

“We recorded the radio signals captured at each of these telescopes at the same time, and then computationally formed a mirror by gathering the data in a central location and combining all the data,” said Lindy Blackburn, a member of the EHT collaboration and an astrophysicist at the Center. of Astrophysics | Harvard & Smithsonian, account Reverse.

The data must be time-stamped precisely, which astronomers do using atomic timing. Each participating telescope must send a microwave laser beam (or maser) into hydrogen gas, which, as the most basic element, is abundant in the sky. Because hydrogen atoms have a known frequency, astronomers can trace the oscillation to calculate the moment the laser was fired. Masers are fairly stable, only losing a second every 100 million years.

Blackburn clarified that it is not impossible for the observatories to work together simultaneously. However, it is easier to ship the hard drives to MIT’s Haystack Observatory and the Max Planck Institute for Radio Astronomy due to the volume of data taken from remote observatories.

“Then when we bring them [the datasets] together, we’re freezing the light in these telescopes,” says Blackburn. “We put it together, and then we reproduce the data digitally, on the same hard drives, and then we combine it in software.”

This image of Sagittarius A* marks the first direct image of the Milky Way’s supermassive black hole event horizon. Event Horizon Telescope Collaboration

What are the challenges of imaging a black hole?

While operating multiple observatories simultaneously during a global pandemic is challenging enough, the technical problems of trying to image a black hole are almost as great as the target itself.

“We’re pushing the extremes of what can be done from the ground, as far as radio frequencies are concerned,” says Blackburn.

The observing frequency is around a millimeter, he says. That, unfortunately, shares a similar frequency to water vapor, which can be abundant in Earth’s atmosphere. If there is too much water vapor, the EHT observations will experience interference.

“A big challenge is just running when the weather is good enough at all of our sites to be able to see the source and take data,” says Blackburn. “So there’s a lot of coordination effort to try to find the night when the weather is pretty decent, and we have a good chance of getting the campaign done.”

But once the telescopes have good weather, their equivalent resolution is several times better than what NASA’s James Webb Space Telescope can see from deep space. The challenge, however, is that distant black holes are very small sources. For example, Sgr A* is about the size of the radius of the orbit of Mercury from the Sun, seen from 25,000 light-years away.

“It’s the sharpest image ever made in this industry,” Blackburn says of the Sgr A* photos. “We hope to improve the image a bit in the future. We are going to move to higher frequencies next year.”

Further down the road, perhaps within the next two decades, Blackburn says there are visions to further expand the EHT by adding more observatories and distance. Some people have even thought of placing a network of radio telescopes in space to get a better view, though such a view may be further in the future.

The ultimate goal, says Blackburn, is “to get longer baselines and that bigger virtual mirror, until we can see ever sharper images.”

Why is the Sgr A* image important?

Sgr A* is a highly variable black hole, changing frequency about every half hour, and is relatively quiet in terms of activity. EHT solved these problems by performing “snapshot images” of the target, or by having all observatories snap an image simultaneously. Blackburn says that if the EHT collaboration could double the number of clays on the ground in future campaigns, it would allow them to follow the dynamics of Sgr A* even more closely.

Blackburn says that both EHT and the Laser Interferometer Gravitational-Wave Observatory (LIGO), which tracks gravitational waves from large cosmic events such as black hole collisions, have been instrumental in mapping out the characteristics of black holes.

“So far, we haven’t seen anything that has been contrary to what is expected from general relativity,” he says, citing Einstein’s work on how space and time behave. He says the implications of a better understanding of black holes extend to cosmology for mapping galactic evolution, since most large galaxies have supermassive black holes like Sgr A*.

Blackburn says his team is focused on tasks like verifying the simulations, which aim to chart the buildup of dust and gas around black holes as matter spirals toward the center. “The EHT is a great way of trying to make sure that our hydrodynamic simulations, which are run on supercomputers, [are verified to] see to what extent they are accurate, credible and extensible”.

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