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

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

Imaging a black hole like this is extremely challenging, requiring astronomers to identify a small target in the sky while dealing with amounts of data so vast that observatory personnel have to ship 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 a scattering of 11 radio telescope installations located around the world using a technique called very long baseline interferometry. The goal is to have these multiple observatories work together to create a single virtual mirror powerful enough to view a distant black hole.

The EHT Collaboration ran 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 record the radio signals captured at each of these telescopes at the same time and then computationally form a mirror, bringing the data together in a central location and combining all the data,” Lindy Blackburn, EHT Collaboration Member and Astrophysicist at the Center of Astrophysics | Harvard & Smithsonian, account inverse.

The data must be accurately timestamped, which astronomers do using atomic time. 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 map the wobble to calculate the time the laser was fired. Masers are quite stable, losing just a single second every 100 million years.

Blackburn clarified that it is not impossible to have the observatories working together simultaneously. However, it is easier to ship the hard drives to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy because of the volume of data obtained from remote observatories.

“So when we bring them [the datasets] together, we’re kind of freezing the light in these telescopes,” says Blackburn. “We gathered them and then reproduced the data digitally, on the same hard drives, and combined them into software.”

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

What are the challenges of imagining a black hole?

While operating simultaneously at multiple observatories during a global pandemic is challenging enough, the technical problems of trying to visualize 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 frequency of observation is about a millimeter, he says. This, unfortunately, shares a similar frequency with water vapor, which can be plentiful in Earth’s atmosphere. If there is too much water vapor, the EHT observations will be interfered with.

“A big challenge is just running when the weather is good enough on all of our sites to actually be able to see the source and get 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 running the campaign.”

But since 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 approximately the size of the radius of the distance from Mercury’s orbit to the Sun, as seen from 25,000 light-years away.

“It’s the sharpest image ever taken in the industry,” says Blackburn of the Sgr A* photos. “We hope to improve the image a little in the future. We will switch to higher frequencies next year.”

Further down the road, perhaps in the next two decades, Blackburn says there are visions of extending the EHT even further, adding more observatories and distance. Some people have even considered putting a network of radio telescopes in space to get a better view, although that view may be further away in the future.

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

Why is the Sgr A* image important?

Sgr A* is a highly variable black hole, changing frequency every half hour, and it is relatively silent in terms of activity. The EHT got around these problems by taking “snapshots” of the target, or having all observatories take an instant image simultaneously. Blackburn says that if the EHT collaboration could double the number of dishes on the ground in future campaigns, it would allow them to track the dynamics of Sgr A* even more closely.

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

“So far, we haven’t seen anything that is contrary to what is expected of 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 to map galactic evolution, as 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 map the accumulation of dust and gas around black holes as matter spirals towards the center. “The EHT is a great way to try to ensure that our hydrodynamic simulations, done on supercomputers, [are verified to] see how accurate, credible and extensible they are”.

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