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

Last weekthe planet-wide Event Horizon Telescope revealed a fresh look at the supermassive black hole at the center of our galaxy. The historic first image of Sagittarius A* (Sgr A*) showed its shape and activity in submillimetre waves, based on 3.5 petabytes of data from several telescopes.

Imaging such a black hole is an enormous challenge, as astronomers must locate a small target in the sky while dealing with amounts of data so large that observatories have to send hard drives to other facilities for analysis. So how did the EHT get the job done?

Event Horizon telescope technical details

In 2022, EHT scattered 11 radio telescope facilities around the world using a technique called very long baseline interferometry. The goal is to make these different observatories work together to create a single virtual mirror powerful enough to image a distant black hole.

The EHT partnership undertook the just-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 several targets, including Sgr A*, but the results are still being processed.

“We record the radio signals captured in each of these telescopes at the same time, and then we mirror computationally by bringing the data to a central location and combining all the data,” said Lindy Blackburn, member of the EHT collaboration and astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, says: inverse

The data must be accurately time-stamped, which astronomers do using atomic timing. Each participating telescope must emit a microwave laser (or maser) beam on hydrogen gas, which is the most elemental element abundant in the air. 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 only one second every 100 million years.

Blackburn clarified that it is not impossible to have the observatories work together at the same time. However, it is easier to send the hard drives to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy because of the amount of data coming from outside observatories.

“If we bring them then” [the datasets] together we freeze the light at these telescopes’, says Blackburn. “We put it together, and then we play the data digitally, on the same hard drives, and then we combine them into software.”

This image of Sagitarrius 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 in imaging a black hole?

While working simultaneously on several observatories during a global pandemic is already challenging, the technical difficulties of imaging a black hole are almost as great as the target itself.

“We’re going to push the limits of what’s possible from the ground, down to the radio frequencies,” Blackburn says.

The sighting frequency is about 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 observations of EHT will experience interference.

“A big challenge is just running when the weather in all our locations is good enough to actually see the source and collect data,” Blackburn says. “So there’s a big coordination effort to try and find the night when the weather is pretty good, and we have a good chance of leading the campaign.”

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

“It’s the sharpest image ever shot in the industry,” said Blackburn of the Sgr A* photos. “We hope to sharpen the picture a bit in the future. Next year we will go to higher frequencies.”

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

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

Why is the Sgr A* image important?

Sgr A* is a highly variable black hole that changes frequency every half hour and is relatively quiet in terms of activity. EHT solved these problems by performing “snapshot imaging” of the target, or by having all observatories create an image immediately. Blackburn says that if the EHT partnership could double the number of dishes on the ground in future campaigns, it would allow them to follow the dynamics of Sgr A* even better.

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 fundamental in mapping the characteristics of black holes.

“So far we haven’t seen anything that conflicts with what is expected from general relativity,” he says, referring to Einstein’s work on how space and time behave. He says the implications of better understanding black holes extend into cosmology to map galactic evolution, as most large galaxies have supermassive black holes like Sgr A*.

Blackburn says his team is engaged in tasks such as verifying the simulations, which aim to map the accretion of dust and gas around black holes as matter spirals toward the center. “The EHT is a great way to ensure that our hydrodynamic simulations, which are run on supercomputers, are [are verified to] see how accurate, credible and extensible they are.”

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