The scientists behind the first picture of a black hole are squeezing everything they can from the data they’ve got.
A year after presenting a portrait of the supermassive black hole in the galaxy M87 (SN: 4/10/19), the Event Horizon Telescope team faces a two-year data drought, thanks to technical snafus, security snags and a global pandemic.
“The coronavirus has set us back a bit,” says astrophysicist Shep Doeleman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the founding director of the EHT. “Nothing is immune, not even black holes.”
The delay may push back answering questions about how black holes shoot speedy jets of charged particles into space, and could postpone our getting a clear view of the black hole at the center of the Milky Way. But the team is still analyzing existing data and making plans to expand the observatory in the future.
The EHT is a global network of radio telescopes that, together, form a telescope that’s effectively the size of the Earth (SN: 4/10/19). Spanning the globe lets the EHT zoom in on a black hole’s event horizon, the region beyond which not even light can escape.
Last year, the EHT team revealed the first direct image of a black hole’s event horizon: an asymmetric ring of light representing the shadow of the black hole on a glowing disk of plasma that orbits it (SN: 3/18/20). The image, of the event horizon of M87’s supermassive black hole, was made entirely from data the observatory took in 2017, with seven telescopes at five sites around the world. (An eighth telescope, the South Pole Telescope in Antarctica, was part of the EHT at the time, but M87 is not visible from the South Pole.)
The results provided stunning confirmations of parts of Einstein’s theory of general relativity and bolstered physicists’ understanding of black holes across the universe. But some surprising pieces were missing: M87’s supermassive black hole shoots jets of plasma light-years into space, yet the EHT saw no sign of them.
And although the telescope observed the black hole at the center of the Milky Way galaxy, Sagittarius A*, the results were so confusing that team chose not to publish them at the time. Sgr A* is much smaller than the black hole at the center of M87, which means that the material in the bright disk around Sgr A* whips around much faster than M87’s does. As a result, Sgr A* changed its appearance too quickly for the researchers to release a single still image in 2019, although they still hope to make a movie (SN: 12/16/19).
In 2020, the scientists had hoped to bring the total number of observatories in the EHT network to 11, adding the Kitt Peak observatory in Arizona, the NOEMA array in the French Alps and the Greenland Telescope. The team also planned to start observing at higher frequencies of radio emission, which can penetrate the plasma surrounding black holes more easily.
“I think that’s kind of a game changer,” says EHT team member Kazu Akiyama, an astrophysicist at the MIT Haystack Observatory in Westford, Mass., of those future observations. Adding more sites would provide sharper images, letting the team zoom in on M87’s jets. And peering through plasma in the Milky Way that’s between Earth and Sgr A*, about 25,640 light-years away, could help clarify astronomers’ view of our own neighboring supermassive black hole.
But the 2020 observations, scheduled for March 25 through April 5, were canceled due to the coronavirus pandemic. The team had performed a dry run as recently as January, sending astronomers to some of the most remote parts of the globe to make sure the telescopes were ready. But by the time astronomers would have had to set out to the observatories again, travel restrictions and lockdowns were in effect around the world.
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“It was only with the greatest reluctance, but with safety paramount, that we canceled the observations,” Doeleman says. “We’re happy to be part of the solution, actually, by not traveling. But it is still heartbreaking.”
The 2019 run was also canceled for a variety of reasons, from technical issues to bad weather to fuel thieves blocking the road to the Large Millimeter Telescope in Mexico. “It was the right decision, given that we expected we would run in 2020,” Doeleman says.
So the team is left sifting through data that are at least two years old. Luckily, there is still a lot to learn from that info.
“We still have a lot of excellent science even in the 2017 dataset,” says EHT deputy director Michael Hecht of Haystack Observatory. And “the 2018 dataset is mostly unutilized to this point.”
One result from the 2017 data is the sharpest view yet of a pair of jets zipping away from a supermassive black hole in a galaxy called 3C 279. Unlike M87, which is a relative neighbor at about 55 million light-years from Earth, 3C 279 sends its light from about 5 billion light-years away. That great distance let astronomers see features of the black hole’s jet that are too close to see at M87.
“We’re really looking at the deepest region of the jet … because we have a very sharp eye,” Akiyama says. At just a few light-years from the black hole’s maw, the jet is already moving at about 99.5 percent the speed of light, the EHT team reports April 7 in Astronomy & Astrophysics. Astronomers have seen such jets zooming along at extremely high speeds far from their black holes, but it was thought that the jets needed more of a runway to accelerate to such great speeds.
“What was surprising is that, even for the inner region of the jet, the jet is already accelerated very close to lightspeed,” the maximum speed at which matter can move, Akiyama says.
The team is also still working on parsing the structure of the magnetic field near M87’s giant black hole, which will be a clue to how that black hole’s jets form. The researchers are hoping to publish details about Sgr A* by the end of 2020, Akiyama says, although he couldn’t say what.
Meanwhile, Doeleman is looking to the future. Over the next decade, he and his colleagues hope to add another 10 or so telescopes to the network to build the “next-generation” EHT.
The present-day EHT uses telescopes that already existed, and the team had to make its observations work from wherever those telescopes happen to be. “We were a little opportunistic at the beginning,” Doeleman says. Now that the scientists know that the EHT works, they can place smaller telescopes wherever they want — including, perhaps, in space — to get even better views of black holes.
“The announcement last year showed everyone what the EHT is capable of,” Doeleman says. “We have a freedom we didn’t have before.”