Astrophysicists have gotten their first direct view of a supermassive black hole’s appearance changing over time.
The black hole at the center of the galaxy M87, about 55 million light-years away from Earth, was the first black hole to get its picture taken (SN: 4/10/19). That image, created using Event Horizon Telescope data from April 2017, showed a lopsided ring of light: the black hole’s shadow on the accretion disk of hot, glowing plasma swirling into it. A new comparison of that image with earlier Event Horizon Telescope data reveals that the brightest spot on the ring changes location, due to turbulence in the violent eddy of material around the black hole, researchers report online September 23 in the Astrophysical Journal.
“It’s a very exciting result,” says astrophysicist Clifford Will of the University of Florida in Gainesville, who was not involved in the study. “The first image that they produced was just a snapshot. What we would really like to do is understand more of the dynamics of what’s going on at the center of that galaxy.”
The Event Horizon Telescope, or EHT, is a network of radio telescopes that, together, make much higher resolution observations than any observatory could alone (SN: 4/10/19). An early version of the EHT began observing M87’s black hole, dubbed M87*, in 2009. Back then, the network included telescopes at only three sites in Arizona, Hawaii and California. In 2013, an observatory in Chile joined the team. But the network did not have enough telescopes to create a complete black hole image until 2017, when the EHT peered at M87* with seven observatories across North America, South America, Hawaii and Europe.
Using the 2017 image of M87* as a starting point for the black hole’s appearance, along with preliminary data from 2009 to 2013 to fill in some of the details, the EHT team was able to get a rough idea of what M87* looked like during the early years of EHT observation.
Although the black hole’s diameter remained the same, the brightest spot on the ring swiveled. The ring’s right side was brightest in 2013, while the bottom was brightest in 2017. “I think many people in the [EHT] collaboration were surprised by the amount of variability,” says EHT team member Maciek Wielgus, an astrophysicist at Harvard University. The ring’s uneven glimmer arises from the tumultuous flow of superhot plasma around the black hole.
This turbulence in the accretion disk — and therefore the variation in the ring’s appearance — is expected to depend on factors like how fast the black hole is spinning, the tilt of its rotation and the strength of its magnetic fields, says astrophysicist Priyamvada Natarajan of Yale University, who was not involved in the study (SN: 1/30/17). Monitoring changes in M87*’s appearance may reveal new information about the nature of the black hole.
The new results show the promise of using the EHT to probe the tempest around M87*, says Harvard University astrophysicist Avi Loeb, who was not involved in the work. But the rough sketches of the black hole’s appearance from 2009 to 2013 do not contain enough information to draw firm conclusions about what is going on in this chaotic region, he says.
The EHT team will need more full images of M87*, like the one created from 2017 data, to uncover detailed changes over time. That series of still images could also be used to create an M87* movie (SN: 12/16/19).
The EHT team is currently analyzing data collected in 2018, including observations from a newcomer to the EHT network, the Greenland Telescope. The EHT did not observe in 2019 or 2020, but “we will be observing in 2021, COVID permitting,” says EHT team member Geoffrey Bower, an astrophysicist at the Academia Sinica Institute of Astronomy and Astrophysics in Hilo, Hawaii (SN: 4/10/20). “We expect to have incredible imaging quality out of those 2021 data,” he says, because by then the EHT will have two more eyes on the sky: the Kitt Peak observatory in Arizona and the NOEMA array in the French Alps. “I think that’s really going to get at the heart of turbulence in the accretion region,” he says.