Even black holes have bad hair days

The Event Horizon Telescope (EHT) collaboration has unveiled new, detailed images of M87*, the supermassive black hole at the center of the galaxy M87, that reveal a dynamic environment with changing polarization patterns near the black hole. The new images, constructed and validated by researchers from the University of Waterloo and Perimeter Institute, show how the environment around the black hole may be changing more than we previously thought.

In 2017, the EHT observed a spiralling polarization pattern that is the signature of a large-scale twisted magnetic structure, confirming long-held ideas about how black holes interact with, and impact, their environments. But in 2018, the polarization all but disappeared. In 2021, the meager remnant began to spiral in the opposite direction. Astrophysicists are now wrestling with a solitary question: why?

New, highly detailed images of M87* from the Event Horizon Telescope (EHT) collaboration have revealed that the supermassive black hole’s magnetic fields are changing direction over time. The new images show that M87*’s magnetic fields appeared to be turning in one direction in 2017, but then settled in 2018, and experienced a reversal of direction in 2021. Scientists believe this flip in direction may be the result of internal magnetic structure and external effects, like a Faraday screen. Credit: EHT Collaboration, Amy C. Oliver/CfA

Blockbuster movies tell us that black holes are fantastic traps where things go in and never get out. But M87* is showing us that black holes can also take energetic material from their surroundings, caught up in a powerful electromagnetic field, and launch it outward in spectacular fashion. M87*’s jet starts near the event horizon, eventually reaching 90 per cent the speed of light. These new observations offer the first tentative hint of connective tissue between the chaotic ring of plasma around the black hole and the engine at the base of this powerful jet. But exactly how black holes perform this magic trick, and what it means for the fundamental nature of gravity, is only beginning to be revealed.

“Black holes hold their mysteries tight, but we are now prying the answers from their grasp,” says Dr. Avery Broderick, a professor in the Department of Physics and Astronomy at the University of Waterloo, and associate faculty at Perimeter Institute. “Our team at Waterloo was central to reconstructing the images from the EHT data, and determining what we can be confident is real and what could be merely an instrumental artefact. We have been at the forefront of understanding how EHT images, and especially their evolution, can reveal the astrophysical dramas unfolding on gravity’s most extreme stage.”

Year after year, the EHT collaboration goes back to M87* to capture moments that show how it is evolving, knowing that each time, they will gain more insight into its long-guarded secrets.

“What’s remarkable is that while the ring size has remained consistent over the years, confirming the black hole’s shadow predicted by Einstein’s theory, the polarization pattern changes significantly,” says Dr. Paul Tiede (BSc ’15, MMath ’18, PhD ’21), an astronomer at the Center for Astrophysics | Harvard & Smithsonian, and a graduate of Waterloo and Perimeter. “This tells us that the magnetized plasma swirling near the event horizon is far from static; it’s dynamic and complex, pushing our theoretical models to the limit.”

The stability of M87*’s shadow can be taken as evidence that “black holes have no hair,” a decades-old metaphor meaning that black holes are simple geometric objects with no descriptive parameters beyond their mass, spin and charge.

“It’s one of the reasons why they’re so interesting as gravitational objects. You can make very crisp, clear predictions, and all the astrophysical phenomena don’t seem to matter a lot,” Broderick says. “But the stuff around it can have hair, and these magnetic fields are a striking example. We’ve had a clear sense for what kind of magnetic hairstyles should be allowed for a long time, but now we’re seeing that, like with humans, you can get a lot of different hairstyles over four years.”

Back in 2009, Broderick wrote the first paper to propose imaging M87*, exploring what we could learn about black holes, their jets and their accretion disks by observing the variability of their magnetic fields. Now, the team has taken that idea and turned it into a reality.

“That first paper talked about how the polarization of M87* could reveal information about the magnetic fields within,” Broderick says. “Now we’re rapidly moving on toward extracting information about how jets accelerate and what that implies about how they’re powered. This is a very exciting time for the EHT and the upcoming data we have.”

With three years of observations under their belts, the EHT has no plans to stop. Future observations will only improve as new telescopes get added to the array, making future multi-year analyses even more detailed. And if one thing is certain, it’s that M87*’s ever-changing hairdo will keep black-hole watchers coming back for more.

Banner image credit: EHT Collaboration 

The paper, “Horizon-scale variability of from 2017--2021 EHT observations,” was published in Astronomy and Astrophysics.