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The only black hole we’ve ever seen has a shadow that wobbles

The supermassive black hole at the center of the M87 galaxy has a shadow crescent that moves, like a dancer in the dark.
September 23, 2020
A simulation of the accretion disk of the M87* supermassive black hole.
A simulation of the accretion disk of the M87* supermassive black hole.EHT / Hotaka Shiokawa

Over a year ago, scientists unleashed something incredible on the world: the first photo of a black hole ever taken. By putting together radio astronomy observations made with dishes across four continents, the collaboration known as the Event Horizon Telescope managed to peer 53 million light-years away and look at a supermassive black hole, which is 6.5 million times the mass of the sun and sits at the center of the galaxy Messier 87 (M87). The fiery historic image showed off a bright crescent of ultra-hot gas and debris orbiting the black hole’s event horizon, the pitch-black central point-of-no-return that traps anything that goes over, even light. 

The EHT team had just made one of the most impressive achievements in the history of astronomy, but this was only the beginning. On Wednesday, members of the EHT collaboration published new findings in the Astrophysical Journal about M87’s supermassive black hole (known as M87*), revealing two new major insights.

First, the shadow diameter of the event horizon doesn’t change over time, which is exactly what Einstein’s theory of general relativity predicts for a supermassive black hole of M87*’s size. However, the second insight is that the bright crescent adorning this shadow is far from stable: it wobbles. There’s so much turbulent matter surrounding M87* that it makes sense the crescent would bug out and get fidgety. But the fact that we can watch it over time means we now have an established method for studying the physics of one of the most extreme kinds of environment in the entire universe.

“We want to understand physics in the extreme conditions in the vicinity of a black hole and learn about how the black hole interacts with the matter in its immediate environment,” says Maciek Wielgus, an astronomer with the Harvard-Smithsonian Center for Astrophysics and the lead author of the new study. “Studying the dynamics of the crescent-like appearance of a black hole is a way to probe this fascinating environment.”

Before the EHT, scientists didn’t have the sensitive tools needed to study the structural changes a black hole goes through. “It was like watching a movie with a 1-pixel resolution,” says Wielgus. “You see that the brightness is changing in time—clearly something is going on there—but good luck figuring out what the movie is about.” 

Event Horizon Telescope

The new findings don’t make new observations of M87*, but rather characterize the shadow crescent through a new analysis of data collected from 2009 to 2013 during the EHT’s early days, combined with the 2017 data set that led to the image of the black hole in the first place. The older data was less detailed because of software constraints and more limited hardware, but it spanned a longer period of time. Meanwhile, the newer data set consisted of just four observations of M87* over one week, but it was much richer and more nuanced. Wielgus and his team were able to use details from the new data to fill in gaps in the old, as you might add a new corrective filter to an old photo to make it sharper. Bam—they had a high-quality time-lapse of M87*, over time scales stretching for several weeks.

The EHT is still processing the 2018 observations and plans to run new observations of M87 next year, using 10 telescopes in total. Those observations, which will involve a deeper study of the crescent, could reveal new insights into the spin of a black hole, the strength of its magnetic field, and the plasma microphysics of the surrounding matter. In turn, researchers hope those insights can be part of a bigger body of work that solves the mystery behind some of the wildest phenomena involved in supermassive black holes, like what drives the ejection of highly ionized matter from their center.

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