M87's black hole crescent wobbles

George Wong for Illinois Physics

In April 2019, the Event Horizon Telescope (EHT) Collaboration published the first images of a black hole, the one at the center of the nearby galaxy M87. Now, a new analysis of unexplored archival data from as early as 2009 has shown that although the size and shape of the crescent-like asymmetry present in the original image is a consistent feature of the data, its orientation varies. The crescent wobbles. The full results have been published in The Astrophysics Journal.

The early observations do not contain enough data to produce fully resolved images like the ones published in 2019, so the EHT team had to rely on statistical modeling and simple geometric models in their analysis. Nevertheless, the historical data provide an invaluable tool to understand how the accretion flow evolves with time.

"Last year we saw an image of the shadow of a black hole," says Maciek Wielgus, an astronomer at the Harvard-Smithsonian Center for Astrophysics, a Black Hole Initiative Fellow, and lead author of the paper. "But those results were based only on observations performed throughout a one-week window in April 2017, which is far too short to see a lot of changes. Based on last year’s results we asked the following questions: is this crescent-like morphology consistent with the archival data? Would the archival data indicate a similar size and orientation of the crescent?"

Scientists at the University of Illinois at Urbana-Champaign working under Professor of Physics and Astronomy Charles Gammie contributed to the new research by providing the numerical black hole accretion simulations that were used to explore whether the early EHT data had a  high enough signal-to-noise ratio that information about the source could be obtained. Studying numerical simulations is important because they provide a physics-accurate, internally consistent model of the small-scale features and time variability that geometric models miss. Moreover, numerical simulations help connect the statistics of the variations to underlying physical processes like the onset of turbulence and the strength of magnetic fields.

From this author's perspective as a graduate student at Illinois and one of the people who produced the simulations, it's incredibly exciting to begin to see what a supermassive black hole looks like over time. So much more can be learned from a movie than an image, and the extra data will help us understand what is really going on near the hole's event horizon. We're finally able to compare theoretical models to the real world!

Being able to study the flow dynamics near the horizon can drive new tests of the theory of General Relativity and may hold the key to understanding phenomena like relativistic jet launching.

The EHT Collaboration is continuing to expand, even as its scientists are digging into the archival data. With each new telescope added to the EHT system, the image accuracy improves drastically.

EHT Project Scientist Geoffrey Bower, research scientist of the Academia Sinica, Institute of Astronomy and Astrophysics (ASIAA), notes, "Monitoring M87* with an expanded EHT array will provide new images and much richer data sets to study the turbulent dynamics. We are already working on analyzing the data from 2018 observations, obtained with an additional telescope located in Greenland. In 2021 we are planning observations with two more sites, providing extraordinary imaging quality. This is a really exciting time to study black holes!"



An animation representing one year of M87* image evolution according to numerical simulations. Measured position angle of the bright side of the crescent is shown, along with a 42 microarcsecond ring. For a part of the animation, image blurred to the EHT resolution is shown. Credit: G. Wong, B. Prather, C. Gammie, M. Wielgus & the EHT Collaboration

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