The iconic first photo of a black hole made M87* famous. Now new data and analysis from the Event Horizon collaboration shows how the supermassive black hole at the center of the galaxy M87 has changed over the course of a year. Although the bright ring of light has remained the same size, its brightest region has shifted counterclockwise – against the direction of rotation of the black hole, according to astronomers. This corresponds to theoretical predictions and confirms that the accretion disk of hot gases rapidly orbiting the black hole is characterized by turbulence. The annual comparison also provides strong evidence that this supermassive black hole is tilted against our line of sight. Its axis of rotation therefore points slightly away from the Earth. These results help to decipher the processes surrounding such gravity giants in more detail.
In 2019, astronomers released the first photo of a black hole – a significant milestone. The image of the supermassive black hole M87* created by the Event Horizon Telescope (EHT) radio telescope network confirmed many theoretical predictions about the appearance of such objects. The image showed the dark central shadow of the gravity giant surrounded by the bright ring of light from hot gases orbiting the event horizon. But this was only a snapshot, based on a few days of observation in spring 2017. In order to find out how the accretion disk made of hot gases in particular changes over time, the EHT collaboration also examined this black hole repeatedly targeted in subsequent years.
Year-on-year comparison confirms key data…
Now, six years after the first image of M87* appeared, the astronomers in the EHT collaboration are presenting a sequel: comparative analyzes based on data obtained in April 2018 – almost exactly one year after the first observation campaign. “The accretion environment of black holes is turbulent and dynamic. Because we can consider the 2017 and 2018 observations as independent measurements, we have a new perspective to study the black hole environment,” says co-author Hung-Yi Pu of National Taiwan Normal University. For their study, the astronomers analyzed and compared observation data from both years using computer models and simulations. “By integrating multi-epoch data with advanced models, we can better understand the dynamic processes that drive the observed brightness fluctuations at M87*,” explains EHT researcher Christian Fromm from the University of Würzburg and the Max Planck Institute for Radio Astronomy in Bonn.
Thanks to these analyses, it is now possible to see how the supermassive black hole M87* has changed within a year. They show that the glowing ring around the black hole’s event horizon has not changed in size. As measured in 2017, it has a diameter of around 43 microarcseconds. This corresponds to the theoretical predictions for the shadow of a black hole with a mass of 6.5 billion solar masses, as is the case with M87*. “Black holes as gigantic as M87* only change on very long time scales. “It is therefore not surprising that we see much of what we measured in 2017 in 2018,” explains Luciano Rezzolla from Goethe University Frankfurt.
But the new data also shows changes. “The small differences we found are very important for understanding what is actually happening near M87*,” says Rezzolla.
…and shows significant differences
One of the noticeable year-on-year changes concerns a particularly bright region in the light ring of M87*. Over the course of a year, it shifted around 30 degrees counterclockwise, the analyzes showed. “The shift in the brightest region is a natural consequence of the turbulence in the accretion disk around the black hole,” explains Abhishek Joshi from the University of Illinois Urbana-Champaign. As gas spirals into a black hole, it can align itself along the black hole’s rotation axis or rotate in opposite directions. This direction also influences the extent and dynamics of the turbulence – and thus the displacement of the brighter areas in the light ring. “In our original theoretical interpretation of the 2017 observations, we predicted that the brightest region would most likely shift counterclockwise. We are very pleased that the observations in 2018 confirmed this prediction!” continued Joshi. According to the astronomers, the shift, which can be seen within a year, indicates that the accretion disk of M87* is probably orbiting against the direction of rotation of the black hole.
The observational data also provides further clues about how the supermassive black hole is oriented. “The location of the brightest region in 2018 reinforces our previous interpretation of the black hole’s orientation from the 2017 observations: the black hole’s rotation axis points away from Earth,” reports Bidisha Bandyopadhyay of the University of Concepción. Accordingly, the black hole is slightly tilted as seen from us. Overall, these results contribute to a better understanding of how matter is attracted to the black hole M87* and what properties M87* actually has. “In the coming years we will carry out more observations of this kind with increasing precision, with the aim of creating a film of what is actually happening near M87*,” says Rezzolla. Analyzes of the EHT data from 2021 and 2022 are already underway. They could then provide even deeper insights into the behavior of this gravity giant.
Source: Event Horizon Telescope Collaboration, Astronomy and Astrophysics; doi: 10.1051/0004-6361/202451296