Astronomers from the Event Horizon Telescope (EHT) collaboration have found strong magnetic fields spiraling around the supermassive black hole at the center of our galaxy, named Sagittarius A* (Sgr A*). In 2022, the EHT Collaboration released the first image of Sgr A*, revealing that it looked very similar to the much larger black hole at the center of Messier 87 galaxy (M87*). This prompted the team to study Sgr A* in polarized light, which helped them study magnetic fields surrounding the black hole.
Sagittarius A* is located 27,000 light-years from Earth and has a mass about four million times that of the Sun. The Event Horizon Telescope observed the black hole in April 2017. The EHT Collaboration used these observations for both the original image from 2022 and the current study.
“What we’re seeing now is that there are strong, twisted, and organized magnetic fields near the black hole at the center of the Milky Way galaxy,” said Sara Issaoun of the Center for Astrophysics | Harvard & Smithsonian, Cambridge, Massachusetts. “Along with Sgr A* having a strikingly similar polarization structure to that seen in the much larger and more powerful M87* black hole, we’ve learned that strong and ordered magnetic fields are critical to how black holes interact with the gas and matter around them.”
Location of Sagittarius A* in the Milky Way, imaged in polarized light. (Credit: S. Issaoun, EHT Collaboration)
Because of its distance, studying Sagittarius A* requires more than just an ordinary telescope. The Event Horizon Telescope is a global collaboration of radio telescopes linked together to operate in unison to study black holes. This collaboration effectively creates an Earth-sized virtual telescope, which allows astronomers to observe black holes and their surroundings directly.
Even with the EHT, studying black holes is hard. The plasma surrounding Sgr A* orbits the black hole in only a few minutes, which means the astronomers could not get a clear picture unless they found a way to compensate for that rotation. To generate the first image from 2022, the astronomers had to create an average of thousands of other images that all accurately fit the EHT observations.
What’s more, the signal is disturbed by interstellar plasma between Earth and the black hole, making it even harder to study, especially in polarized light.
“Because Sgr A* moves around while we try to take its picture, it was difficult to construct even the unpolarized image,” said Geoffrey Bower of the Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan. “We were relieved that polarized imaging was even possible. Some models were far too scrambled and turbulent to construct a polarized image, but nature was not so cruel.”
Just like the image from 2022, the newly released picture of Sagittarius A* is generated from an average of multiple datasets obtained from the EHT observations using different methods. To create the final picture, the astronomers processed the image to emphasize regions with strong polarization and overlaid it on top of the image from 2022.
But how does polarized light help astronomers study magnetic fields?
When light is emitted in hot, magnetized regions of space, it becomes polarized in a pattern perpendicular to the magnetic field lines. Such is the case in the hot plasma surrounding a black hole. This makes polarized light particularly useful to astronomers interested in magnetic fields.
“By imaging polarized light from hot glowing gas near black holes, we are directly inferring the structure and strength of the magnetic fields that thread the flow of gas and matter that the black hole feeds on and ejects,” said Harvard black hole initiative fellow Angelo Ricarte. “Polarized light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds.”
Messier 87* (left) and Sagittarius A* (right) in polarized light, as imaged by the Event Horizon Telescope. (Credit: EHT Collaboration)
The new images reveal even more similarities Between Sgr A* and M87* than the previous. An earlier study of M87* revealed the black hole launched jets of material into space and the current results suggest that the same might be happening at Sgr A*. Moreover, the similarities suggest that some processes are similar for all black holes, regardless of differences in mass and size.
“The fact that the magnetic field structure of M87* is so similar to that of Sgr A* is significant because it suggests that the physical processes that govern how a black hole feeds and launches a jet might be universal among supermassive black holes, despite differences in mass, size, and surrounding environment,” said Mariafelicia De Laurentis of the University of Naples Federico II, Italy. “This result allows us to refine our theoretical models and simulations, improving our understanding of how matter is influenced near the event horizon of a black hole.”
The Event Horizon Telescope is scheduled to observe Sagittarius A* again in April, but this time more telescopes will participate. This allows astronomers to observe the black hole in more frequencies.
The EHT Collaboration’s findings were reported in two papers (one and two), published on March 27 in The Astrophysical Journal Letters.
(Lead image: Sagittarius A* in polarized light as imaged by the Event Horizon Telescope. Credit: EHT Collaboration)
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