New View Reveals Magnetic Fields Around Our Galaxy’s Giant Black Hole

Fresh imagery from the Event Horizon Telescope traces the lines of powerful magnetic fields spiraling out from the edge of the supermassive black hole at the center of our Milky Way galaxy, and suggests that strong magnetism may be common to all supermassive black holes.

The newly released image showing the surroundings of the black hole known as Sagittarius A* — which is about 27,000 light-years from Earth — is the subject of two studies published today in The Astrophysical Journal Letters. This picture follows up on an initial picture issued in 2022. Both pictures rely on radio-wave observations from the Event Horizon Telescope’s network of observatories around the world.

Sagittarius A* wasn’t the first black hole whose shadow was imaged by the EHT. Back in 2019, astronomers showed off a similar picture of the supermassive black hole at the center of the galaxy M87, which is more than a thousand times bigger and farther away than the Milky Way’s black hole.

In 2021, the EHT team charted the magnetic field lines around M87’s black hole by taking a close look at the black hole in polarized light, which reflects the patterns of particles whirling around magnetic field lines. Researchers used the same technique to determine the magnetic signature of Sagittarius A*, or Sgr A* for short.

Getting the image wasn’t easy, largely due to the fact that Sgr A* was harder to pin down than M87. The EHT team had to combine multiple views to produce a composite image.

“Making a polarized image is like opening the book after you have only seen the cover,” EHT project scientist Geoffrey Bower, an astronomer at Academia Sinica in Taiwan, explained in today’s news release. “Because Sgr A* moves around while we try to take its picture, it was difficult to construct even the unpolarized image. … 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.”

The resulting picture met the research team’s expectations, and then some.

“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 project co-leader Sara Issaoun, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “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.”

The structure of the magnetic fields around Sgr A* suggests that the black hole is launching a jet of material into the surrounding environment. Previous research has shown that to be the case for M87’s black hole.

“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 EHT deputy project scientist Mariafelicia De Laurentis, a professor at the University of Naples Federico II in Italy.

In the seven years since the EHT began gathering observations, the collaboration has been adding to its array of radio telescopes, which is resulting in the production of higher-quality imagery. The EHT team plans to observe Sgr A* again next month — and in the years ahead, the researchers aim to produce high-fidelity movies of Sgr A* that may reveal a hidden jet. They’ll also look for evidence of similar polarization features around other supermassive black holes.

More than 300 researchers are part of the EHT collaboration that produced the two studies published today in The Astrophysical Journal Letters:

• “First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring”
• “First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring”

More explanatory videos from the Event Horizon Telescope:

• An Introduction to the EHT
• What Is Polarization?
• The Power of Polarized Imaging

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