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New view of our galaxy's black hole reveals a swirling magnetic field

This is the supermassive black hole at the centre of our galaxy as we have never seen it before. The image reveals the swirling magnetic fields around Sagittarius A* (Sgr A*) and hints that it may be producing a jet of high-energy material, which astronomers have yet to see.

The picture was taken by a network of observatories around the world operating as a single, giant telescope, called the Event Horizon Telescope (EHT). In 2022, it produced the first image of Sgr A*, revealing light coming from whirling hot plasma set against the dark background of the black hole’s event horizon, where light can’t escape its extreme gravity.

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Sleeping black hole is way more massive than it should be

Now, EHT researcher Ziri Younsi at University College London and his colleagues have measured how this light is polarised, or the orientation of its electromagnetic field, showing the direction and strength of the magnetic field around Sgr A*.

The image is remarkably similar to the magnetic field of M87*, the first black hole EHT studied. Given that M87* is around 1500 times more massive than Sgr A*, it suggests that supermassive black holes may have similar structures regardless of their size, says Younsi.

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One major difference between M87* and our galaxy’s black hole is the lack of visible high-energy jets seen from Sgr A*. This absence has long puzzled astronomers, but the fact that Sgr A* has a magnetic field like M87* suggests that there could be jets in our galaxy’s black hole, too.

“There’s this really exciting hint that there may be some additional structure,” says Younsi. “There might be something going on that’s quite exciting in the centre of the galaxy, and I think that these results we’re going to need to follow up.”

This would make sense given other pieces of evidence we see for a jet that may have existed long ago in our galaxy’s history, such as the Fermi bubbles, large spheres of X-ray-producing plasma above and below the Milky Way.

As well as revealing a potential hidden jet, the magnetic field’s properties could help solve other astrophysical mysteries, such as how particles like cosmic rays and neutrinos are accelerated to extremely high energies, says Younsi. “The magnetic fields are the bedrock of all of this. Anything which gives us more insight into how black holes and magnetic fields interact is just foundationally important for astrophysics.”

Younsi and his colleagues hope to take more images of Sgr A* with a larger network of telescopes and more advanced equipment, which will improve their understanding of the nature of the magnetic field and how it might be producing jets. The EHT will begin these observations in April, although the data is likely to take several years to process.

Being able to see the magnetic field of a supermassive black hole in such detail is incredibly rare, says Christine Done at Durham University, UK. “Magnetic fields are one of those things in astrophysics, kind of like saying the tooth fairy arrived.” As for the similarities between M87* and Sag A*, it may be that looking at both of them in greater detail could reveal differences that aren’t so apparent at the resolution we can see them at now, she says.

References: The Astrophysical Journal Letters, DOI: 10.3847/2041-8213/ad2df0 & DOI: 10.3847/2041-8213/ad2df1

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