New image of black hole at Galactic center reveals swirling magnetic field

The black hole in the center of our galaxy has now also been photographed in polarized light. The photo reveals the magnetic field around Sagittarius A*.

A new image of Sagittarius A*, the supermassive black hole at the center of the Milky Way, reveals a swirling magnetic field around the dark goblin. This suggests that the black hole may be producing a previously invisible jet of high-energy material.

The photo was taken with the Event Horizon Telescope (EHT). This is a worldwide network of observatories that together function as one gigantic telescope.

In 2022, the EHT group unveiled the first image of Sagittarius A* (or Sgr A* for short). On it you see light that comes from swirling hot plasma. That plasma revolves around the dark region within the black hole’s event horizon, the place where light can no longer escape the extreme gravity.

Jet or no jet?

EHT researcher Ziri Younsi from University College London and his colleagues have now measured the polarization, or direction of vibration, of this light. This shows how the magnetic field around Sgr A* is oriented and how strong that field is. The results have been published in two articles in the scientific journal The Astrophysical Journal Letters.

The magnetic field of Sgr A* is remarkably similar to that of M87*, the first black hole imaged with the EHT. M87* is about 1500 times heavier than Sgr A*. According to Younsi, this indicates that supermassive black holes have similar structures regardless of their size.

A major difference between M87* and the black hole in our galaxy is that no visible high-energy jets emerge from Sgr A*. This has long puzzled astronomers. The fact that Sgr A* has a magnetic field similar to M87* indicates that the black hole in our galaxy may also produce a jet. “There may be something very exciting going on at the center of the galaxy,” Younsi says.

Astronomers have previously found evidence that SgrA* emitted a jet long ago. This is partly due to the Fermi bubbles: large plasma bubbles above and below the Milky Way that emit X-rays.

Tooth fairy

The properties of the magnetic field not only potentially reveal a hidden jet. They can also help solve other astrophysical mysteries. For example, it is unclear how certain cosmic particles are accelerated to extremely high energies. Younsi: ‘The magnetic fields form the foundation of all this. Anything that gives us more insight into how black holes and magnetic fields interact is of fundamental importance for astrophysics.’

With a larger network of telescopes and more advanced equipment, Younsi and his colleagues hope to be able to take more images of Sgr A*. That would give them more insight into the nature of the magnetic field and how it can produce jets. The EHT will start making these observations this month, but processing the data will probably take several years.

It is incredibly rare to be able to see the magnetic field of a supermassive black hole in such detail, says astronomer Christine Done of Durham University in the United Kingdom. “Magnetic fields are one of those things in astrophysics that you talk about a bit like you’re talking about the tooth fairy.”

Looking at both M87* and SgrA* in more detail, Done says, may reveal differences that aren’t as obvious at current resolution.