The magnetic field surrounding the supermassive black hole at the centre of the Milky Way has been observed for the first time. Astronomers using the Event Horizon Telescope (EHT) have been surprised by the orderly nature of the field, which exists in the extremely violent environment surrounding the black hole Sagittarius A*. The study could lead to a better understanding of the crucial role that the magnetic field plays in how the black hole feeds on surrounding matter.

This is the second time that the EHT has observed the magnetic field of a supermassive black hole. In 2021 it detected the field of the black hole at the centre of the galaxy Messier 87 (M87).

Supermassive black holes are believed to be surrounded by plasma that is swirling into the gravitational abyss. This creates a powerful magnetic field, which can then interact with the infalling material. This accelerating material emits copious amounts of radiation including radio waves that are polarized by the local magnetic field.

Global network

The EHT is a global network of radio telescopes that can measure this polarization and therefore map-out the magnetic field surrounding a black hole.

Sagittarius A* weighs in at about 6.6 million solar masses – which is one thousand times less massive than the gargantuan M87. Despite this huge difference, EHT astronomers were surprised by the similarity of the two objects’ magnetic fields.

“We expected to find some signature of the magnetic field simply because we know Sagittarius A* is still feeding, just very slowly,” says Ziri Younsi of University College London, who is a member of the EHT team. “What we didn’t anticipate was that the pattern of the polarization would be so similar in morphology to M87.”

All supermassive black holes that accrete matter are expected to have a magnetic field that is embedded in their accretion discs. The field is anchored in the plasma just outside the event horizon and is then amplified by the rotation of the black hole. The M87 black hole is very active with a large accretion disc of plasma, compared to Sagittarius A*.

Controlling the flow

The magnetic fields of both objects have magnetic field lines in vortex-like configurations (see figure). The closer the lines are to each other, the stronger and more organized the magnetic field is. Younsi estimates that the magnetic field strength of Sagittarius A* is on par with that of a refrigerator magnet. While that might not sound like much, it is strong enough to affect the inflow of accreting plasma – thereby helping to control how the black hole feeds.

The apparent similarity in the structures of the two magnetic fields has some astronomers wondering about other possible similarities.

M87’s black hole is notably for its relativistic jet. This is a tightly collimated beam of particles that are swept up from the accretion disc by the magnetic field and accelerated outwards to close to the speed of light. One jet is visible along the object’s axis of rotation and it is possible that another extends in the opposite direction.

Given the similarity in magnetic structure, it is possible that Sagittarius A* could also host relativistic jets that have thus far gone undetected.

Mysterious bubbles

Indeed, such jets could be the source of the Milky Way’s mysterious Fermi Bubbles. These are two huge plumes of charged particles that rise 25,000 light–years above and below the plane of the galaxy. Estimated to be just a few million years old, they originate from the galactic centre, but their cause is uncertain.

However, Younsi points out that a jet is highly collimated, whereas the Fermi Bubbles span a wider area and are almost like an explosion. And while he considers the similarities between the two black holes as “curious”, Younsi tells Physics World of his scepticism that our galaxy’s black hole has a jet.

“One could take some liberty and over-interpret this and say maybe it’s evidence that there could be a jet,” he says. “Or it could be that we need to have better data in the future at higher resolution and maybe we’ll see that the polarization pattern changes a little bit.”

Rapid change

M87 is 53 million light–years away, and its black-hole accretion disc is huge, so those two factors mean we do not see it change very much over short time frames. Sagittarius A* is much closer to us at a distance of about 26,000 light–years, and its far smaller accretion disc means that the EHT can see the accretion disc changing over the course of minutes and hours.

The first image of Sagittarius A* (brightness, not polarization), released in 2022, was therefore, a time-averaged view of the black hole, and Younsi points out that it could just be a coincidence that the time-averaged image of the magnetic field looks similar to M87, meaning searches for jets could be futile.

“Sagittarius A* is changing very rapidly, so there is a lot more uncertainty in the structure seen in the image,” says Younsi. “We need some long-term monitoring, because what we’re looking at right now could just be a fluke that happens to look like M87 and actually it’s not representative of the general time-averaged state. It could be that this image changes a lot in the next few years.”

Weather permitting, the EHT observes Sagittarius A* every year, most recently this April. It is also continuing to keep tabs on M87’s black hole and is trying to detect supermassive black holes in other galaxies. The more black holes are observed, the more we will know whether Sagittarius A* and M87’s black hole really are typical examples.

The observations are described in two papers in the The Astrophysical Journal Letters. One paper covers the polarization measurements and the other describes their implications.