Strong Magnetic Fields Swirl Near Milky Way’s Black Hole

The Event Horizon Telescope collaboration has released another image of our galaxy’s central black hole, Sagittarius A* — this time revealing the behavior of the magnetic fields that thread the surrounding gas. The results appear today in two papers in Astrophysical Journal Letters.

The images that the EHT produces are reconstructions of the black hole’s silhouette, a dark shadow framed by the glow of light that’s evaded the event horizon. The team has so far imaged both Sgr A* and M87*, the behemoth squatting at the center of the elliptical galaxy M87 in Virgo. Both the new image and the previous ones are based on observations taken in 2017 by the EHT’s worldwide network of radio telescopes, the data scrupulously spliced to reveal glimmers of what a planet-size telescope would see.

The radio emission that the EHT observes is called synchrotron radiation. It’s emitted by electrons corkscrewing along magnetic field lines — think of it like the particles’ scream of glee as they whiz along.

By nature, this light is polarized. Light is an electromagnetic wave, and it oscillates perpendicularly to the direction of motion. Usually, the oscillation’s orientation fluctuates randomly with time. But when light is polarized, the orientation is fixed, whether in a single plane or rotating in a predictable way.

As electrons spiral around magnetic field lines, the particles emit photons in directions perpendicular to the field. Thus the amount and pattern of polarization in the emission provides key information about the magnetic fields, enabling astronomers to map the fields’ structure and strength. That matters because magnetic fields are powerful players in the environment around an accreting black hole — more so than astronomers had appreciated, the new results confirm.

The polarization detected in Sgr A*’s emission indicates that the magnetic fields in its accretion flow aren’t the tangled mess once expected. Rather, they’re strong, twisted, and orderly, producing a notably strong polarization signal.

Fields like these can control and even choke the gas flowing into a black hole, manipulating the black hole’s diet. They can also serve as a pipeline for outflowing plasma jets. This kind of accretion environment is called a magnetically arrested disk, or MAD. Analysis of the first Sgr A* images had suggested the black hole might be MAD, but the team needed to unpack the polarization data to confirm.

EHT astronomers found signs of a MAD environment around M87*, too. That both black holes would be surrounded by strong, orderly magnetic fields surprised astronomers back in 2022 when the first Sgr A* image came out. These two black holes are seemingly polar opposites: Sgr A* is a quiet, finicky cat lapping from a trickle of gas; M87* is a roaring lion that shoots out plasma jets thousands of light-years long. (In fact, the jet pointing toward us is so prominent that it was discovered in 1918.)

It could be that strong, orderly magnetic environments are a universal feature of the gas skirting supermassive black holes. Theoretically, such fields should also power a jet. Observations have yet to find one from Sgr A*, but data like these raise hopes that a jet exists, even if only a stubby one.

Using a variety of simulations and calculations, the team speculated on what the polarization data may mean specifically for the properties of Sgr A*. Of the models tested, the only one that works suggests the black hole is spinning nearly as fast as physically possible and is tilted away from us such that we’re looking up at its chin.

Previous measurements, both by EHT astronomers and others, have also hinted that the black hole is leaning. But we’ve known little about its spin. The few supermassive black holes for which astronomers have calculated a spin all whirl similarly fast, but they’re all actively gobbling down gas, which can spin up a black hole like water from a hose shot at a ball. There was no particular reason to expect Sgr A* would spin quickly, too — especially since black holes likely power jets with their spin, and there’s no jet to be seen.

The team warns, however, that these conclusions about tilt and spin are based on simplified simulations, and “values inferred from our best-bet model should not be interpreted as measurements.” They’ll need more data, and the planned addition of higher-frequency observations, to know more.

References:

The Event Horizon Telescope Collaboration. “First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring.” Astrophysical Journal Letters. March 26, 2024.

The Event Horizon Telescope Collaboration. “First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring.” Astrophysical Journal Letters. March 26, 2024.