The magnetic field swirling around an enormous black hole, located about 55 million light-years from Earth, has unexpectedly switched directions. This dramatic reversal challenges theories of black hole physics and provides scientists with new clues about the dynamic nature of these shadowy giants.
The supermassive black hole, nestled in the heart of the M87 galaxy, was first imaged in 2017. Those images revealed, for the first time, a glowing ring of plasma — an accretion disk — encircling the black hole, dubbed M87*. At the time, the disk’s properties, including those of the magnetic field embedded in the plasma, matched theoretical predictions.
But observations of the accretion disk in the years that followed show that its magnetic field is not as stable as it first seemed, researchers report in a paper to appear in Astronomy & Astrophysics. In 2018, the magnetic field shifted and nearly disappeared. By 2021, the field had completely flipped direction.
“No theoretical models we have today can explain this switch,” says study coauthor Chi-kwan Chan, an astronomer at Steward Observatory in Tucson. The magnetic field configuration, he says, was expected to be stable due to the black hole’s large mass — roughly 6 billion times as massive as the sun, making it over a thousand times as hefty as the supermassive black hole at the center of the Milky Way.
The accretion disk is the first stop for material falling into any black hole. There, material becomes superheated as friction and gravitational forces squish and squeeze it. The blazingly hot material emits X-rays and radio waves that telescopes on Earth can detect. Since the black hole itself emits no light, imaging the accretion disk is one of the best ways to study a black hole.
In the new study, astronomers analyzed images of the accretion disk around M87* compiled by the Event Horizon Telescope, a global network of radio telescopes. The scientists focused on a specific component that’s sensitive to magnetic field orientation called polarized light, which consists of light waves all oscillating in a particular direction.
By comparing the polarization patterns over the years, the astronomers saw that the magnetic field reversed direction. Magnetic fields around black holes are thought to funnel in material from their surrounding disks. With the new findings, astronomers will have to rethink their understanding of this process.
While researchers don’t yet know what caused the flip in this disk’s magnetic field, they think it could have been a combination of dynamics within the black hole and external influences.
“I was very surprised to see evidence for such a significant change in M87’s magnetic field over a few years,” says astrophysicist Jess McIver of the University of British Columbia in Vancouver, who was not involved with the research. “This changes my thinking about the stability of supermassive black holes and their environments.”