Polarized light reveals secrets of black hole's magnetic field
Two years ago, astronomers revealed the first-ever direct images of a black hole. Now, the collaboration behind the historic image has released a new version that shows the polarization of the light around the object.
Black holes are notoriously hard to see – after all, light itself can’t escape from their extreme gravitational pull. But there is a way to at least see their silhouette, thanks to the intense environment these objects create around themselves. As black holes pull in huge amounts of dust and gas, this material heats up and glows brightly in what’s called an accretion disc, leaving a circular shadow in the center.
And that’s just what the Event Horizon Telescope (EHT) Collaboration captured in the image released in April 2019. That now-iconic image shows the supermassive black hole at the center of the galaxy M87, located in the Virgo cluster about 55 million light-years away.
Now, the collaboration has conducted follow-up analysis on the data to measure the polarization of the light across the ring. Light becomes polarized as it passes through magnetic fields, which are present around a black hole. So by measuring the orientation of the polarized light, astronomers can get a better understanding of what’s going in this enigmatic environment.
"Polarization is a powerful tool available to astronomers to probe the physical conditions in one of the most extreme environments in the universe,” says Colin Lonsdale, chair of the Event Horizon Telescope Board. “It can provide clues not only to the strength and orientation of magnetic fields, but also how well-ordered those fields are, and possibly even something about the otherwise invisible material that lies between us and the material that is emitting the radio waves.”
Specifically, the team was looking to learn more about the powerful jets that some of these supermassive black holes emit. For such infamously ravenous objects, it’s unclear why huge amounts of material would be thrown off instead of sucked in. By observing the polarization of light near the event horizon, and then running models to find the best fit, the team found that strongly magnetized gas could explain these jets.
“New polarization images suggest that the powerful jet is formed by plasma flow arrested by aligned magnetic fields in the vicinity of the black hole, resisting its strong gravitational pull,” says Kotaro Moriyama, a researcher on the team.