When astronomers look deep into space, black holes often appear as inscrutable giants, swallowing everything that crosses their path. But a new study has finally revealed details about an elusive feature of these cosmic entities: their superheated corona. Using data from NASA’s IXPE (Imaging X-ray Polarimetry Explorer) mission, researchers have, for the first time, mapped the shape of this plasma region, helping to clarify how black holes consume surrounding matter.
These new findings show that these coronas, regions of gas heated to billions of degrees, align with the accretion disks that swirl matter into the black holes. The breakthrough offers new insights into the feeding mechanisms of black holes, from stellar-mass systems like Cygnus X-1 to supermassive giants at the centers of distant galaxies.
The Mysterious Glow of a Black Hole’s Corona
If you’ve ever witnessed a solar eclipse, you might recall the Sun’s corona — its outermost atmosphere — a halo of glowing light visible only when the bright disk is obscured. Black holes, it turns out, have their own version of this corona, albeit far more intense.
For years, scientists had a theoretical understanding that black holes, much like stars, possess superheated coronae — except they’re on overdrive. These regions consist of hot, ionized gas that flares to ungodly temperatures in the order of billions of degrees Celsius. For comparisson, the Sun’s corona is about one to two million degrees Celsius.
But observing such a corona directly posed a significant challenge. The accretion disks around black holes, formed by matter spiraling inward, shine so brightly that they overwhelm the faint glow of the corona.
“Scientists have long speculated on the makeup and geometry of the corona,” explained Lynnie Saade, the study’s lead author and a postdoctoral researcher at NASA’s Marshall Space Flight Center. “Is it a sphere above and below the black hole, or an atmosphere generated by the accretion disk, or perhaps plasma located at the base of the jets?”
The answer, it seems, lies in using X-ray polarization. By examining how X-rays scatter and align, scientists can map structures that are otherwise invisible. Using IXPE’s highly sensitive instruments, Saade’s team analyzed the high-energy X-rays emitted from a dozen black holes. These include well-known stellar-mass systems like Cygnus X-1 and Cygnus X-3 in our Milky Way, along with supermassive black holes in distant galaxies.
Revealing the Geometry of Black Hole Accretion
After decades of research and thousands of papers, astronomers have pieced together a general model of black holes. Surrounding a black hole is often a torus — a donut-shaped region of gas and dust. At the heart of this chaotic structure lies an accretion disk, a swirling plane of superheated material spiraling toward oblivion. Above and below this disk, jets of ionized gas can shoot out into space at nearly the speed of light.
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But where does the corona fit into this picture? Thanks to IXPE’s new observations, scientists now believe that the corona aligns with the plane of the accretion disk, rather than encircling the black hole in a spherical cocoon.
“IXPE demonstrated that, among all black holes for which coronal properties could be directly measured via polarization, the corona was found to be extended in the same direction as the accretion disk,” said Saade. These findings ruled out earlier models suggesting that the corona resembles a lamppost hovering above the disk.
This new understanding also sheds light on how black holes consume matter and power their jets. The researchers found that regardless of the black hole’s size — whether it’s a stellar-mass black hole 10 times the mass of our Sun or a supermassive black hole millions of times that size — the geometry of the accretion disk and corona appears to be strikingly similar.
Black Hole Diets
Stellar-mass black holes, like Cygnus X-1, feed by ripping gas from their companion stars. Meanwhile, supermassive black holes, like those in galaxies NGC 1068 and NGC 4151, feast on gas and stars in their galactic centers. Yet, despite these differences in how they feed, both types seem to produce coronas and accretion disks that look alike. Their differences just come down to scale.
“Stellar-mass black holes rip mass from their companion stars, whereas supermassive black holes devour everything around them,” said Philip Kaaret, principal investigator for the IXPE mission. “Yet the accretion mechanism functions much the same way.”
This suggests that insights gleaned from studying closer, smaller black holes may help scientists unlock the secrets of their far more massive, distant cousins. It’s an exciting prospect that could bridge our understanding of black holes across vastly different scales.
The findings were reported in The Astrophysical Journal.
