For the first time, astronomers have confirmed the existence of an intense magnetic field surrounding the event horizon of a supermassive black hole. The much-anticipated discovery fits an important piece into our understanding of the way black holes affect the space around them.

Unsurprisingly, black holes are not easy to observe. However, we can detect their existence based in part on the powerful plasma jets the supermassive black holes spit out. These are directed along the rotation axis of the holes' accretion disks—the material on which they feed.

As the paper in Science notes, however, “The mechanism that forms such a jet and guides it over scales from a few light-days up to millions of light-years remains uncertain, but magnetic fields are thought to play a critical role.”

Until this paper, the only magnetic fields detected around black holes were relatively weak and several light-years from the hole itself. Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team from Chalmers University of Technology, Sweden, examined the light emerging near the event horizon of the black hole at the heart of galaxy PKS 1830-211.

They found the light to be strongly polarized. The polarization of light, used for 3D films and by certain animals, got a lot of attention last year when scientists thought they had found evidence for gravitational waves from the birth of the universe based on the way the cosmic background radiation is polarized. 

That research turned out to be wrong; a lot of things can induce polarization. One of these is powerful magnetic fields. “When produced naturally, polarization can be used to measure magnetic fields, since light changes its polarization when it travels through a magnetized medium,” says lead author Dr. Ivan Marti-Vidal.

“Of particular importance is the observation of the rotation measure, RM, defined as the change of polarization angle as a function of wavelength squared,” the authors write. “This quantity is directly related to the plasma density and the strength of the magnetic field along the line of sight.”

Much of the light emitted from the inner regions of the area around black holes is absorbed before reaching us, so it is only in the submillimeter wavelengths that we can detect signs of this polarization. ALMA, only in full operation for two years, represents the first instrument capable of detecting the polarization in this part of the electromagnetic spectrum with the sensitivity required.

The rotation detected is hundreds of times higher than anything previously observed. “At least a few tens of Gauss, and possibly much higher,” the authors conclude.

“In this case, the light that we detected with ALMA had been traveling through material very close to the black hole, a place full of highly magnetized plasma," says Marti-Vidal. Where previous observations struggled to see anything within a few light-years of black holes, the fields measured here were just a few light-days from the event horizon.

Understanding this field, the authors argue, will help explain both the operation of black hole accretion disks and the jets produced.