Some of the brightest light in the universe even comes from supermassive black holes. Well, not actually the black holes themselves; it is the material around them as they actively suck up massive amounts of matter from their immediate environment.
Among the brightest of these maelstroms of swirling hot material are galaxies known as blazars. Not only do they glow with the heat of a swirling jacket, but they also channel material into “flaming” beams that buzz through the cosmos, giving off electromagnetic radiation with energies that are difficult to fathom.
Scientists have finally discovered the mechanism that produces the incredible high-energy light that reached us billions of years ago: Shocks in the black hole‘s jets that increase the speed of particles to amazing speeds.
“This is a 40-year-old mystery that we have solved,” says astronomer Yannis Liodakis of the Finnish Center for Astronomy with ESO (FINCA). “We finally had all the pieces of the puzzle and the picture they made was clear.”
Most galaxies in the universe are built around a supermassive black hole. These mind-bogglingly large objects reside in the galactic center and sometimes do very little (such as Sagittarius A*the black hole in the heart of the Milky Way) and sometimes does a lot.
This activity consists of the growth of material. A huge cloud gathers in an equatorial disk around the black hole and, as it were, circles around it water around a drain. The frictional and gravitational interactions at play in the extreme space around a black hole cause this material to heat up and shine brightly across a range of wavelengths. That is a source of light from a black hole.
The other – the one that plays in blazars – are two jets of material launched from the polar regions outside the black hole, perpendicular to the disk. These jets are believed to be material from the inner rim of the disk that, instead of falling toward the black hole, is accelerated along external magnetic field lines to the poles, where it is launched at very high velocities, close to the speed of the light. .
To classify a galaxy as a blazar, these jets must be aimed almost directly at the viewer. That’s us, on Earth. Thanks to extreme particle acceleration, they beam with light across the entire electromagnetic spectrum, including high-energy gamma and X-rays.
How exactly this jet accelerates the particles to such high speeds has been a huge cosmic question mark for decades. But now there’s a powerful new X-ray telescope called the Imaging X-ray Polarimetry Explorer (IXPE), launched in December 2021, gave scientists the key to solving the mystery. It is the first space telescope to reveal the orientation or polarization of X-rays.
“The first X-ray polarization measurements from this class of sources allowed direct comparison for the first time with the models developed by observing other frequencies of light, from radio to very high energy gamma rays,” says astronomer Immacolata Donnarumma of the Italian Space Agency.
IXPE was flipped to the brightest high-energy object in our sky, a blazar called Markarian 501, 460 million light-years away in the constellation of Hercules. In March 2022, the telescope spent a total of six days collecting data on the X-ray light emitted by the blazar’s jet.
At the same time, other observatories measured light from other wavelength ranges, from radio to optical, that had previously been the only data available for Markarian 501.
The team soon noticed a curious difference in the X-ray light. The orientation was significantly more warped or polarized than the lower energy wavelengths. And the optical light was more polarized than the radio frequencies.
However, the direction of polarization was the same for all wavelengths and aligned with the direction of the beam. This, the team found, is consistent with models in which impacts in the jets produce shock waves that provide additional acceleration along the length of the jet. Closest to the shock, this acceleration is highest and produces X-rays. Further down the beam, the particles lose energy, producing lower energy optical and then radio emission, with lower polarization.
“As the shock wave traverses the region, the magnetic field gets stronger and the energy of particles gets higher,” says astronomer Alan Marscher from Boston University. “The energy comes from the kinetic energy of the material that creates the shock wave.”
It’s not clear what causes the jerks, but one possible mechanism is that faster material in the jet overtakes slower-moving clumps, resulting in collisions. Future research could help confirm this hypothesis.
Since blazars are among the most powerful particle accelerators in the universe, and one of the best labs for understanding extreme physics, this research marks a pretty important piece of the puzzle.
Future research will continue to observe Markarian 501 and switch IXPE to other blazars to see if similar polarization can be detected.
The research has been published in Nature Astronomy.