For something that doesn’t emit as much light as we can detect, black holes like to hide themselves in glare. In fact, the brightest light in the universe comes from supermassive black holes. It’s not actually the black holes themselves; material around them as they actively siphon large amounts of matter from their immediate surroundings.

Among the brightest of these red-hot material swirls are galaxies known as blazars. Not only do they glow with the heat of the rotating sheet, but they also emit electromagnetic radiation of inconceivable energy, directing the material into “burning” rays radiating through space. Scientists have finally figured out the mechanism behind the incredibly high-energy light that reached us billions of years ago: shocks in the jets of a black hole that accelerate the speed of particles to incredible speeds.

“It’s a 40-year mystery that we’ve unraveled,” says astronomer Yannis Liodakis from ESO’s Finnish Center for Astronomy (FINCA). “We finally had all the pieces of the puzzle and the picture they created was clear.”

Most galaxies in the universe are built around a supermassive black hole. These surprisingly large objects are at the center of the galaxy and sometimes do little (like Sagittarius A*, the black hole in the center of the Milky Way) and sometimes a lot. This activity consists of material accumulation. A large cloud gathers in the equatorial disk around the black hole and swirls it like water around a sewer. The friction and gravitational interactions that occur in the extreme space surrounding a black hole cause this material to heat up and shine brightly at a range of wavelengths. This is one of the light sources of a black hole.

Another is that involved in blazars, twin jets of material ejected perpendicular to the disk from polar regions outside the black hole. Rather than falling towards the black hole, these jets are thought to be material from the inner edge of the disk accelerating toward the poles along the outer magnetic field lines, and ejected from there at very high speeds, close to its velocity. light. .

To classify a galaxy as a blazar, these jets must be directed almost directly at the viewer. This is us on Earth. Due to the extraordinary acceleration of particles, they shine with light across the electromagnetic spectrum, including gamma and high-energy X-rays.

How exactly this jet accelerated particles to such high velocities remained a huge cosmic question mark for decades. But now, a powerful new X-ray telescope called the Imaging X-ray Polarimetry Explorer (IXPE), launched in December 2021, has given scientists the key to solving the mystery. It was the first space telescope to detect the direction or polarization of X-rays.

“The first X-ray polarization measurements of sources in this class allowed for the first time a direct comparison with models developed from observations of other light frequencies, from radio to very high-energy gamma rays,” says Italian astronomer Immacolata Donnarumma. Space Agency.

IXPE looked at the brightest high-energy object in our sky, a blazar called Markarian 501 located 460 million light-years away in the constellation of Hercules. The telescope collected data on X-ray light emitted by the blazar jet for a total of six days in March 2022. Image showing IXPE’s observation of the Markarian 501 as it loses energy as it moves away from the light shock front. (Pablo Garcia/NASA/MSFC)

At the same time, other observatories were measuring light in other wavelength ranges, from radio to optics, which was the only data previously available for the Markarian 501. The team soon noticed an interesting difference in the X-ray light. Its orientation was significantly more bent or polarized than lower energy waves. And optical light became more polarized than radio frequencies.

However, the polarization direction was the same for all wavelengths and was consistent with the direction of the jet. The team found that this is consistent with models where shocks in jets create shock waves that provide additional acceleration along the length of the jet. This acceleration, which is closest to the impact, is greatest and generates the X-ray emission. During the ejection, the particles lose energy creating lower-energy optical radiation followed by radio radiation with less polarization.

“As the shock wave passes through the region, the magnetic field gets stronger and the energy of the particles increases,” says Boston University astronomer Alan Marsher. “The energy comes from the energy of motion of the material that makes up the shock wave.”

It’s not clear what creates the shocks, but one possible mechanism is that the faster material in the jet traps the slower moving clumps, leading to collisions. Future research may help confirm this hypothesis.

This research is a very important piece of the puzzle, as blazars are among the most powerful particle accelerators in the universe and one of the best laboratories for understanding extreme physics. Future studies will continue to observe the Markarian 501 and turn the IXPE to other blazars to see if similar polarization can be detected.