We finally know how black holes produce the brightest light in the universe: ScienceAlert

For something that does not emit light that we can detect, black holes they just like to mask themselves in a glow.

Some of the brightest lights in the universe actually come from supermassive black holes. Well, not actually the black holes themselves; it is the material around them as they actively siphon vast amounts of matter from their immediate environment.

Among the brightest of these eddies of spinning hot material are galaxies known as blazars. Not only do they glow with the heat of a spinning envelope, but they channel the material into “flaming” rays that sweep through space, emitting electromagnetic radiation at energies that are hard to fathom.

Scientists finally understand the mechanism producing the incredible high-energy light that reaches us billions of years ago: Strikes in Black holejets that increase the speed of particles to mind-boggling speeds.

“It’s a 40-year-old mystery we’ve solved,” says astronomer Yanis Liodakis of the Finnish Center for Astronomy at ESO (FINCA). “We finally got all the pieces of the puzzle and the picture they painted was clear.”

Most of the galaxies in the universe are built around a supermassive black hole. These mind-bogglingly large objects sit at the galactic center, sometimes doing very little (eg Sagittarius A*the black hole at the heart of the Milky Way) and sometimes it does a lot.

This activity consists of accumulating material. A huge cloud gathers into an equatorial disk around the black hole, orbiting it as water around drain. The interactions of friction and gravity in the extreme space surrounding a black hole cause this material to heat up and glow brightly over a range of wavelengths. This is one source of light from a black hole.

The other – the one at play in blazars – are twin jets of material shot out from the polar regions outside the black hole, perpendicular to the disk. These jets are thought to be material from the inner edge of the disc that, instead of falling towards the black hole, is accelerated along the lines of the external magnetic field towards the poles, where it is shot out at very high speeds close to the speed of light.

For a galaxy to be classified as a blazar, these jets must be almost directly pointed at the viewer. This is us, on Earth. Thanks to the extreme acceleration of the particles, they glow with light across the entire electromagnetic spectrum, including high-energy gamma and X-rays.

How exactly this jet accelerates particles to such high speeds has been a huge cosmic question mark for decades. But now, a new powerful 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 of this class of sources allowed for the first time a direct comparison with models developed from observations of other frequencies of light, from radio to very high-energy gamma rays,” says astronomer Immaculate Donnarumma of the Italian Space Agency.

IXPE was addressed to the brightest high-energy object in our sky, a blazar called Markarian 501 located 460 million light years away in the constellation Hercules. For a total of six days in March 2022, the telescope collected data on the X-ray light emitted by the blazar jet.

At the same time, other observatories were measuring light from other wavelength ranges, from radio to optical, which had previously been the only data available for Markarian 501.

The team soon noticed a curious difference in the X-ray light. Its orientation was significantly more twisted or polarized than the lower-energy wavelengths. And optical light was more polarized than radio frequencies.

However, the direction of polarization was the same for all wavelengths and aligned with the direction of the jet. This, the team found, is consistent with models where shocks in jets produce shock waves that provide additional acceleration along the length of the jet. Closest to the shock, this acceleration is highest, producing X-ray radiation. Further along the jet, the particles lose energy, producing optical and then lower-energy, less-polarized radio radiation.

“When the shock wave crosses the region, the magnetic field becomes stronger and the energy of the particles becomes higher,” says astronomer Alan Marcher from Boston University. “The energy comes from the energy of motion of the material creating the shock wave.”

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

Since blazars are among the most powerful particle accelerators in the universe and some of the best laboratories for understanding extreme physics, this research marks a pretty important piece of the puzzle.

Future studies will continue to observe Markarian 501 and point the IXPE at other blazars to see if similar polarization can be detected.

The study was published in Natural astronomy.