Black holes simply adore engulfing themselves in radiance, which is amazing considering that they cannot be seen to emit any light.
Supermassive black holes in fact produce some of the universe’s brightest light. The matter around black holes, which aggressively slurp down enormous amounts of matter from their immediate surrounds, is what causes the problem, not the black holes themselves.
Blazar galaxies are some of these hot, churning, maelstrom-like masses that are brightest. They emit electromagnetic radiation at energies that are difficult to comprehend, and in addition to glowing with the heat of a whirling coat, they can also channel material into “blazing” beams that go beyond space.
The mechanism behind the extraordinary high-energy light that reached Earth billions of years ago has finally been identified by scientists: shocks in the black hole’s jets that cause particles to travel at amazing speeds.
“This is a 40-year-old mystery that we’ve solved,” says astronomer Yannis Liodakis of the Finnish Centre for Astronomy with ESO (FINCA). “We finally had all of the pieces of the puzzle, and the picture they made was clear.”
A supermassive black hole is the center of the vast majority of galaxies in the universe. These enormous celestial bodies are located in the galactic center, where they occasionally perform relatively little work (like Sagittarius A*, the Milky Way’s central black hole), and occasionally perform a great deal.
That activity entails the accumulation of matter. A huge cloud forms an equatorial disk that encircles the black hole like water does a drain. This material heats up and shines brilliantly across a range of wavelengths due to the frictional and gravitational interactions at work in the extreme vacuum surrounding a black hole. That is one place where a black hole gets its light.
The other, which also occurs in blazars, is a pair of material jets that are fired perpendicular to the disk from the polar regions outside the black hole. These jets are believed to be made of material from the inner rim of the disk that, rather than collapsing into the black hole, is propelled to the poles by acceleration along lines of the external magnetic field at speeds that are nearly as fast as light.
These jets must be virtually directly aimed at the observer for a galaxy to be categorized as a blazar. We are that on Earth. They emit light over the electromagnetic spectrum, including high-energy gamma- and X-rays, as a result of the intense particle acceleration.
For many years, it has been unclear exactly how this jet accelerates the particles to such high speeds. But today, scientists have the solution thanks to the Imaging X-ray Polarimetry Explorer (IXPE), a potent new X-ray telescope that was launched in December 2021. It is the first space telescope to show how X-rays are oriented, or polarized.
According to astronomer Immacolata Donnarumma of the Italian Space Agency, “the first X-ray polarization measurements of this class of sources allowed, for the first time, a direct comparison with the models developed from observing other frequencies of light, from radio to very high-energy gamma rays.”
IXPE was pointed at the Markarian 501 blazar, which is 460 million light-years away in the constellation of Hercules and is the brightest high-energy object in our sky. The telescope recorded data on the X-ray photons generated by the blazar’s jet for a total of six days in March 2022.
The light from other wavelength ranges, from radio to optical, was being measured simultaneously by other observatories, which were previously the only sources of information for Markarian 501.
The group picked up on a strange change in the X-ray light right away. Compared to the lower-energy wavelengths, its polarization was noticeably more twisted. And radio frequencies were less polarized than optical light.
However, the polarization’s orientation was consistent across all wavelengths and coincided with the jet’s path. The scientists discovered that this is in line with theories in which shocks in the jets result in shockwaves that give the jet additional acceleration throughout its length. This acceleration is the greatest right before the shock, which results in X-radiation. The particles lose energy as they travel further along the jet, creating lower-energy optical and later radio emissions with less polarization.
“As the shock wave crosses the region, the magnetic field gets stronger, and the energy of particles gets higher,” says astronomer Alan Marscher of Boston University. “The energy comes from the motion energy of the material making the shock wave.”
The cause of the shocks is unknown, although one potential mechanism is that faster jet material catches up to slower-moving clumps, causing collisions. Future investigations might support this theory.
This study adds a significant element to the puzzle because blazars are among the Universe’s most potent particle accelerators and one of the best labs for studying extreme physics.
In the future, observations of Markarian 501 will continue, and IXPE will be directed to other blazars to determine if similar polarization can be found.