For something that doesn’t emit light that we can detect, black holes love to veil themselves in radiance.
In fact, some of the brightest light in the universe comes from supermassive black holes. Well, not actually the black holes themselves; It is the matter around them as they actively scatter huge amounts of matter from their immediate surroundings.
Among the brightest of these swirling floats of hot material are galaxies known as blazars. Not only do they glow with the heat of the vortex shell, but they also channel matter into “flaring” beams that travel across the universe, emitting electromagnetic radiation at energies that are difficult to comprehend.
Scientists have finally discovered the mechanism for producing the astonishingly high-energy light reaching us billions of years ago: shocks in a black hole’s jets that accelerate particles to dizzying speeds.
“This is a 40-year-old mystery that has been solved,” says astronomer Yannis Liodakis of the Finnish Center for Astronomy with ESO (FINCA). “We finally got all the pieces of the puzzle in, 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 center of the galaxy, sometimes doing very little (like Sagittarius A*, the black hole at the heart of the Milky Way) and sometimes doing a lot.
This activity consists of cumulative material. A huge cloud gathers into an equatorial disk around the black hole, swirling it around like water around a drain. Frictional and gravitational interactions in the extreme space surrounding the black hole cause this matter to heat up and shine brightly across a range of wavelengths. This is one of the black hole’s light sources.
The other—which plays out in blazars—is twin jets of material shot 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 disk, and instead of falling towards the black hole, it is accelerated along the outer magnetic field lines to the poles, where it is shot at very high speeds, close to the speed of light.
To classify a galaxy as a plazar, these jets must be directed almost directly at the viewer. This is us on Earth. Thanks to the particles’ intense acceleration, they glow with light across the electromagnetic spectrum, including high-energy gamma rays and X-rays.
The exact way this jet accelerates particles to such high speeds has been a giant cosmological question mark for decades. But now, a powerful new X-ray telescope called the X-ray Imaging Explorer (IXPE), which was launched in December 2021, has given scientists the key to solving the mystery. It is the first space telescope to detect the direction 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 high-energy gamma rays,” says astronomer Immaculata Donnarumma. Italian Space Agency.
IXPE has been transformed into the brightest high-energy object in our sky, a blazar called Markarian 501, located 460 million light-years away in the constellation of Hercules. For six days in March 2022, the telescope collected data on the X-ray light emitted by the Blazar plane.
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 strange difference in the X-ray light. Their orientation was significantly more skewed or polarized than at lower energy wavelengths. Optical light was more polarized than radio frequencies.
However, the polarization direction was the same for all wavelengths and aligned with the plane orientation. The team found that this is consistent with models in which shocks in aircraft produce shock waves that provide additional acceleration along the jet. Closer to the shock, this acceleration is at its highest, producing x-rays. Along the plane, the particles lose energy, producing lower energy light and then radio emission, with less polarization.
“When the shock wave crosses the region, the magnetic field gets stronger, and the energy of the particles increases,” says astronomer Alan Marcher of Boston University. “The energy comes from the kinetic energy of the material creating the shock wave.”
It is not clear why the shocks occur, but one possible mechanism is for faster material in the jet to catch up with the slower-moving aggregates, creating collisions. Future research can help confirm this hypothesis.
Since blazars are among the most powerful particle accelerators in the universe, and one of the best laboratories for understanding extreme physics, this research is a very important piece of the puzzle.
Future research will continue to monitor Markarian 501, and refer IXPE to other blazars to see if similar polarizations can be detected.
Research published in natural astronomy.
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