Blazars are active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight. Most of the light from these extremely luminous objects is produced by high-energy particles. Although the jets are known to be ultimately powered by a supermassive black hole, how these particles are accelerated to such high energies has been an unanswered question. The Imaging X-Ray Polarimetry Explorer (IXPE), a collaboration between NASA and the Italian Space Agency, has helped astronomers get closer to an answer.
This illustration shows the IXPE spacecraft (right) observing the blazar Markarian 501 (left). A blazar is a black hole surrounded by a disk of gas and dust with a bright jet of high-energy particles pointed toward Earth. The inset illustration shows high-energy particles in the jet (blue). When the particles hit the shock wave, depicted as a white bar, the particles become energized and emit X-rays as they accelerate. Moving away from the shock, they emit lower-energy light: first visible, then infrared, and radio waves. Farther from the shock, the magnetic field lines are more chaotic, causing more turbulence in the particle stream. Image credit: NASA / Pablo Garcia.
“This is a 40-year-old mystery that we’ve solved. We finally had all of the pieces of the puzzle, and the picture they made was clear,” said Dr. Yannis Liodakis, an astronomer at the Finnish Centre for Astronomy (FINCA).
In the study, Dr. Liodakis and colleagues used IXPE to observe X-rays from Markarian 501 (Mrk 501), a blazar located 483 million light-years away in the constellation of Hercules.
They watched the blazar for three days in early March of 2022, and then again two weeks later.
During these observations, they also used other space- and ground-based telescopes to gather information about Mrk 501 in a wide range of wavelengths of light including radio, optical, and X-ray.
While other studies have looked at the polarization of lower-energy light from blazars in the past, this was the first time scientists could get this perspective on a blazar’s X-rays, which are emitted closer to the source of particle acceleration.
“Adding X-ray polarization to our arsenal of radio, infrared, and optical polarization is a game changer,” said Dr. Alan Marscher, an astronomer at Boston University.
The study authors found that X-ray light is more polarized than optical, which is more polarized than radio.
But the direction of the polarized light was the same for all the wavelengths of light observed and was also aligned with the jet’s direction.
After comparing their information with theoretical models, they realized that the data most closely matched a scenario in which a shock wave accelerates the jet particles.
A shock wave is generated when something moves faster than the speed of sound of the surrounding material, such as when a supersonic jet flies by in our Earth’s atmosphere.
“As the shock wave crosses the region, the magnetic field gets stronger, and energy of particles gets higher. The energy comes from the motion energy of the material making the shock wave,” Dr. Marscher said.
“As particles travel outward, they emit X-rays first because they are extremely energetic.”
“Moving farther outward, through the turbulent region farther from the location of the shock, they start to lose energy, which causes them to emit less-energetic light like optical and then radio waves.”
“This is analogous to how the flow of water becomes more turbulent after it encounters a waterfall — but here, magnetic fields create this turbulence.”
This research is described in a paper in the journal Nature.
I. Liodakis et al. 2022. Polarized blazar X-rays imply particle acceleration in shocks. Nature 611, 677-681; doi: 10.1038/s41586-022-05338-0
Source : Breaking Science News