Blazars are some of the brightest celestial bodies. They consist of a supermassive black hole that can produce two strong jets perpendicular to the disc on either side, feeding off material that is swirling around it in a disc.
This illustration shows NASA’s IXPE spacecraft, at right, observing blazar Markarian 501, at 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
A blazar is particularly bright because one of its strong jets of fast particles is aimed directly at Earth. How the particles in these jets become accelerated to such high energies has long been a mystery to scientists.
Astronomers have advanced in their search for an answer with the help of NASA’s Imaging X-Ray Polarimetry Explorer, or IXPE. An extensive international team of astronomers has published a new study in the journal Nature that asserts that a shock wave within the jet provides the most plausible explanation for particle acceleration.
This is a 40-year-old mystery that we have solved. We finally had all of the pieces of the puzzle, and the picture they made was clear.
Yannis Liodakis, Study Lead Author and Astronomer, Finnish Centre for Astronomy
The Earth-orbiting IXPE satellite, a joint project of NASA and the Italian Space Agency, was launched on December 9th, 2021, and it offers a unique type of data that has never before been available from space.
IXPE detects the average direction and strength of the electric field of the light waves that make up X-Rays, and the new data also includes the measurement of the polarization of X-Ray light. Since the atmosphere absorbs X-Rays from space, telescopes on Earth are unable to determine the orientation of the electric field and the degree of polarization.
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 will continue to provide new evidence as the current data is analyzed and additional data is acquired in the future.
Immacolata Donnarumma, Researcher, High Energy Astrophysics, Italian Space Agency
The new study focused on Markarian 501, a blazar in the constellation Hercules, using data from IXPE. This massive elliptical galaxy’s active black hole system is located in its center.
Early in March 2022, IXPE observed Markarian 501 for three days before returning two weeks later. Other telescopes in orbit and on Earth were utilized by astronomers during these observations to collect data on the blazar in a variety of light spectrums, including radio, optical, and X-Ray.
This was the first time researchers could obtain this viewpoint on a blazar’s X-Rays, which are emitted closer to the source of particle acceleration. While other studies have previously examined the polarization of lower-energy light from blazars, this was the first time scientists could.
Adding X-ray polarization to our arsenal of radio, infrared, and optical polarization is a game changer.
Alan Marscher, Astronomer, Boston University
Researchers discovered that X-Ray light is more polarized than optical and that radio light is more polarized than both. The polarized light, however, was always pointed in the same general direction and coincided with the jet’s direction for all wavelengths of light that were observed.
The team of astronomers discovered after comparing their data with theoretical models that the data most closely matched a scenario in which a shock wave accelerates the jet particles. A shock wave is created when an object moves faster than the surrounding material’s sound speed, such as when a supersonic jet passes by in the Earth’s atmosphere.
The study was not intended to look into the still-unknown causes of shock waves. However, researchers believe that a change in the jet’s flow is what causes a portion of it to accelerate above the speed of sound. Both abrupt pressure changes at the jet boundary and high-energy particle collisions within the jet could cause this.
Marscher added, “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.”
Particles emit X-rays first because of their high energy as they move outward. They begin to lose energy as they move farther away from the shock, through the turbulent region, and as a result, they start to emit less-energetic light, such as optical and then radio waves.
Similar to how a waterfall causes the flow of water to become more turbulent, but in this case, the turbulence is caused by magnetic fields.
To determine whether the polarization of the Markarian 501 blazar changes over time, researchers will keep observing it. Throughout its two-year prime mission, IXPE will also look into a larger group of blazars, uncovering more enduring cosmological mysteries.
“It is part of humanity’s progress toward understanding nature and all of its exoticness,” further stated Marscher.
Liodakis, I., et al. (2022) Polarized blazar X-rays imply particle acceleration in shocks. Nature. doi:10.1038/s41586-022-05338-0