Blazars are some of the brightest objects in the sky. They are made up of a supermassive black hole that feeds off material swirling around it in a disk, creating two powerful jets perpendicular to the disk on each side.
A blazar is particularly bright because one of its strong jets of fast particles is directed 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 thanks to NASA’s Imaging X-Ray Polarimetry Explorer (IXPE). The best explanation for the particle acceleration, according to a recent study by a sizable international collaboration of astronomers, is a shock wave inside the jet.
“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 lead author Yannis Liodakis, astronomer at FINCA, the Finnish Centre for Astronomy with ESO.
X-ray Polarization Data
Illustration shows NASA’s IXPE spacecraft, at right, observing blazar Markarian 501, at left.
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.
Credit: NASA/Pablo Garcia
The Earth-orbiting IXPE satellite, a joint project of NASA and the Italian Space Agency, was launched on December 9, 2021. It offers a unique type of data that has never before been available from space.
This new data includes a measurement of the polarization of X-ray light – in other words, IXPE detects the average direction and intensity of the electric field of light waves that comprise X-rays. Because the Earth’s atmosphere soaks up X-rays from space, telescopes on Earth can’t tell how the electric field is oriented or how much polarization there is in X-ray light.
“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,”
said Immacolata Donnarumma, IXPE project scientist at the Italian Space Agency.
As the current data is analyzed and new data is gathered in the future, IXPE will continue to present fresh evidence.
Blazar Markarian 501
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.
Markarian 501 was observed by IXPE for three days in early March 2022, and then again two weeks later. During these observations, astronomers used other telescopes in space and on Earth to gather data on the blazar in a variety of light wavelengths, including radio, optical, and X-ray.
While previous research has looked at the polarization of lower-energy light from blazars, this was the first time scientists could get this perspective on blazar 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 Alan Marscher, who leads the group studying giant black holes with IXPE.
Shock Wave Origins
Researchers discovered that X-ray light is more polarized than optical light, which is more polarized than radio light. However, the polarized light’s direction was the same for all wavelengths of light observed and was also aligned with the jet’s direction.
When the astronomers compared their data to theoretical models, they discovered that the data most closely matched a scenario in which a shock wave accelerates the jet particles. When something moves faster than the speed of sound in the surrounding material, such as when a supersonic jet flies by in our Earth’s atmosphere, a shock wave is produced.
The study was not intended to help clarify the origins of shock waves, which remain a mystery. However, scientists believe that a disturbance in the jet’s flow causes a section of it to become supersonic. This could be caused by high-energy particle collisions within the jet or by sudden pressure changes at the jet’s boundary.
“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,”
Marscher said.
Exploring X-ray Exoticness
Because X-rays are extremely energetic, they are emitted first as particles travel outward.
Moving outward, through the turbulent region farther from the shock, they begin to lose energy, causing them to emit less-energetic light such as optical and then radio waves. This is similar to how the flow of water becomes more turbulent after passing through a waterfall, but in this case, magnetic fields cause the turbulence.
Scientists will continue to monitor the polarization of the Markarian 501 blazar to see if it changes over time. During its two-year prime mission, IXPE will also investigate a broader collection of blazars, attempting to solve more long-standing mysteries about the universe.
“It’s part of humanity’s progress toward understanding nature and all of its exoticness,”
Marscher said.
Reference: Liodakis, I., Marscher, A.P., Agudo, I. et al. Polarized blazar X-rays imply particle acceleration in shocks. Nature 611, 677–681 (2022).