NASA’s Imaging X-Ray Polarimetry Explorer, or IXPE, has helped astronomers get closer to an answer. In a new study in the journal Nature, authored by a large international collaboration, including a number of researchers at the Finnish Centre for Astronomy with ESO and the University of Turku, astronomers find that the best explanation for the particle acceleration is a shock wave within the jet.

“This is a 40-year-old mystery that we’ve solved,” said Yannis Liodakis, lead author of the study and astronomer at the Finnish Centre for Astronomy with ESO. “We finally had all of the pieces of the puzzle, and the picture they made was clear.”

Launched Dec. 9, 2021, the Earth-orbiting IXPE satellite, a collaboration between NASA and the Italian Space Agency, provides a special kind of data that has never been accessible from space before. This new data includes the measurement of X-ray light’s polarization, meaning IXPE detects the average direction and intensity of the electric field of light waves that make up X-rays. Information about the electric field orientation in X-ray light, and the extent of polarization, is not accessible to telescopes on Earth because the atmosphere absorbs X-rays from space.

The new study used IXPE to point at Markarian 501, a blazar in the constellation Hercules. This active black hole system sits at the center of a large elliptical galaxy.

IXPE watched Markarian 501 for three days in early March of 2022, and then again two weeks later. During these observations, astronomers used other telescopes in space and on the ground to gather information about the blazar in a wide range of wavelengths of light including radio, optical, and X-ray. Particularly important were the observations in optical polarization conducted at the Nordic Optical Telescope – NOT, owned by the University of Turku, that allowed the X-ray observations to be put into perspective. “NOT has always been a corner stone in polarization studies of blazars”, said Jenni Jormanainen, PhD student at the University of Turku, who led the analysis of the NOT observations. Further information was obtained with the in-house-built optical polarimeter DIPol-2 installed on the 60cm Tohoku telescope at Haleakala observatory in Hawaii that is operated remotely from Turku. “This instrument has been a workhorse in our program of high-precision polarimetric measurements of various astronomical sources“, pointed out Vadim Kravtsov, PhD student at the University of Turku, in charge of the DIPoL-2 observations.

This illustration shows the 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: Pablo Garcia (NASA/MSFC)

Scientists 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, the team of astronomers 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.

The study was not designed to investigate the origins of shock waves, which are still mysterious. But scientists hypothesize that a disturbance in the flow of the jet causes a section of it to become supersonic. This could result from high-energy particle collisions within the jet, or from abrupt pressure changes at the jet boundary.

“As the shock wave crosses the region, the magnetic field gets stronger, and energy of particles gets higher,” said Alan Marscher, an astronomer at Boston University who leads the group studying giant black holes with IXPE. “The energy comes from the motion energy of the material making the shock wave.”

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.

Scientists will continue observing the Markarian 501 blazar to see if the polarization changes over time. IXPE will also investigate a broader collection of blazars during its two-year prime mission, exploring more longstanding mysteries about the universe.

“It’s part of humanity’s progress toward understanding nature and all of its exoticness,” Marscher said.