Determined how particle acceleration occurs in the jets of a supermassive black hole

Blazars, a type of active galaxy, are the most powerful sources of continuous energy in the universe. Like all other active galaxies, they show a structure consisting of a central supermassive black hole surrounded by a disc of matter feeding it, but they are among the 10% of active galaxies that show a jet of matter emerging from both poles at extremely high speed, and among the even smaller percentage of cases where their orientation allows us to see the jet almost head-on. Now, a study answers a question that has been unanswered for decades: how do the particles in these jets accelerate to such high energies?

An international scientific team has used data from the X-ray Polarimetry Explorer (IXPE) to sketch the answer, published today in the journal Nature, which suggests that the explanation for the acceleration of the particles lies in a shock wave within the jet. "This is a forty-year-old mystery that we have solved", says Yannis Liodakis, lead author of the study and an astronomer at the Finnish Centre for Astronomy with the European Southern Observatory. "We finally have the pieces of the puzzle, and the picture that has been created is clear”.

Launched on 9 December 2021, the Earth-orbiting IXPE satellite, a collaboration between NASA and the Italian Space Agency with the participation of scientists from the IAA-CSIC in Spain, provides a type of data that has never been accessible before. These new data include the measurement of the polarisation of X-ray light, which means that IXPE detects the direction and average intensity of the electric field of X-ray light.

The new study used IXPE to observe Markarian 501, a blazar at the centre of a large elliptical galaxy. During the observations, the scientific team used not only space-based observatories but also ground-based telescopes capable of detecting polarised radiation to gather information about the object in a wide range of wavelengths, including microwaves, visible light and X-rays. Observations made by the IAA-CSIC team at the IRAM 30m millimetre radio telescope and the 1.5m optical telescope at the Sierra Nevada Observatory (both in Granada), and with the 2.2m telescope at Calar Alto (in Almería), were essential to interpret the IXPE data.

"Adding X-ray polarisation to our arsenal of optical, infrared and millimetre polarisation is a game-changer", says Alan Marscher, an astronomer at Boston University who coordinates the group studying giant black holes with IXPE.

This is the first time that the polarisation of X-ray light has been observed in the regions close to the source of particle acceleration, and analysis of the object at different wavelengths has revealed that X-ray light is more polarised than optical light, which is more polarised than microwave light. However, the direction of the polarisation coincides at all observed wavelengths and is aligned with the direction of the jet.

Comparison of the data with theoretical models showed that the most viable scenario involves the existence of a shock wave that accelerates the jet particles. A shock wave is generated when a pressure wave moves faster than the speed of sound in the surrounding material, such as when a supersonic aircraft passes through the Earth's atmosphere.

Although shock waves have been considered one of the viable scenarios to favour the acceleration of particles in relativistic jets for decades, there have always been other equally possible scenarios that could explain the same phenomenon", says Iván Agudo, coordinator of the IAA-CSIC group participating in the study. “The novelty and relevance of this result lies in the fact that the new polarisation-sensitive X-ray observations, combined with our ground-based observations, only favour the shock-wave particle acceleration mechanism”.

As the accelerated particles move outwards from the shock, they emit X-rays first, as they are extremely energetic. Moving outwards, the energised region becomes increasingly turbulent as it moves away from the shock site, and the particles begin to lose energy, resulting in emission at less energetic wavelengths, such as visible and microwave. This is analogous to the way the flow of water becomes more turbulent after encountering a waterfall.