An international team of astrophysicists has measured the polarization of X-rays from an extragalactic supermassive black hole for the first time using the Imaging X-ray Polarimetry Explorer (IXPE), a new satellite from NASA and the Italian Space Agency. These measurements shed light on decades-old mysteries about the most massive objects in the Universe.
Black holes are among the most powerful and mysterious objects in the Universe. They come in different sizes, ranging from one to a few billion times the mass of our Sun. But although researchers have been studying these objects for decades, there are still many fundamental questions about them: “How can they radiate across the entire electromagnetic spectrum at such phenomenal powers? What is the geometry, intensity and turbulence of the magnetic fields around them? How do they accelerate particles such as electrons at relativistic speeds into interstellar space?” enumerates Frédéric Marin from the Astronomical observatory of Strasbourg, responsible for the “radio-quiet AGNs and the Galactic center” topical working group for IXPE. These are the questions that astrophysicists will elucidate for the first time with IXPE, the very first satellite capable of measuring the polarization of light in X-rays with unprecedented precision.
Artist representation of the IXPE satellite observing a supermassive black hole.
Polarization is a property of light. More precisely, it is the average direction of the electric field of the electromagnetic waves which compose the light. This orientation of the waves and the degree of polarization of this light contain valuable information about the magnetic field that cannot be obtained by any other means available to scientists.
The first active galactic nucleus observed by IXPE is Markarian 501, in the constellation of Hercules. Markarian 501 (or Mrk 501 for short) has a supermassive black hole (a few billion times the mass of the Sun) located at the heart of an elliptical galaxy. Gas from the galaxy flows towards the black hole through an accretion disk. A small fraction of particles from the accreted gas is ejected toward Earth in a narrow, collimated stream called a jet. The gas is in an extremely hot state called plasma and moves through the jet at speeds close to the speed of light. As the plasma flows through the jet’s magnetic field, it is excited and emits light, including X-rays and gamma rays. Active galactic nuclei whose jet is directed towards us are called blazars and are the brightest objects in the night sky. However, how the particles in the plasma are energized was still a mystery until now.
Mrk 501 was observed in early March 2022 for three consecutive days. A second, similar observation was made two weeks later. As the IXPE “watched” Mrk 501, astronomers used numerous space and ground-based telescopes around the world to observe the blazar in radio, optical and X-rays. The data was then combined like the pieces of a puzzle. Similar efforts had been undertaken in the past, however, radio and optical polarizations alone were not able to differentiate competing models. X-ray polarization provided the final piece of this puzzle.
The IXPE team found that X-ray light is more polarized than in optics, which itself is more strongly polarized than in radio. At the same time, the orientation of the polarized light is the same from radio band to the X-rays and aligned with the direction of the jet on the sky. This provided crucial information for astrophysicists to understand what was going on.
What the IXPE consortium observed can only be explained if the particles that produce the X-ray light are energized by shock waves that form inside the jet. Shocks are formed either by plasma moving at different speeds colliding with themselves or when the pressure at the edges of the jet changes abruptly. The energized particles then emit X-rays immediately after leaving the shock zone, while they emit optical and radio photons as they move away, where the plasma becomes more turbulent. These new observations made it possible to eliminate many models based on purely spectroscopic or temporal considerations and clarified the role of turbulence in the jets.
“We finally had all the pieces of the puzzle, and the picture they made is clear,” said Yannis Liodakis of ESO’s Finnish Center for Astronomy, lead author of the Nature paper. “Although there were many competing models before the IXPE observations, this result clearly shows that shock waves are responsible for the X-ray light from blazars.” IXPE will observe more blazars over the next two years to determine if shocks and turbulence dominate the jets in all blazars in the sky, or if other processes such as the reconnection of magnetic fields from opposite directions may also be important.
Article published in Nature : Liodakis et al. 2022, Polarized blazar X-rays imply particle acceleration in shocks.
Contact : Frédéric Marin frederic.marin@astro.unistra.fr
For more information :
- News Nature
- Press release : NASA
- CNRS-INSU (in french)
- Savoir(s), Université de Strasbourg (in french)
- News by OSUG (in french)