We now know why black hole jets produce high-energy radiation

Enlarge / The jets of material ejected around black holes can be enormous.

Active galactic nuclei, powered by the supermassive black holes they contain, are the brightest objects in the universe. The light comes from jets of material that are flung out at nearly the speed of light by the environment around the black hole. In most cases, these active galactic nuclei are called quasars. But in rare cases where one of the jets is aimed directly at Earth, they are called a blazar and appear brighter.

While the general outline of how a blazar works has been worked out, several details remain poorly understood, including how the fast-moving material generates so much light. Now researchers have turned a new space-based observatory called the Imaging X-ray Polarimetry Explorer (IXPE) into one of the brightest blazars in the sky. Its data and other observations combined indicate that light is produced when the black hole’s jets impact slower-moving materials.

Jets and light

The IXPE specializes in detecting the polarization of high-energy photons – the orientation of the coils in the light’s electric field. Polarization information can tell us something about the processes that created the photons. For example, photons originating from a turbulent environment will have essentially random polarization, while a more structured environment will produce photons with a limited number of polarizations. Light passing through material or magnetic fields can also change polarization.

This proves to be useful for studying blazars. The high-energy photons emitted by these objects are generated by charged particles in the jets. When these objects change course or slow down, they must release energy in the form of photons. Because they move at nearly the speed of light, they have a lot of energy to give up, allowing blazars to emit across the spectrum from radio waves to gamma rays — some of the latter remaining at those energies despite billions of years. of redshift.

So the question then becomes what causes these particles to slow down. There are two leading ideas. One is that the environment inside the jets is turbulent, with chaotic accumulations of materials and magnetic fields. This slows down the particles and the cluttered environment would mean that the polarization becomes largely random.

The alternative idea involves a shock wave, where material from the jets collides with slower-moving material and slows itself down. This is a relatively orderly process and produces a polarization that is relatively limited in range and becomes more pronounced at higher energies.

Enter IXPE

The new series of observations is a coordinated campaign to capture the blazar Markarian 501 using a variety of telescopes that capture polarization at longer wavelengths, with IXPE processing the highest-energy photons. In addition, the researchers searched the archives of several observatories to obtain previous observations of Markarian 501, which would allow them to determine whether the polarization is stable over time.

Across the entire spectrum from radio waves to gamma rays, the measured polarizations were generally within a few degrees of each other. It was also stable over time and its alignment increased at higher photon energies.

There’s still a bit of variation in the polarization, suggesting there’s a relatively small amount of distortion at the collision site, which isn’t really a surprise. But it’s much less disorderly than you might expect from a turbulent material with complicated magnetic fields.

While these results provide a better understanding of how black holes produce light, that process ultimately relies on the production of jets, which occur much closer to the black hole. How these jets form is still not really understood, so people who study black hole astrophysics still have a reason to get back to work after the holiday weekend.

Nature2022. DOI: 10.1038/s41586-022-05338-0 (About DOIs).

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