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

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

While the general scheme 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 observatory called An imaged X-ray polarimetric explorer (IXPE) to one of the brightest blazars in the sky. His data and other observations combined show that the light is produced when black hole jets collide with slower-moving materials.

Streams and light

IXPE specializes in detecting the polarization of high-energy photons—the orientation of the wobble in the electric field of light. Polarization information can tell us something about the processes that created the photons. For example, photons originating from a turbulent medium will have an essentially random polarization, while a more structured medium will tend to produce photons with a limited range of polarizations. Light that passes through material or magnetic fields can also change its polarization.

This turns out to be useful for studying blazars. The high-energy photons these objects emit are generated by charged particles in the jets. When these objects change course or slow down, they must give up energy in the form of photons. Because they travel at speeds close to the speed of light, they have a lot of energy to release, allowing blazars to emit across the spectrum from radio waves to gamma rays—some of the latter remaining at these 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 of them is that the environment in the jets is turbulent, with chaotic accumulation of materials and magnetic fields. This slows down the particles and a cluttered medium would mean that the polarization becomes largely random.

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

Enter IXPE

The new set of observations is a coordinated campaign to record the Markarian blazar 501 using different telescopes capturing polarization at longer wavelengths, with IXPE processing the highest-energy photons. In addition, the researchers searched the archives of several observatories for earlier observations of Markarian 501, allowing them to determine whether the polarization was stable over time.

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

There is still little variation in polarization, suggesting that there is some relatively minor disorder at the collision site, which is not really a surprise. But it is much less disordered than you would expect from a turbulent material with complex magnetic fields.

Although these results provide a better understanding of how black holes produce light, this process ultimately relies on jet production taking place much closer to the black hole. How these jets form is still not fully understood, so people studying black hole astrophysics still have reason to get back to work after the holiday weekend.

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