We Now Know Why Black Hole Jets Produce High-Energy Radiation

Enlarge / The jets of matter ejected around black holes can be huge. NASA, ESA

Active galactic nuclei, powered by the supermassive black holes they contain, are the brightest objects in the Universe. The light comes from jets of matter projected at a speed close to 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 pointing directly at Earth, they are called a blazar and they appear brighter.

While the 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 observatory called the Imaging X-ray Polarimetry Explorer (IXPE) into one of the brightest blazars in the sky. Data from this and other observations combined indicate that light is produced when jets from the black hole collide with slower moving material.

Jets and light

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

This is 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 give up energy in the form of photons. Since they move at near 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, with some of the latter remaining at these energies despite billions of years. redshift.

So the question then becomes what is causing these particles to decelerate. There are two main ideas. One of them is that the environment in the jets is turbulent, with chaotic stacks of materials and magnetic fields. This slows down the particles and the disordered environment would mean that the polarization becomes largely random.

The alternative idea involves a shock wave, where material from the jets hits slower material and decelerates. This is a relatively ordered process, and it produces a polarization that is relatively limited in scope and becomes more pronounced at higher energies.

Enter IXPE

The new set of observations is a coordinated campaign to record Markarian blazar 501 using various telescopes capturing polarization at longer wavelengths, with the IXPE handling the more energetic photons. In addition, the researchers searched the archives of several observatories to obtain previous observations of Markarian 501, allowing them to determine if the polarization is stable over time.

Overall, 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 some variation in the polarization, suggesting that there is a relatively minor mess at the collision site, which isn't much of a surprise. But it's much less messy than you might expect from turbulent material with complicated magnetic fields.

While these results provide insight into how black holes produce light, this process ultimately relies on jet production, which takes place much closer to the black hole. The formation of these jets is still not fully understood, so people who study black hole astrophysics always have a reason to get back to work after the holiday weekend.

Nature, 2022. DOI: 10.1038/s41586-022-05338-0 (About DOIs).