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

Zoom in / The jets of material ejected from 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 originates from jets of material that are 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 on rare occasions where one of the jets is pointed directly at Earth, it is called a blazar and appears much brighter.

While the general outline of how the blazar works has been worked out, many details are still poorly understood, including how fast-moving matter generates so much light. Now, researchers have turned a new space observatory called the X-ray Imaging Polarimetry Explorer (IXPE) toward one of the brightest blazars in the sky. Taken together, the data from it and other observations indicate that the light is produced when black hole jets collide with slow-moving matter.

Planes and light

IXPE specializes in detecting the polarization of high-energy photons – the direction of vibrations in the electric field of light. The polarization information can tell us something about the processes that created the photons. For example, photons originating in a disordered environment will have essentially random polarizations, while a more ordered environment tends to produce photons with a limited range of polarizations. Light passing through materials or magnetic fields can also change its polarization.

This has been shown to be useful in the study of blazars. The high-energy photons that these objects emit are generated by the charged particles in the jets. When these objects change trajectory or slow down, they have to give up energy in the form of photons. Because they move at close to the speed of light, they have a lot of energy to give up, allowing blazars to emit across the entire spectrum from radio waves to gamma rays – some of the latter staying 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 such factor is that the environment in aircraft is turbulent, with chaotic accumulations of material and magnetic fields. This causes the particles to slow down, and a chaotic environment will mean that the polarization becomes largely random.

An alternative idea involves a shock wave, where material from the jets collides with slow-moving material, slowing it down. This is a relatively orderly process, producing a relatively band-limited polarization that becomes more pronounced at higher energies.

Enter IXPE

The new set of observations is a coordinated campaign to record Blazar Markarian 501 using a variety of telescopes that capture polarization at longer wavelengths, with IXPE handling the highest energy photons. In addition, the researchers searched the archives of several observatories for earlier observations of Markarian 501, which allowed them to determine whether the polarization was stable over time.

In general, across the entire 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 a bit of a difference in polarization, which indicates a relatively slight perturbation at the collision site, which isn’t really a surprise. But it is much less turbulent than you would expect from turbulent matter with complex magnetic fields.

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

nature2022. DOI: 10.1038/s41586-022-05338-0 (about DOIs).

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