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

Zoom in / 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 ejected from the surroundings around the black hole at nearly the speed of light. In most cases, these active galactic nuclei are called quasars. However, on rare occasions when one of the jets is aimed 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, scientists have converted a new space observatory called the Polarizing X-ray Imaging Explorer (IXPE) into one of the brightest bursts in the sky. Taken together, its data and other observations indicate that the light is produced when jets from black holes collide with slow-moving matter.

Airplane and light

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

This has proven useful in the study of blazars. The high-energy photons emitted by these objects are generated by the charged particles in the beams. When these objects change orbit 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 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 such factor is that the environment in aircraft is turbulent with chaotic accumulations of material and magnetic fields. This slows down the particles and in a chaotic environment the polarization becomes largely random.

An alternative idea involves a shock wave in which material from the jets collides with slow-moving material and slows it down. This is a relatively well-ordered process that produces a relatively band-limited polarization that becomes more pronounced at higher energies.

Enter IXPE

The new series of observations is a coordinated campaign to capture the blazar Markarian 501 using an array of telescopes that capture polarization at longer wavelengths where IXPE processes the highest-energy photons. In addition, the researchers searched the archives of several observatories for previous observations of Markarian 501, which allowed them to determine whether the polarization was stable over time.

In general, the measured polarizations across the spectrum from radio waves to gamma rays 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 small difference in polarization, suggesting a relatively small perturbation at the collision site, which is not really a surprise. But it is much less turbulent than you would expect from turbulent matter with complex magnetic fields.

Although these results provide a better understanding of how black holes produce light, this process ultimately depends on the production of jets that 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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