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 projecting at near-light speed from the environment around the black hole. In most cases, these active galactic nuclei are called quasars. But on rare occasions when one of the jets is headed directly for Earth, it is called a blazar and appears much brighter.
Although the outlines of how blazars work have been worked out, many details remain poorly understood, including how fast-moving matter generates so much light. Now researchers have converted a new space observatory called X-ray polarization exploration (IXPE) to one of the brightest flames in the sky. The data from these and other observations taken together indicate that light is produced when jets from the black hole collide with slow-moving matter.
Planes and light
IXPE specializes in detecting the polarization of high-energy photons, that is, the direction of ripples in the electric field of light. The polarization information can tell us something about the processes that created the photons. For example, photons that come from 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 is useful for studying blazars. The high-energy photons emitted by these objects are generated by the charged particles in the jets. When these objects change trajectory or slow down, they must give up energy in the form of photons. Because they move 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 though billions of years away. redshift.
So the question then becomes what causes these particles to slow down. There are two main ideas. One is that the environment in planes is turbulent, with chaotic stacks of material and magnetic fields. This slows the particles down and the turbulent environment will mean that the polarization becomes largely random.
An alternative idea involves a shock wave, where material from the jets collides with slower material and slows 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 Markarian blazar 501 using a variety of telescopes that capture polarization at longer wavelengths, with IXPE dealing with the most energetic 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 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 further aligned at higher photon energies.
There is still a bit of a difference in polarization, which indicates a relatively slight disorder at the collision site, which is not much of a surprise. But it’s much less messy than you might expect from turbulent matter with complex magnetic fields.
While these findings provide insight into how black holes produce light, this process ultimately depends on jet production, which occurs near the black hole. How these jets form is still not fully 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).