Cosmic beacon: Astronomers have discovered how quasars generate their intense radiation spanning billions of light-years. Accordingly, the highest-energy portion of this radiation occurs when particles accelerated by these black holes encounter a shock front and decelerate abruptly. It mainly releases synchrotron radiation in the X-ray range. Only later do other, longer-wave radiation components arise, as the researchers report in “Nature.”
Quasars are the brightest objects in the universe. Intense cones of radiation from these active galaxy cores may shine just as brightly hundreds of billion suns and rich billion light years far away in space. The source of this immense glow is the supermassive black hole at the center of these distant galaxies: it sucks in vast amounts of material and emits energy in the form of accelerated particles and radiation. Quasars, whose radiation and particle jets point directly toward Earth, are also called blazars.
Shock front or disturbance?
But the details of how quasars generate their radiation have not been fully elucidated. Observations and models suggest that giant jets of highly accelerated particles are the source of the high-energy emission. Similar to synchrotron systems of particle accelerators or X-ray lasers, such particles can release excess energy in the form of radiation if they are decelerated or deflected.
However, it remained unclear by what mechanism the fast particles in the quasar’s jet are slowed down – whether distributed over the jet in a sudden shock front or in turbulence. This can be distinguished, among other things, by the polarization of the radiation: the more directed the radiation from the quasar, the more concentrated and uniform the jet must be at the source.
The problem, however, is that until now the polarization of quasar radiation could only be measured in the range of radio waves and optical light – and this pointed to more distributed, turbulent regions of origin. Such measurements were not available for high-energy X-rays.
First X-ray polarimetry in a blazar
That has now changed: In December 2021, a new space telescope was launched that, for the first time, can measure the polarization of cosmic X-rays. “The Imaging X-ray Polarimetry Explorer (IXPE7) can thus provide a more complete picture of the quasar’s emission region than previously possible,” explain Ioannis Liodakis of the Finnish Center for Astronomy in Turku and his colleagues.
For their study, the astronomers used the IXPE7 satellite to analyze radiation from the Markarian 501 blazar. This active galactic core is “only” about 450 million light-years away from us, so its radiation appears particularly intense and is easy to measure. So now in March 2022 it is the first blazar to be examined with X-ray polarimeter. In parallel, several other observatories captured the radiation of the remaining wavelengths of this quasar.
Various sources of X-rays and other radiation
The measurements revealed: in the lower-energy ranges of the spectrum, the radiation from the quasar is poorly and unevenly polarized. However, it is different in the high-energy X-ray range: there, the polarizer registered a degree of polarization of more than ten percent and an angle that matches the orientation of the quasar jet, as Liodakis and his team report. .
Thus this data provides important information about the origin of this X-ray radiation. “This points to a shock front as the source of the particle acceleration,” explain the researchers. According to this, this high-energy radiation is released as particles in a jet driven by the black hole’s magnetic field collide with a region of slower particles. At this shock front, they suddenly slow down and X-rays are released.
Behind the shock front of the quasar jet, the particles continue to race, but have lost energy. “As a result, they now emit radiation of longer wavelengths as they move away from this region,” Liodakis and his colleagues say. From the non-uniform polarization of this low-energy radiation, they conclude that the jet is becoming increasingly turbulent in this region.
“a turning point in the understanding of the Blazer”
For the first time, astronomers have gained insight into the mechanisms behind the brightest radiation sources in the universe. “Our results suggest that multi-wavelength polarimetry can uniquely detect the physical conditions around supermassive black holes,” Liodakis and his team said. Further measurement data from IXPE and other instruments may reveal even more details of these processes in the future.
Yale University astrophysicist Lee Marcotulli, who was not involved in the study, also sees these results as an important breakthrough. “They mark a turning point in our understanding of blazers,” she writes in an accompanying commentary. “This is a huge leap forward in our effort to understand these extreme particle accelerators.” X-ray polarimetry can now also clarify whether the mechanisms are the same in all quasars and what role the different particles – electrons and protons – play in the jet beam generation. (Nature, 2022; doi: 10.1038/s41586-022-05338-0)
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