How quasars generate their light


Cosmic Beacons: Astronomers have discovered how quasars generate their intense radiation spanning billions of light-years. As a result, the most energetic part of this radiation occurs when the particles accelerated by these black holes encounter a shock front and are abruptly decelerated. This releases synchrotron radiation, mostly in the X-ray range. It’s only later that other components of longer-wave radiation appear, as the researchers report in “Nature.”

Quasars are the brightest objects in the cosmos. The intense cones of radiation from these active galactic nuclei can shine as bright as hundreds of trillions of suns and reach billions of light-years into space. The source of this enormous radiation is the supermassive black hole at the center of these distant galaxies: it sucks in large amounts of matter and emits energy in the form of accelerated particles and radiation. Quasars, whose radiation and particle jets point directly at the earth, are also called blazars.

Frontal impact or turbulence?

But until now, how quasar radiation occurs in detail has not been clear. Observations and models suggest that the huge jets of highly accelerated particles are the source of the high-energy emissions. Similar to particle accelerators or synchrotron systems of X-ray lasers, these particles can release excess energy in the form of radiation if slowed down or deflected.

It was not known, however, by what mechanism the fast particles in the jet of quasars are slowed down – whether suddenly during a shock front or distributed over the turbulent jet. This is distinguished, among other things, by the polarization of the radiation: the more the radiation of the quasar is directed, the more the source in the jet must be concentrated and uniform.

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 seemed to indicate more distributed and turbulent areas 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, which can measure the polarization of cosmic X-rays for the first time. “The Imaging X-ray Polarimetry Explorer (IXPE7) can thus provide a more complete picture of the quasar emission region than was previously possible,” explains Ioannis Liodakis of the Finnish Center for Astronomy in Turku and his colleagues.

For their study, the astronomers used the IXPE7 satellite to analyze the radiation from blazar Markarian 501. This active galaxy nucleus is “only” about 450 million light-years from us, so its radiation appears particularly intense and easy to measure. It is therefore now the first blazar to be examined with an X-ray polarmeter in March 2022. In parallel, many other observatories have captured the radiation of the remaining wavelengths of this quasar.

When particles accelerated in the quasar jet hit a shock front, they are abruptly decelerated and release high-energy X-rays. When they fly, they then generate other radiation of lower energy. © Pablo Garcia/NASA/MSFC

Source of X-rays and other different radiations

The measurements showed: In the lower energy ranges of the spectrum, the quasar radiation is poorly and unevenly polarized. However, it’s different in the high-energy X-ray range: there, the polarimeter recorded a degree of polarization of more than ten percent and an angle that matches the orientation of the quasar’s jet, as reported by Liodakis. and his team.

These data thus provide crucial information on the origin of this X-radiation. “This indicates a shock front as the source of the acceleration of the particles”, explain the researchers. According to this, this high-energy radiation is released as particles in the jet, accelerated by the magnetic fields of the black hole, collide with an area of ​​slower-moving particles. In this shock front, they are suddenly slowed down and the X-rays are released.

Behind the shock front of the quasar jet, the particles continue to run, but have lost energy. “As a result, they now emit radiation of longer and longer wavelengths as they move away from this area,” Liodakis and his colleagues explain. From the non-uniform polarization of this lower energy radiation, they conclude that the jet becomes increasingly turbulent in this area.

“Turning point in understanding blazars”

For the first time, astronomers have been able to better understand the mechanisms behind the brightest sources of radiation in the cosmos. “Our results demonstrate that multi-wavelength polarimetry can uniquely explore the physical conditions surrounding supermassive black holes,” Liodakis and his team say. Further measurement data from the IXPE and other instruments may reveal even more details about these processes in the future.

Yale University astrophysicist Lea Marcotulli, who was not involved in the study, also sees the results as an important step forward. “They mark a turning point in our understanding of blazars,” she wrote in an accompanying comment. “This is a big step forward in our attempt to understand these extreme particle accelerators.” X-ray polarimetry could 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 for them play in beam generation. (Nature, 2022; doi: 10.1038/s41586-022-05338-0)

Source: Nature

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