How quasars generate their light – X-ray polarization reveals mechanism behind the strongest radiation source in the cosmos

Cosmic beacons: Astronomers have discovered how quasars generate their intense radiation spanning billions of light-years. Accordingly, the highest-energy 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, primarily 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 cosmos. The intense cones of radiation from these active galaxy cores can shine as brightly as hundreds of billions of suns and rich billion light years far into space. The source of this enormous radiance is the supermassive black hole at the center of these distant galaxies: It sucks in large amounts of material 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.

Shock front or turbulence?

But how the radiation of the quasars arises in detail has so far been largely unclear. Observations and models suggest that the enormous jets of highly accelerated particles are the source of the high-energy emissions. Similar to particle accelerators or the synchrotron systems of X-ray lasers, such particles can release excess energy as radiation if they are decelerated or deflected.

It remained unclear, however, by which mechanism the fast particles in the jet of the quasars are slowed down – whether abruptly at a shock front or distributed over the jet 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 source in the jet must be.

The problem, however, is that until now the polarization of the quasar radiation could only be measured in the range of radio waves and optical light – and this seemed to indicate more distributed, turbulent zones of origin. Such measurements were not available for high-energy X-rays.

First x-ray polarimetry at 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 emission region of quasars than was previously possible,” explain Ioannis Liodakis from the Finnish Center for Astronomy in Turku and his colleagues.

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

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

Source of x-rays and other radiation different

The measurements showed: In the lower-energy ranges of the spectrum, the radiation from the quasar is poorly and unevenly polarized. However, this is different in the high-energy X-ray range: there, the polarimeter registered a degree of polarization of more than ten percent and an angle that corresponds to the orientation of the quasar jet, as Liodakis and his team report.

This data thus provides crucial information about the origin of this X-ray radiation. “This indicates a shock front as the source of the particle acceleration,” the researchers explain. According to this, this high-energy radiation is released because the particles in the jet, accelerated by the magnetic fields of the black hole, collide with a zone of slower particles. In this shock front, they are abruptly slowed down and the 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 ever longer wavelengths as they move away from this zone,” say Liodakis and his colleagues. From the non-uniform polarization of this lower-energy radiation, they conclude that the jet is becoming increasingly turbulent in this area.

“Turning point in the understanding of blazars”

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

The astrophysicist Lea Marcotulli from Yale University, who was not involved in the study, also sees these results as an important breakthrough. “They mark a turning point in our understanding of blazars,” she writes in an accompanying comment. “This is a big leap 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 different particles – electrons and protons – in the jet play for them play beam generation. (Nature, 2022; doi: 10.1038/s41586-022-05338-0)

Source: Nature

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