Quasar jets: cosmic particle accelerators

Composite image of Centaurus A, revealing the jets emerging from the central black hole of the galaxy, and the associated gamma emission. © ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray), H.E.S.S. collaboration (Gamma)

Using the H.E.S.S. telescope array in Namibia, an international collaboration of more than 200 scientists from 13 countries has demonstrated that plasma jets from quasars accelerate electrons to relativist speeds. This work, involving researchers from the Leprince Ringuet Laboratory, revealing the role of quasar jets in accelerating cosmic particles, was published in Nature on 18 June 2020.

Gamma rays to observe the universe

Over the past few years, scientists have observed the Universe using gamma rays, which are very high-energy photons. Gamma rays, which form part of the cosmic rays that constantly bombard the Earth, originate from regions of the Universe where particles are accelerated to huge energies unattainable in human-built accelerators. Gamma rays are emitted by a wide range of cosmic objects, such as quasars, which are active galaxies with a highly energetic nucleus. The intensity of the radiation emitted from these systems can vary over very short timescales of up to one minute. Scientists therefore believed that the source of this radiation was very small and located in the vicinity of a supermassive black hole, which can have a mass several billion times that of the Sun’s. The black hole is thought to gobble up the matter spiralling down into it and eject a small part of it in the form of large jets of plasma, at relativistic speeds, close to the speed of light, thus contributing to the redistribution of matter throughout the Universe.

H.E.S.S.: Eyes for Gamma Rays

Using the H.E.S.S. observatory in Namibia, an international astrophysics collaboration observed a radio galaxy (a galaxy that is highly luminous when observed at radio wavelengths) for over 200 hours at unparalleled resolution. As the nearest radio galaxy to Earth, Centaurus A is favourable to scientists for such a study, enabling them to identify the region emitting the very high-energy radiation while studying the trajectory of the plasma jets. They were able to show that the gamma-ray source extends over a distance of several thousand light-years. This extended emission indicates that particle acceleration does not take place solely in the vicinity of the black hole but also along the entire length of the plasma jets. Based on these new results, it is now believed that the particles are reaccelerated by stochastic processes along the jet. The discovery suggests that many radio galaxies with extended jets accelerate electrons to extreme energies and might emit gamma-rays, possibly explaining the origins of a substantial fraction of the diffuse extragalactic gamma background radiation.

A redistribution of energy in the universe

These findings provide important new insights into cosmic gamma-ray emitters, and in particular about the role of radio galaxies as highly efficient relativistic electron accelerators. Due to their large number, it would appear that radio galaxies collectively make a highly significant contribution to the redistribution of energy in the intergalactic medium. The results of this study required extensive observations and optimized analysis techniques with H.E.S.S., the most sensitive gamma-ray observatory to date. Next-generation telescopes (Cherenkov Telescope Array, or CTA) will no doubt make it possible to observe this phenomenon in even greater detail.

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Gamma rays : another look at the universe

Very high energy gamma ray detection in Nature