African Physics Newsletter

Quasar Jets Are Particle Accelerators Thousands of Light-Years Long

An important discovery about the behavior of a nearby galaxy has been made thanks to a unique gamma ray observatory built in Namibia.

In June 2020, the H.E.S.S. Collaboration published a breakthrough result in Nature, showing that the very high energy gamma ray emission from nearby galaxy Centaurus A (NGC 5128) originates from the relativistic plasma jets extending thousands of light-years out from the center.

It had previously been accepted that gamma rays in the TeV energy range are the result of particle acceleration in regions close to a black hole at the center of a galaxy. The H.E.S.S. network of Imaging Atmospheric Cherenkov Telescopes in Namibia has the resolution to prove that the center of this galaxy is not the dominant source.

A bright source of radio waves

Centaurus A is the fifth-brightest galaxy in the sky and is about 10 to 16 million light-years away – relatively close to Earth. It is a bright source of radio waves, known to originate from synchrotron emission from relativistic electrons in the jets, which extend out for hundreds of kiloparsecs.

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 energies unattainable in human-built accelerators. Gamma rays are emitted by a wide range of cosmic objects, including quasars, which are active galaxies with a highly energetic nucleus.

A supermassive black hole

The intensity of the radiation emitted from these systems can vary over very short timescales of down to one minute. Scientists therefore believed that the source of this radiation was very small, and therefore located in the vicinity of a supermassive black hole, which can have a mass several billion times that of our Sun.

The new publication shows that this is not the dominant source. The discovery shakes up current scenarios for the behaviour of plasma jets.

Operations during the pandemic

The work, published in the journal Nature, was carried out as part of the H.E.S.S. collaboration, involving in particular the CNRS (National Centre for Scientific Research in France) and CEA (French Alternative Energies and Atomic Energy Commission), the Max Planck Society and a group of research institutions and universities in Germany, along with universities from South Africa and the University of Namibia (UNAM).

Thanks to the Namibian on-site crew and students from the University of Namibia, H.E.S.S. is the only gamma ray observatory worldwide that could keep operating during the current pandemic. Besides regularly manning the telescopes with observers, the H.E.S.S. group at UNAM is mostly involved in studying radio galaxies like Centaurus A and globular clusters, as well as characterizing the telescope cameras and improving the data analysis methods. In the context of the latter, members of the group make use of UNAM’s High-Performance Computing cluster and are active in UNAM’s Virtual Institute for Scientific Computing and Artificial Intelligence.

In addition to H.E.S.S., the UNAM Department of Physics is also involved in other large-scale international projects like the Cherenkov Telescope Array (CTA), the Southern African Large Telescope (SALT), the Square Kilometre Array (SKA), the African VLBI Network (AVN), and the Africa Millimetre Telescope (AMT).

H.E.S.S. Press Release:
Stefan Wagner, Director of H.E.S.S.
Mathieu de Naurois, Deputy Director of H.E.S.S.
Michael Backes, Head of Namibian H.E.S.S. Group

This article has first been published by the African Physics Newsletter - © American Physical Society, 2020

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