Thanks to specialised gamma-ray telescopes, scientists have gained unprecedented insights into the Universe's largest particle accelerators. The international collaboration bringing together over 200 scientists from 13 countries has shown that the very high-energy gamma-ray emission from galaxies with a highly energetic nucleus is not concentrated in the region close to their central black hole but in fact extends over several thousand light-years along the jets of plasma emitted by these objects. This discovery shakes up current understanding of the maximum energy attainable by the particle acceleration processes. The work, published in the journal Nature, was carried out as part of the H.E.S.S collaboration, involving in particular the French research centre CNRS, the Max Planck Institute for Nuclear Physics and DESY in Germany, and the University of Innsbruck in Austria.

Over the past few years, scientists have observed the Universe using X-rays and gamma rays, which are very high-energy photons. “These photons originate from systems like accreting supermassive black holes at the heart of certain galaxies, where electrons are accelerated to huge energies unattainable in human built accelerators”, said H.E.S.S. scientist Andrew Taylor from DESY, one of the corresponding authors of the publication. In these active galaxies, the central black hole is surrounded by a so-called accretion disc, where matter gathers like in the swirl of a bathtub's plug hole before plunging into oblivion. Often, these accreting black holes shoot two gigantic jets of hot plasma perpendicular to the disc far into the cosmos.The intensity of the gamma radiation emitted from these systems can vary over very short timescales of up to one minute, suggestive of an origin close to the central black hole. Additionally, scientists have debated the origin of the X-ray emission in the outflows of these objects, with one scenario requiring extremely energetic electron acceleration. Since electrons accelerated within the jet lose energy quickly, they therefore need to be kept energised to exist all along the jet.

Using the High Energy Stereoscopic System (H.E.S.S.) observatory in Namibia, the 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 in gamma rays. “As the nearest radio galaxy to Earth, Centaurus A, was favourable for such a study, because it enabled us to identify the region emitting the very high-energy radiation while studying the trajectory of the plasma jets,” explained H.E.S.S. Deputy Director Mathieu de Naurois from the Centre National de la Recherche Scientifique (CNRS) in France, also one of the corresponding authors of the publication.

Based on dedicated analysis efforts by groups in Innsbruck and Paris, the scientists 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, as groups in Heidelberg and Zeuthen near Berlin have demonstrated – making the largest particle accelerators in the cosmos even bigger.

The discovery suggests that many radio galaxies with extended jets do indeed efficiently accelerate particles up to extreme energies. This finding therefore brings crucial new information to the debate on the origin of the X-ray emission. “This discovery revolutionises our understanding of large-scale jets and brings our understanding of cosmic particle acceleration a huge step forward,” said Taylor. “It is deeply satisfying to see long-term observational efforts paying off dividends like this. Indeed, we continue to enjoy being surprised by our close by cosmic friends when looked at in a different way.”

The results of this study required extensive observations and optimised analysis techniques. The next-generation observatory Cherenkov Telescope Array (CTA) will no doubt make it possible to observe this phenomenon in even greater detail. The H.E.S.S. International Observatory, consisting of five telescopes located in Namibia, involves laboratories from thirteen countries (France, Germany, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria, Sweden, the United Kingdom, the Netherlands and Japan).

 

Reference:
Resolving acceleration to very high energies along the jet of Centaurus A; The H.E.S.S. Collaboration; Nature, 2020; DOI: 10.1038/s41586-020-2354-1

 

Further information:
H.E.S.S. homepage at DESY