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Very high energy gamma ray detection in Nature

The giant telescope of the H.E.S.S. has detected for the first time the very high energy gamma rays emitted by an astronomical cataclysm. Involving researchers from the Leprince-Ringuet Laboratory for Particle Physics and Astrophysics, this discovery is published in Nature on November 20, 2019.

Vue d'artiste d'un sursaut gamma. Crédit : ESO/A. RoquetteArtistic depiction of a gamma burst. Credit: ESO / A. Roquette

The most violent events in the universe, such as the collapse of stars or the fusion of neutron stars, are responsible for the emission of gamma-ray bursts. The H.E.S.S. Telescope Network first detected a very high-energy gamma-ray burst in July 2018 and helped to change the understanding of this astronomical phenomenon. This research is published in Nature on November 20, 2019.

A delayed observation

On July 20, 2018, two detectors, aboard the Fermi satellite and the Swift space telescope, detected the first traces of a gamma-ray burst. Following this alert, several terrestrial observatories have oriented their detectors in the direction of the burst. The H.E.S.S. Telescope Network could only observe the region of the sky concerned 10 hours after initial detection. This observation could have been useless because the emission of very high energy gamma radiation is supposed to last only a few seconds in current models. Nevertheless, after searching for more than 10 years the signature of such a spatial cataclysm, H.E.S.S. eventually detected the emission of gamma rays of very high energy several hours after the initial burst.

Refine astronomical models

The burst of July 20, 2018 was very intense and lasted about 50 seconds, a relatively long duration which probably indicate the death of a massive star. In this process, the heart of the star collapses and becomes a black hole in rapid rotation. The interaction between the surrounding gas and gas jets from the collapse would be responsible for the gamma-ray bursts: accelerated particles at relativistic velocities interact with the material and/or radiation and then produce numerous gamma photons.

The detection carried out by H.E.S.S. demonstrates for the first time the existence of accelerated particles at extreme energies in gamma-ray bursts, but also reveals that these bursts are still poorly understood. Since these particles exist, or even are created long after the initial burst, it could be the signature of a shock wave created by the explosion. The most likely underlying hypothesis would be that this shock wave behaves like a cosmic particle accelerator by generating gamma rays of very high energy several hours after the burst. H.E.S.S. measurements thus impose new constraints on astrophysical theories in order to advance the knowledge of astronomical phenomena.

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HESS (High Energy Stereoscopic System) is a network of Cherenkov telescopes at 1800m altitude on a desert in Namibia, which studies cosmic gamma rays from 100 GeV to about 100 TeV. With four 13m and one 28m diameter telescopes, it is the world's most sensitive ultra-high energy cosmic gamma ray detector. Operated by an international collaboration of more than 260 scientists from 13 countries (Namibia, South Africa, Germany, France, United Kingdom, Ireland, Austria, Netherlands, Poland, Sweden, Armenia, Japan and Australia), H.E.S.S. involves in France CNRS and CEA. The experience H.E.S.S. was directed by Mathieu de Naurois, Research Director at the Leprince-Ringuet Laboratory (École Polytechnique / CNRS) and lecturer at the École Polytechnique between 2016 and 2019.