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Gamma rays: another look at the universe

Under the direction of the Director of Research at the Leprince-Ringuet Laboratory, Mathieu de Naurois, the international collaborative network known as H.E.S.S. has published the results of fifteen years of gamma ray observations of the Milky Way in the Astronomy & Astrophysics journal. This compilation of data will serve as a reference for the international scientific community.

Many locations in our Universe, such as supernovae, pulsars or the environment of some black holes, are the sites of violent phenomena that create intense shockwaves and electromagnetic fields. These fields then accelerate the cosmic particles of the surrounding area until they become highly charged with energy. The particles, in turn, interact with the matter and radiation around them, and can generate gamma rays, thus revealing their existence in outer space.

When these gamma rays enter the Earth’s atmosphere at an altitude of 10,000 meters, they become absorbed, producing an ephemeral cascade of secondary particles. For a few billionths of a second, this shower of particles emits a faint blue light, known as Cherenkov light.

Detecting the gamma rays that reach the Earth

In order to detect this bluish light, 200 researchers from 14 countries have been collaborating on the H.E.S.S. network, a collection of five telescopes set up in the Namibian desert. The analysis of the light, captured by extremely sensitive telescopic cameras, allows the scientists to trace the direction and energy of gamma photons that reach the Earth. By accumulating each of these images, photon by photon, H.E.S.S. is able to map out astronomical objects in gamma light.

The center of our galaxy and remnants from the explosion of a massive star, known as the Crab Nebula, are among the most iconic of the first sources identified by the H.E.S.S. Cherenkov telescopes. On cloudless, moonless nights that allow the researchers to make out the faint Cherenkov light, H.E.S.S. has been bringing the field of gamma-ray astronomy consistently closer to our planet through its exploration of the Milky Way and beyond. A total of 2,700 hours of observation have allowed for the identification of 78 cosmic sources of gamma rays, more than all the other observatories in the world put together.

Results long-anticipated by the scientific community

The H.E.S.S. Galactic Plane Survey (HGPS) catalog, published in the Astronomy & Astrophysics journal, will prove valuable for the entire astrophysics community. It comprises a review of fifteen years of studies that have allowed, in particular, for the characterization of the most abundant types of gamma sources, such as pulsar wind nebulae and supernova remnants; and precision measurements of individual sources like entire areas of the Milky Way. The considerable extent of this H.E.S.S. review demonstrates the incredible progress that has been made in high-energy gamma-ray astronomy.

In 2006, the H.E.S.S. network was awarded the Descartes Prize by the European Commission, and then went on to receive the Rossi Prize from the American Astronomy Society (AAS) in 2010. Furthermore, a study published in 2009 cited H.E.S.S. among the most 10 most influential astronomical observatories in the world. Mathieu de Naurois, director of the experiment and researcher at the Leprince-Ringuet Laboratory (École Polytechnique/CNRS Joint Research Unit), has just been given the CNRS silver medal for his work, initially within, then at the head of this international experiment.