Newly discovered supernova remnants only reveal themselves at the highest gamma-ray energies
Astrophysicists of the University of Tübingen publish new results on the occasion of the 15th anniversary of the Gamma Telescope System in Namibia that is operated by the H.E.S.S. collaboration
The H.E.S.S. telescopes have surveyed the Milky Way for the past 15 years searching for sources of gamma radiation. The H.E.S.S. collaboration includes scientists of the Institute of Astronomy and Astrophysics of the University of Tübingen led by Professor Andrea Santangelo and Dr. Gerd Pühlhofer. They are interested in sources of very high energy gamma radiation in the TeV energy range, i.e. in the range of 1012 electron volts, corresponding to a trillion of the energy of visible light photons. For the first time they have been able to classify celestial objects using only the emission of this kind of radiation: very likely they are supernova remnants, which are celestial objects that emerge after the explosion of massive stars. The results are published in a special edition of the scientific journal Astronomy & Astrophysics, which appears on the occasion of the 15th anniversary of the H.E.S.S. telescopes with the largest set of science results of the project to date.
Over 200 sources of TeV radiation are known to date, both Galactic and Extragalactic. “We can often relate the radiation to known astrophysical objects that have been studied before with conventional telescopes in lower frequency bands, e.g. in optical or radio wavebands”, says Gerd Pühlhofer. “Interestingly, however, with the survey observations along the Galactic plane that have been conducted with the H.E.S.S. telescopes, many new sources have been discovered which are not or not clearly associated with objects in lower frequencies.” And the TeV gamma-ray data alone is usually not sufficient to attribute a source to a particular astrophysical type of object. “Those unidentified sources continue to remain a big puzzle in gamma-ray astronomy.”
But the H.E.S.S. telescopes delivered data that are detailed enough that the scientists could get further. “For the first time, we are now able to classify unidentified TeV sources to be members of a particular object class, using only the TeV data”, says Pühlhofer. “Three particular sources are now classified with high probability as supernova remnants.”
A supernova remnant is a celestial object that forms after the explosion of a massive star at the end of its lifetime. The matter that is expelled in such an explosion leads to shock waves that propagate into the interstellar medium. There, the matter is heated and particles are accelerated to relativistic speeds. The particles interact with light and gas in the neighbourhood of the sources and thus produce very high energy gamma rays. “We have already known since well over a decade that some of the 300 known supernova remnants in our Galaxy shine brightly in TeV gamma-rays”, explains Daniel Gottschall, PhD student in Pühlhofer’s research group. “But all these objects have been known before from observations in other wavebands and have been classified as supernova remnants”, adds Massimo Capasso, also PhD student in the research group.
More research questions
The question remains, states Gerd Pühlhofer, why these supernova remnants have escaped detection so far. “They are as large as the full moon, but totally invisible to the eye or to conventional, e.g. optical, telescopes”, explains the astrophysicist. He considers it possible that in previous sky surveys, because of their position in the Milky Way and because of their large extension, they were indistinguishable from the many other objects or they are partially covered by foreground gas. “A more exciting possibility would be if the new supernova remnants substantially differ from the other known big remnants that have been investigated with the H.E.S.S. telescopes before”, he adds. “They may belong to a special flavour of supernova remnants whose gamma-ray emission is induced by hadrons.”
The community of gamma-ray astronomers is currently preparing the much more sensitive next-generation instrument for TeV gamma-ray astronomy, the Cherenkov Telescope Array CTA. Scheduled to move into regular operations in the 2020’s, it will provide a much more detailed and sensitive image of our Milky Way in gamma-rays.
A search for new supernova remnant shells in the Galactic plane with H.E.S.S.
Corresponding authors: G. Pühlhofer, D. Gottschall, M. Capasso.
H. Abdallah et al. (H.E.S.S. collaboration), Astronomy & Astrophysics, Vol. 612,
A&A special issue:
Press release of the H.E.S.S. collaboration about the special issue:
University of Tübingen
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H.E.S.S. II press release of the University of Tübingen:
The H.E.S.S. Telescopes
The collaboration: The High Energy Stereoscopic System (H.E.S.S.) team consists of scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria, Sweden, and the Netherlands. The University of Tübingen is part of the H.E.S.S. collaboration through the High Energy Astrophysics Section of the Institute for Astronomy and Astrophysics Tübingen (IAAT), financially supported by the Federal Ministry for Education and Research.
The instrument: The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes – a couple of years ago complemented with the huge 28 m H.E.S.S. II telescope – is one of the most sensitive detectors of very high-energy gamma rays. These are absorbed in the atmosphere, where they create a short-lived shower of particles. The H.E.S.S. telescopes detect the faint, short flashes of bluish light which these particles emit (named Cherenkov light, lasting a few billionths of a second), collecting the light with big mirrors which reflect onto extremely sensitive cameras. The H.E.S.S. telescopes have been operating since late 2002. H.E.S.S. has discovered the majority of the known cosmic objects emitting very high-energy gamma rays. A study performed in 2009 listed H.E.S.S. among the top 10 observatories worldwide.
The Earth is constantly bombarded by high energy particles of cosmic origin. Those particles are protons, electrons and atomic nuclei and comprise the so-called “cosmic radiation”. Since more than a century, the origin of the cosmic rays remains one of the most enduring mysteries of science. The problem is: these “cosmic rays” are electrically charged, and are hence strongly deflected by the interstellar magnetic fields that pervade our galaxy. Their path through the cosmos is randomised by these deflections, making it impossible to directly identify the astrophysical sources responsible for their production. Cosmic rays interact with light and gas in the neighbourhood of their sources and thus produce high energy gamma rays. These gamma rays travel in straight lines to Earth, undeflected by magnetic fields. This gamma radiation can therefore be used to identify the sources of the cosmic rays at the sky.
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