First image of TeV-energy gamma-rays of a cosmic source : a supernova remnant

The existence of high energy particles bombarding constantly the Earth (cosmic rays) is known since nearly a century. If it is clear that this radiation consists of particles, mainly protons of energy up to 1020 eV and electrons accelerated to nearly the light velocity in the Galaxy, the origin of cosmic rays and the localization of their acceleration sites is still debated. The shock fronts of supernovae, observed in their shell-like remnants are generally believed to be the principal accelerators of galactic cosmic rays. At the time of explosion, the external layers of the star are ejected at relativistic speeds. They form a piston producing, in the interstellar medium, a shock wave in which effective mechanisms of particle acceleration are at work. We thus expect interactions of protons or of high energy ions with the surrounding medium as well as inverse-Compton emission with relativistic electrons, both processes leading to the production of extreme energy gamma rays. Up to now, the TeV emission of supernovae remnants has been detected or suspected in three cases only (RX J1713.7-3946, Cas A and SN 1006) and with a rather large uncertainty on the position of the emitting source because of the low spatial resolution of the instruments of the generation preceding HESS, but also because these sources are extended. The telescope array H.E.S.S. could obtain for the first time in August 2003 (during its development phase with only 2 telescopes), a map of the supernova remnant RX J1713.7-3946 between 800 GeV and 10 TeV. Thanks to the stereoscopic observation of the Cherenkov blue light of the cosmic showers produced in the upper Earth’s atmosphere by the extreme gamma rays coming from this source, it was possible to derive the direction of the electromagnetic signal with a precision of about one arc minute and to reveal the shell structure of this very high energy emission.

Figure 1 Image in linear color scales of TeV gamma-rays from the supernova remnant RX J1713.7-3946. The spatial resolution from H.E.S.S. is of 3 arcmin. The superposed contours, in linear scales, are the X-rays of energy 1-3 KeV observed by the satellite ASCA, with a comparable resolution.

In the figure, on the TeV gamma-rays colour map are superimposed the one as dtected in X-rays with the Japanese satellite ASCA. The fact that these maps observed at different energies coincide tends to prove that very high energy particles are indeed accelerated in situ in the shell. However various emission processes can contribute to the gamma radiation observed. Now that the network of 4 telescopes is fully operational, it will be possible to study spectroscopically different zones of the shell and in consequence to obtain even more precise information on the mechanisms at work. The TeV map presented here represents, not only one important step in the understanding of the origin of galactic cosmic radiation, but shows the very new imagery capabilities of a system working at energies 12 orders of magnitude higher than that of the visible light. Probably we assist there to the birth of a new way to observe the sky...

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Reference Direct evidence for high-energy particle acceleration in the shell of a supernova remnant Aharonian et al, Nature, 4 novembre 2004

Contacts Catherine Boisson (Observatoire de Paris, LUTH) Helène Sol (Observatoire de Paris, LUTH)

Dernière modification le 4 mars 2013