High-energy particle acceleration in the shell of a supernova remnant

Abstract

A significant fraction of the energy density of the interstellar medium is in the form of highenergy charged particles (cosmic rays) 1 . The origin of these particles remains uncertain.Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically 2,3 , unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7−3946 (G347.3−0.5), which was discovered by ROSAT 4 in the X-ray spectrum and later claimed as a source of high-energy γ-rays 5,6 of TeV energies (1 TeV=10 12 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that veryhigh-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks. RX J1713.7−3946, together with several other southern hemisphere SNRs, is a prime target for observations with the High Energy Stereoscopic System (H.E.S.S.), a new system of four imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia.H.E.S.S. 7,8 (we note that V. F. Hess discovered cosmic rays) exploits the most effective detection technique for very-high-energy γ-rays, namely, the imaging of Cherenkov light from air showers. This technique, which was pioneered by the Whipple collaboration 9 , makes use of the fact that whenever a high-energy γ-ray hits the Earth's atmosphere it is absorbed and initiates a cascade of interactions with air atoms, leading to the formation of a shower of secondary charged particles.Those travelling faster than the local speed of light in air emit Cherenkov radiation, which results in a brief flash of blue Cherenkov light detectable at ground level. By using a telescope with sufficient mirror area to collect enough of the faint light signal, and a fast camera with fine pixelation, one can image the shower and reconstruct from this image the direction and energy of the primary γ-ray.Combined with the approach of stereoscopic imaging of the cascade using a system of telescopes, as pioneered by the HEGRA collaboration 10 , this yields a very powerful technique for imaging and obtaining energy spectra of astronomical sources at TeV energies.The H.E.S.S. experiment is such a stereoscopic system that consists of four 13-m-diameter telescopes 11 spaced at the corners of a square of side 120 m, each equipped with a 960-phototube camera 12 covering a large field of view of diameter 5°. Construction of the telescope system started in 2001; the full array was completed in December 2003 with the commissioning of the fourth telescope. HESS has an angular resolution of ...

First ground-based measurement of atmospheric Cherenkov light from cosmic rays

“…The High Energy Stereoscopic System (H.E.S.S.) [4] has clearly identified supernova shock waves as sources of high-energy particles [5,6]. However, the nature of these particles -electrons or cosmic-ray nucleons-remains under debate.…”

TeV neutrinos from supernova remnants embedded in giant molecular clouds

“…Only recently, a new generation of atmospheric Cherenkov telescopes, especially HESS, were able to find the smoking-gun of the interaction of CRs accelerated by an active SNR with a close MC. The most interesting HESS observations in this respect are those of RXJ1713.7-3946 [5] and the very recent measurements of the Galactic Centre (GC) ridge emission [6].…”

Observational Signatures of Particle Acceleration in Supernova Remnants

“…Also, X-ray synchrotron emitting TeV electrons were identified at the shock fronts of the SN 1006 supernova remnant and others (Koyama et al, 1995, and see section 2.3). In the last decade, GeV and TeV γ-ray emission has been observed from several supernova remnants (Table 1.1), indicating the presence of particles with GeV and TeV energies (e.g., Aharonian et al, 2004;, and see section 2.13).…”

“…1 , 2 Albert et al (2007b); Acciari et al (2010), 3 Fesen et al (2006), 4 Reynolds et al (2008), 5 H.E.S.S. Collaboration et al (2011), 6 Aharonian et al (2009), 7 Stephenson and Green (2002), 8 Abdo and Fermi LAT Collaboration (2011), 9 Aharonian et al (2004), 10 Wang et al (1997), 11 Tanaka et al (2011), 12 HESS Collaboration: F. A. , 13 Katsuda et al (2008), 14 Acero et al (2010), 15 Park et al (2010), 16 Giordano et al (2011), 17 Acciari et al (2011) Drury, 2001; Schure et al, 2012, for a review). The somewhat softer CR spectrum observed in the Earth's, might be explained by propagation effects of CRs through the Galaxy.…”