Protons with energies up to ~ 10 15 eV are the main component 1 of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking 2 . Electrons are known to be accelerated to cosmic-ray energies in supernova remnants 3,4 , and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (π 0 ), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in γ-rays with a particular spectral-energy distribution 5,6 . Here we report the observation of cascade showers of optical photons resulting fromγ-rays at energies of ~ 10 12 eV hitting Earth's upper atmosphere, in the direction of the supernova remnant RX J1713.7-3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.
In this Letter we report the discovery of TeV gamma-ray emission from a supernova remnant made with the CANGAROO 3.8 m telescope. TeV gamma rays were detected at the sky position and extension coincident with the northeast rim of shell-type supernova remnant (SNR) SN 1006 (Type Ia). SN 1006 has been a most likely candidate for an extended TeV gamma-ray source, since the clear synchrotron X-ray emission from the rims was recently observed by ASCA (Koyama et al.), which is strong evidence for the existence of very high energy (up to hundreds of TeV) electrons in the SNR. The observed TeV gamma-ray flux was (2.4 ע 0.0.7 [systematic]) # 10 3.0 ע 0.9 (4.6 ע 0.6 ע 1.4) # 10 1.7 ע 0.5 from the 1996 and 1997 observations, respectively. Also, we set an upper limit on the TeV gamma-ray emission from the southwest rim, which is estimated to be cm Ϫ2 s Ϫ1 (≥ TeV, 95% confidence level) Ϫ12
The energy spectrum of primary cosmic rays above 1017.0 eV has been updated from data collected with two atensive air shower arrays operating at k e n o , one with area 1 kmz and the other with area 20 kmz. Along with our previous results in the lower energy region, the energy Spectrum has been determined over about five decades of energy from 10L4.5 eV to 1019.8 eV. A change in the index of the power-law energy SpectNm is observed around 1017.s eV, as well as the usual features, namely the lolee around 1015.7 eV and the ankle around 1019.0 CV: The indica of the differential power-law energy spectrumare: (2.62+0.12) below lO"-'eV, (3.02f0.03) for lo".' -1017.8eV and (3.16 f 0.08) for -1018.geV There is an indicalion of a flattening of the spectrum above .. 10t8.' eV with an index of (2.8 f 0.3). The flux above 1018eV is(1.5 -2.4) x m-2s-Lsr-1 and is in good agreement with other experiments.The number of showers above 1019.5cV is seven for an exposure of 80 km'yrsr and lunher investigation by a new Akeno giant air shower array (AGASA), whose operation has started, is necessary to determine a cutoff energy in the spectrum, if any.
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