Abstract
Protons with energies up to ∼1015 eV are the main component1 of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking2. Electrons are known to be accelerated to cosmic-ray energies in supernova remnants3,4, 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 (π0), 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 distribution5,6. Here we report the observation of cascade showers of optical photons resulting from γ-rays at energies of ∼1012 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.
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Acknowledgements
We thank H. Tomida for help in understanding the ASCA observational results, M. Seta for comments on molecular cloud density, and F. Aharonian for suggestions. This work was supported by a Grant-in-Aid for Scientific Research by the Japan Ministry of Education, Science, Sports and Culture, and the Australian Research Council.
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Enomoto, R., Tanimori, T., Naito, T. et al. The acceleration of cosmic-ray protons in the supernova remnant RX J1713.7–3946. Nature 416, 823–826 (2002). https://doi.org/10.1038/416823a
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DOI: https://doi.org/10.1038/416823a
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