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A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations

2016; Nature Portfolio; Volume: 531; Issue: 7592 Linguagem: Inglês

10.1038/nature16976

1476-4687

S. Buitink, A. Corstanje, H. Falcke, J.R. Hörandel, T. Huege, A. Nelles, J. P. Rachen, L. Rossetto, P. Schellart, O. Schölten, S. ter Veen, S. Thoudam, T. N. G. Trinh, J. M. Anderson, A. Asgekar, I. M. Avruch, M. E. Bell, Mark Bentum, G. Bernardi, P. N. Best, A. Bonafede, F. Breitling, J. W. Broderick, W. N. Brouw, M. Brüggen, H. R. Butcher, D. Carbone, B. Ciardi, J. E. Conway, F. de Gasperin, E. de Geus, Adam T. Deller, R.‐J. Dettmar, Ger van Diepen, S. Duscha, J. Eislöffel, D. Engels, J. Emilio Enriquez, R. A. Fallows, R. P. Fender, C. Ferrari, W. Frieswijk, M. A. Garrett, J.‐M. Grießmeier, A. W. Gunst, M. P. van Haarlem, Tim Hassall, G. Heald, J. W. T. Hessels, M. Hoeft, A. Horneffer, M. Iacobelli, H. T. Intema, E. Juette, A. Karastergiou, V. I. Kondratiev, M. Krämer, M. Kuniyoshi, G. Küper, J. van Leeuwen, G. M. Loose, P. Maat, G. Mann, Sera Markoff, R. McFadden, D. McKay‐Bukowski, J. P. McKean, M. Mevius, D. D. Mulcahy, H. Munk, M. J. Norden, E. Orrù, H. Paas, M. Pandey-Pommier, V. N. Pandey, M. Pietka, R. Pizzo, A. G. Polatidis, W. Reich, H. J. A. Röttgering, Anna M. M. Scaife, Dominik J. Schwarz, M. Serylak, J. Sluman, O. Smirnov, B. W. Stappers, Matthias Steinmetz, A. Stewart, J. Swinbank, M. Tagger, Y. Tang, C. Tasse, M. C. Toribio, R. C. Vermeulen, C. Vocks, C. Vogt, R. J. van Weeren, R. A. M. J. Wijers, S. J. Wijnholds, M. W. Wise, O. Wucknitz, S. Yatawatta, Philippe Zarka, J. A. Zensus,

Dark Matter and Cosmic Phenomena

High-resolution radio measurements of air showers—cascades of secondary particles in the atmosphere initiated by cosmic rays—reveal that cosmic rays with energies of 1017–1017.5 electronvolts have a mixed composition, with light elements (protons and helium nuclei) making up 80 per cent of their mass. Stijn Buitink et al. report on the mass composition of cosmic rays in the energy range 1017 to 1017.5 electron volts, derived from LOFAR radio telescope measurements of cosmic ray initiated cascades of secondary particles (air showers) in the atmosphere. They find a mixed composition, containing a light-mass fraction of approximately 80%. Unless the extragalactic cosmic ray component becomes significant below 1017.5 electron volts, these findings indicate an additional Galactic component dominating in this energy range. Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 1017–1018 electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal1 comes from accelerators capable of producing cosmic rays of these energies2. Cosmic rays initiate air showers—cascades of secondary particles in the atmosphere—and their masses can be inferred from measurements of the atmospheric depth of the shower maximum3 (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground4. Current measurements5 have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays6,7,8 is a rapidly developing technique9 for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front6,12. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 1017–1017.5 electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 1017.5 electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 1017–1017.5 electronvolt range.