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

S. Buitink 1 A. Corstanje 1 H. Falcke 1, 2 J. R. Hörandel 1 T. Huege 3 A. Nelles 1 J. P. Rachen 1 L. Rossetto 1 P. Schellart 1 O. Scholten 4 S. ter Veen 5 S. Thoudam 1 T. N. G. Trinh 6 J. Anderson 7 A. Asgekar 2 I. M. Avruch 8 M. E. Bell 9 M. J. Bentum 2 G. Bernardi 10 P. Best 11 A. Bonafede 12 F. Breitling 13 J. W. Broderick 14 W. N. Brouw 15 M. Brüggen 12 H. R. Butcher 2 D. Carbone 16 B. Ciardi 17 J. E. Conway 18 F. de Gasperin 19 E. de Geus 20 A. Deller 2 R.-J. Dettmar 21 G. van Diepen 2 S. Duscha 2 J. Eislöffel 22 D. Engels 19 J. E. Enriquez 23 R. A. Fallows 24 R. Fender 14 C. Ferrari 25 W. Frieswijk 2 M. A. Garrett 26, 2 Jean-Mathias Grießmeier 27, 28 A. W. Gunst 2 M. P. van Haarlem 2 T. E. Hassall 29 G. Heald 2 J. W. T. Hessels 16 M. Hoeft 22 A. Horneffer 30 M. Iacobelli 26 H. Intema 26 E. Juette 31 A. Karastergiou 32 V. I. Kondratiev 2 M. Kramer 30, 33 M. Kuniyoshi 30 G. Kuper 2 J. van Leeuwen 16 G. M. Loose 2 P. Maat 2 G. Mann 34 S. Markoff 35 R. Mcfadden 36 D. Mckay-Bukowski 37 J. P. Mckean 2 M. Mevius 2 D. D. Mulcahy 14 H. Munk 36 M. J. Norden 2 E. Orru 2 H. Paas 38 M. Pandey-Pommier 39 V. N. Pandey 40 M. Pietka 41 R. Pizzo 2 A. G. Polatidis 2 W. Reich 30 H. J. A. Röttgering 26 A. M. M. Scaife 42 D. J. Schwarz 43 M. Serylak 27, 28 J. Sluman 2 O. Smirnov 44 B. W. Stappers 29 M. Steinmetz 45 A. Stewart 46 J. Swinbank 35 Michel Tagger 27 Y. Tang 47 C. Tasse 48, 49 M. C. Toribio 50 R. Vermeulen 2 C. Vocks 34 C. Vogt 2 R. J. van Weeren 2 R. A. M. J. Wijers 16 S. J. Wijnholds 2 M. W. Wise 35, 2 O. Wucknitz 51 S. Yatawatta 2 P. Zarka 52 J. A. Zensus 30
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Abstract : 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.
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S. Buitink, A. Corstanje, H. Falcke, J. R. Hörandel, T. Huege, et al.. A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations. Nature, Nature Publishing Group, 2016, 531 (7592), pp.70-73. ⟨10.1038/nature16976⟩. ⟨insu-01321748⟩



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