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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 P. Best 10 A. Bonafede 11 F. Breitling 12 J. W. Broderick 13 W. N. Brouw 14 M. Brüggen 11 H. R. Butcher 2 D. Carbone 15 B. Ciardi 16 J. E. Conway 17 F. de Gasperin 18 E. de Geus 19 A. Deller 2 R.-J. Dettmar 20 G. van Diepen 2 S. Duscha 2 J. Eislöffel 21 D. Engels 18 J. E. Enriquez 22 R. A. Fallows 23 R. Fender 13 C. Ferrari 24 W. Frieswijk 2 M. A. Garrett 25, 2 Jean-Mathias Griessmeier 26, 27 A. W. Gunst 2 M. P. van Haarlem 2 T. E. Hassall 28 G. Heald 2 J. W. T. Hessels 15 M. Hoeft 21 A. Horneffer 29 M. Iacobelli 25 H. Intema 25 E. Juette 30 A. Karastergiou 31 V. I. Kondratiev 2 M. Kramer 29, 32 M. Kuniyoshi 29 G. Kuper 2 J. van Leeuwen 15 G. M. Loose 2 P. Maat 2 G. Mann 12 S. Markoff 33 R. Mcfadden 2 D. Mckay-Bukowski 34 J. P. Mckean 2 M. Mevius 2 D. D. Mulcahy 13 H. Munk 2 M. J. Norden 2 E. Orru 2 H. Paas 35 M. Pandey-Pommier 36 V. N. Pandey 37 M. Pietka 38 R. Pizzo 2 A. G. Polatidis 2 W. Reich 29 H. J. A. Röttgering 25 A. M. M. Scaife 39 D. J. Schwarz 40 M. Serylak 26, 27 J. Sluman 2 O. Smirnov 41 B. W. Stappers 28 M. Steinmetz 42 A. Stewart 43 J. Swinbank 33 Michel Tagger 26 Y. Tang 44 C. Tasse 45, 46 M. C. Toribio 47 R. Vermeulen 2 C. Vocks 12 C. Vogt 2 R. J. van Weeren 2 R. A. M. J. Wijers 15 S. J. Wijnholds 2 M. W. Wise 33, 2 O. Wucknitz 48 S. Yatawatta 2 P. Zarka 49, 50 J. A. Zensus 29 
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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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Submitted on : Tuesday, February 14, 2017 - 2:18:44 PM
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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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