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)

1 Radboud University [Nijmegen]
2 ASTRON - Netherlands Institute for Radio Astronomy
3 Institute for Nuclear Physics (IKP), Karlsruhe Institute of Technology (
4 University of Groningen [Groningen]
5 IMAPP - Institute for Mathematics, Astrophysics and Particle Physics
6 LAGA - Laboratoire Analyse, Géométrie et Applications
7 Institute for Mathematics Applied to Geoscience
8 SRON - SRON Netherlands Institute for Space Research
9 ATNF - Australia Telescope National Facility
10 University of Edinburgh
11 Jacobs University [Bremen]
12 AIP - Leibniz-Institut für Astrophysik Potsdam
13 University of Southampton
14 Kapteyn Astronomical Institute [Groningen]
15 UvA - University of Amsterdam [Amsterdam]
16 Max Planck Institute for Astrophysics
17 Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology
18 Hamburger Sternwarte/Hamburg Observatory
19 Medstar Research Institute
20 Astronomisches Institut der Ruhr-Universität Bochum
21 TLS - Thüringer Landessternwarte Tautenburg
22 SETI Institute
23 Institute of Mathematical and Physical Sciences
24 Département de Géologie
25 Leiden Observatory [Leiden]
26 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
27 USN - Unité Scientifique de la Station de Nançay
28 Jodrell Bank Centre for Astrophysics
29 MPIFR - Max-Planck-Institut für Radioastronomie
30 Ruhr-Universität Bochum [Bochum]
31 Oxford Astrophysics
32 CAL - Columbia Astrophysics Laboratory
33 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
34 University of Oulu
35 Center for Information Technology CIT
36 CRAL - Centre de Recherche Astrophysique de Lyon
37 NRAO - National Radio Astronomy Observatory [Charlottesville]
38 University of Oxford
39 School of Physics and Astronomy [Southampton]
40 Interactions Son Musique Mouvement
41 Rhodes University, Grahamstown
42 DSMZ - Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH / Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures
43 LASP - Laboratory for Atmospheric and Space Physics [Boulder]
44 UP11 - Université Paris-Sud - Paris 11
45 SKA South Africa
46 GEPI - Galaxies, Etoiles, Physique, Instrumentation
47 Finca El Encin
48 AlfA - Argelander-Institut für Astronomie
49 OP - Observatoire de Paris - Site de Paris
50 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique

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.

Keywords: Particle astrophysics; High-energy astrophysics

Domains: Sciences of the Universe [physics]

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, 2016, 531 (7592), pp.70-73. ⟨10.1038/nature16976⟩. ⟨insu-01321748⟩