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

A Corrigendum to this article was published on 20 July 2016

This article has been updated


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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Figure 1: Energy resolution.
Figure 2: Measurements of 〈Xmax〉.
Figure 3: Composition model fits.
Figure 4: p-value distribution for the four-component model.

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We acknowledge financial support from the Netherlands Organization for Scientific Research (NWO), VENI grant 639-041-130, the Netherlands Research School for Astronomy (NOVA), the Samenwerkingsverband Noord-Nederland (SNN) and the Foundation for Fundamental Research on Matter (FOM). We acknowledge funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC (grant agreement no. 227610) and under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 640130). LOFAR, the Low Frequency Array designed and constructed by ASTRON, has facilities in several countries that are owned by various parties (each with their own funding sources) and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy.

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Authors and Affiliations



All authors are part of the LOFAR collaboration and have contributed to the design, construction, calibration and maintenance of LOFAR and/or LORA. The first thirteen authors constitute the Cosmic Ray Key Science Project and have contributed to the acquisition, calibration and analysis of cosmic-ray radio data and LORA data. The manuscript was written by S.B. and subjected to an internal collaboration-wide review process. All authors approved the final version of the manuscript.

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Correspondence to S. Buitink.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Fitted lateral distributions.

Lateral distribution of radio-pulse power for all 118 measured showers (red circles) and the corresponding best-fitting CoREAS simulation (blue squares). The distance to the shower axis is the distance between the antenna and the axis of the air shower. Therefore, a value of 0 corresponds to an antenna that is located at the position where the shower axis reaches the ground. The ID numbers are unique values that are used to label the detected air showers. a.u., arbitrary units.

Extended Data Figure 2 Fitted lateral distributions.

Continuation of Extended Data Fig. 1.

Extended Data Figure 3 Fitted lateral distributions.

Continuation of Extended Data Fig. 2.

Extended Data Figure 4 Fitted lateral distributions.

Continuation of Extended Data Fig. 3.

Extended Data Figure 5 Fitted lateral distributions.

Continuation of Extended Data Fig. 4.

Extended Data Figure 6 Distribution of uncertainty on Xmax.

The distribution of the uncertainty on Xmax for all showers used in this analysis. The mean value is 16 g cm−2.

Extended Data Figure 7 Energy reconstruction.

Distributions of the ratio between true (Etrue) and reconstructed (Ereco) energy for proton (blue solid line) and iron (red dashed line) showers. The two types of showers have a systematic offset of the order of 1%.

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Buitink, S., Corstanje, A., Falcke, H. et al. A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations. Nature 531, 70–73 (2016).

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