Detection and imaging of atmospheric radio flashes from cosmic ray air showers

Top-end cosmic rays The origin of ultrahigh-energy cosmic rays is one of the most profound mysteries in high-energy astrophysics. No standard supernovae, pulsars or black holes can accelerate particles to such enormous energies, so various exotic sources have been proposed. An obstacle to identifying the source is the difficulty of finding out what particles are actually present in the rays: once they enter the Earth's atmosphere they lose their original identity, producing a shower of elementary particles travelling close to the speed of light. A collaboration between radio astronomers and particle physicists may offer a new way of learning more about the nature and structure of ultrahigh-energy cosmic rays. Using low-cost radio receivers it is possible to detect radio flashes coincident with cosmic-ray air showers. The radiation can be explained by the geosynchrotron effect. With radio telescopes and particle detectors trained on the incoming rays, their structure may soon be revealed. The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10 20 eV remains a mystery 1 . They are likely to be of extragalactic origin, but should be absorbed within ∼50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics 2 . Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or γ-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques 3 are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect 4 , 5 , 6 , 7 , 8 . Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing 9 , 10 .