Radio pulses from cosmic rays

Ultrahigh-energy cosmic rays (UHECRs), with energies exceeding 10¹⁹eV, are rare. A random square kilometer on Earth is hit by a few dozen per century. But their trajectories, relatively undeflected by intergalactic magnetic fields, might reveal their enigmatic sources. So astrophysicists build enormous ground-detector arrays. But even the flagship 3000-km² Pierre Auger Observatory in Argentina, which has been recording UHECRs since 2004, has not yet harvested enough to yield robust evidence of anisotropy that might reveal a class of sources (see PHYSICS TODAY, May 2010, page 15 ). Now a serendipitous discovery by the ANITA collaboration opens the prospect of surveilling much larger areas. While the balloon-borne ANITA radio detector was circumnavigating Antarctica at an altitude of 37 km, searching for signs of cosmogenic neutrinos interacting in the ice, it found something quite unexpected. In less than three weeks of monitoring for the nanosecond radio pulses expected from neutrino interactions, ANITA recorded 16 pulses whose polarization and phase characteristics convincingly marked them as coming from atmospheric UHECR showers. Interferometry with the gondola’s antenna array fixed each shower’s direction to within about 2°, revealing that most of the radio pulses had been reflected off the ice (at the red diamonds on the map). But two came directly from atmospheric showers (at the black squares). In either case, one learns the cosmic ray’s incident celestial direction. Year-round radio surveillance from an orbital platform that monitors reflections from the open oceans as well as the polar ice should greatly enhance the rate at which the highest-energy cosmic rays are harvested. (S. Hoover et al., Phys. Rev. Lett. 105, 151101, 2010 .)—Bertram Schwarzschild