Extremely energetic cosmic rays from a preferred direction Available to Purchase

Earth is continuously bombarded by cosmic rays—high-energy protons or nuclei—that come from beyond our galaxy. The energy spectrum falls rapidly at the so-called Greisen-Zatsepin-Kuzmin (GZK) cutoff of about 6 × 10¹⁹ eV, but cosmic rays have been observed with energies up to 3 × 10²⁰ eV. Astrophysicists have long sought to determine what accelerates particles to such extraordinary energies. Possibilities include supernovae and relativistic jets from active galactic nuclei. Now the Telescope Array experiment has provided an enticing clue by identifying a “hotspot” in the northern sky that sends a disproportionate amount of ultrahigh-energy cosmic rays (UHECRs) our way. See the figure; red indicates greater flux. (The Pierre Auger Observatory had previously spotted a weaker hotspot in the southern sky.) In a five-year scan that concluded in May 2013, the Telescope Array identified 72 cosmic rays with energies above the GZK limit, via the shower of particles...