Extremely energetic cosmic rays from a preferred direction

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 created when the UHECRs interact with Earth’s atmosphere. Ground-based scintillators recorded the arrival of shower particles, and fluorescence detectors monitored the energy deposited in atmospheric nitrogen. (See Physics Today, May 2007, page 17 .) About 25% of the observed UHECRs came from the direction of the hotspot, which fills only 6% of the northern sky. The hotspot is large enough to cover many potential UHECR sources; its extent arises in part because galactic and intergalactic magnetic fields bend UHECRs and thus muddle their source locations. Fortunately, particles with energies near and beyond the GZK cutoff bend little enough that a larger and more sensitive array may yet reveal their origins. (R. U. Abbasi et al., Astrophys. J. 790, L21, 2014, doi:10.1088/2041-8205/790/2/L21 .)