IceCube fails to see neutrinos from gamma-ray bursts

IceCube fails to see neutrinos from gamma-ray bursts. Cosmic-ray protons and nuclei with ultrahigh energies exceeding 10¹⁸ eV originate in powerful extragalactic accelerators; gamma-ray bursts (GRBs) are a well-studied possibility. From April 2008 to May 2010, even as the IceCube neutrino observatory was being constructed near the South Pole (see the article by Francis Halzen and Spencer Klein, PHYSICS TODAY, May 2008, page 29 ), it was already looking for the neutrinos that would be produced by the interactions of ultra-high-energy protons with the intense photon field of a GRB. If GRBs were the source of all ultra-high-energy cosmic rays, the detector would have registered at least some neutrinos coming from the bursts—exactly how many is model dependent, but the most popular model gives 8.4. However, the IceCube team has announced it saw none. Evidently, they concluded, either GRBs are not the exclusive source of ultra-high-energy gamma rays, or some basic GRB physics has eluded our understanding. IceCube, shown in the figure and now fully functional, contains 86 strings of photodetectors embedded deep in Antarctic ice. The detectors observe the Cherenkov radiation produced by energetic muons created by neutrino–ice interactions. Armed with a catalog of GRBs that exploded during their data runs—which used 40 and 59 strings of photodetectors—the IceCube team searched in vain for suitable muon tracks that coincided with a GRB and that pointed back to the burst. In addition to its continued search for GRB neutrinos, IceCube is looking for neutrinos from another class of impressive cosmic accelerator, active galactic nuclei. (R. Abbasi et al., IceCube collaboration, Nature 484, 351, 2012.)