The Beginning of Extra-Galactic Neutrino Astronomy

Cosmic rays—the steady shower of high-energy electrons, protons, nuclei, and antiparticles hitting Earth—vary in energy from 109 electron volts (eV) to more than 1020 eV. Scientists have suggested that cosmic rays at energies lower than 1019 eV come from Galactic sources, but no known objects within our Galaxy are capable of accelerating particles to energies beyond 1019 eV[1]. The missing information needed to understand these mysterious, likely extra-Galactic, cosmic accelerators could come from the recent detection of unusually high-energy neutrinos, from 1012 electron volts (teraelectron-volt, or TeV) to 1015 eV (peta-electron-volt, or PeV), by the IceCube collaboration [2, 3], whose latest analysis appears in Physical Review Letters[4]. Although new experiments are needed to confirm that these neutrinos come from the same sources as the highest-energy cosmic rays, IceCube’s signal already points in this direction. The leading explanation for cosmic-ray particles with energies beyond 1019 eV—known as ultrahigh-energy (UHE) cosmic rays—is that they come from gamma-ray bursts (GRBs) or active Galactic nuclei (AGN). Both objects are powered by the rapid accretion of mass onto black holes, and it is this swirling matter, which produces relativistic jets of particles, that is thought to be a source of cosmic rays. But although GRBs and AGNs are understood on a phenomenological level, constructing a model for the structure of these objects and how they act as “cosmic accelerators” is considered an outstanding goal of astrophysics. Neutrino “telescopes” are designed to detect neutrinos that potentially come from the same sources that produce high-energy cosmic rays [5]. Neutrinos emerge when protons or nuclei interact with plasma or with a radiation field within or surrounding the source: the interaction produces pions, whose decay produces one electron neutrino and two muon neutrinos. Each neutrino carries 5% of the energy per nucleon of the parent cosmic-ray particle. But unlike charged cosmic rays, chargeless neutrinos are not deflected by magnetic fields on their trajectory to Earth, which provides the advantage that they can be compared to light from the same potential source. And unlike photons, neutrinos can escape from deep within a source, carrying useful information about its physics. From the observed cosmic-ray flux, and its falloff with energy, we can infer something about the sources of the particles, and hence estimate the flux of neutrinos we can expect to see from the same sources. Above 1019 eV, the cosmic-ray flux is consistent with a cosmological distribution of sources that produce mostly protons at a rate (Q) that is independent of energy (E): dQ/dlogE = QUHE = (0.5 ± 0.15) × 1044 erg/Mpc3yr. This “flat” spectrum is a generic prediction of astrophysical acceleration models. Analyses of data from different experiments differ as to the identity of the ultrahigh-energy cosmic rays [1]. But if they are indeed protons—as inferred by some experiments—and assuming that the protons convert all of their energy to pions, an upper bound on the neutrino flux of E2dΦ/dE < EdΦWB/dE = 3.4 × 10−8 GeV/cm2sr s, known as the Waxman-Bahcall (WB) bound, is obtained [6]. This bound, and the fact that neutrinos interact extremely weakly with matter, implies a detector for neutrinos in the energy range of 1 TeV to 1 PeV has to have a mass of 1 billion tons. A much larger effective mass is required at higher energy. The IceCube detector achieves a 1 billion ton effective mass by burying photomultiplier detectors in a cubic kilometer of ice in the South Pole. When an electron neutrino collides with an atom in the ice, it generates a broad shower of particles, while muon (tau) neutrinos can generate kilometer-long muon (tau) tracks; both produce light that is detected by the photomultipliers (Fig.1). The detectors can pinpoint the origin of a neutrino to within a degree, based on the direction of its associated track. (The sources of neutrino events that result in showers can only be defined to within tens of degrees.) In 2013, IceCube reported on two years of data taking,