Cosmic Neutrino Pevatrons: A Brand New Pathway to Astronomy, Astrophysics, and Particle Physics

TL;DR

This review analyzes IceCube’s evidence for extraterrestrial high-energy neutrinos and systematically evaluates Galactic and extragalactic origin scenarios, linking neutrino fluxes to cosmic ray and gamma-ray constraints. It assesses production channels (pp vs $p\gamma$), source classes (GRBs, AGN/blazars, starbursts, newborn pulsars), and the energetics needed to explain the observed PeV flux, while exploring cosmogenic neutrinos and potential new physics signals. The work emphasizes multimessenger consistency, diffusion-driven spectral shapes, and flavor composition as key discriminants, and outlines future data-driven tests to pinpoint sources or reveal new physics. The analysis highlights that both Galactic and extragalactic origins remain plausible given current statistics, with flavor measurements and anisotropy studies poised to break degeneracies as data accumulate.

Abstract

The announcement by the IceCube Collaboration of the observation of 28 cosmic neutrino candidates has been greeted with a great deal of justified excitement. The data reported so far depart by 4.3σfrom the expected atmospheric neutrino background, which raises the obvious question: "Where in the Cosmos are these neutrinos coming from?" We review the many possibilities which have been explored in the literature to address this question, including origins at either Galactic or extragalactic celestial objects. For completeness, we also briefly discuss new physics processes which may either explain or be constrained by IceCube data.

Figures (16)

• Figure 1: Schematic of the IceCube instrument, which covers a cubic kilometer of Antarctic glacial ice. It detects neutrinos by observing Cherenkov light from secondary charged particles produced in neutrino-nucleon interactions. This light is detected by an array of 5160 DOMs, each of which contains a photomultiplier and readout electronics housed in a clear glass pressure resistant sphere. The DOMs are arranged into an array of 86 vertical strings, with 60 DOMs per string at depths between 1450 m and 2450 m. Outside of the DeepCore low-energy subarray, these DOMs are vertically spaced at 17-meter intervals and the strings are on average 125 m apart horizontally. The DeepCore subarray fills in the center of the detector with a denser array of photomultipliers and provides a lower energy threshold of 10 GeV over a fraction of the IceCube volume. Figure courtesy of the IceCube Collaboration.
• Figure 2: Atmospheric muon and electron neutrino spectrum as function of energy. The open and filled symbols represent measurements of various detectors (Super-Kamiokande GonzalezGarcia:2006ay, Fréjus Daum:1994bf, AMANDA forward folding analysis Abbasi:2009nfa and unfolding analysis Abbasi:2010qv, IceCube (40 strings) forward folding $\nu_\mu$ analysis Abbasi:2011jx, unfolding $\nu_\mu$ analysis Abbasi:2010ie, and $\nu_e$Aartsen:2012uu). Curved lines are theoretical predictions of atmospheric fluxes. The conventional $\nu_e$ (red solid line) and $\nu_\mu$ (blue solid line) from Ref. Sanuki:2006yd, and $\nu_e$ (red dotted line) from Ref. Barr:2006it. The (magenta) band for the prompt flux indicates the theoretical uncertainty of charm-induced neutrinos Enberg:2008te. From Ref. Aartsen:2012uu. It should be noted that this figure contains deceptive spectra which do not consist of measured data points at given energies, but spectra fitted as a whole, and hence the individual "error bars" are misleading at best. The IceCube $\nu_e$ data is an exception. Super-Kamiokande has contained events for muons up to about 10 GeV and electrons up to about 100 GeV. (Frejus had no events in the highest energies in which they report a flux measurement with error bars, for example.) One should note that folding techniques can obscure any unexpected distortions of the tested spectra, which are typically simple power law distributions.
• Figure 3: The two highest energy neutrino events reported by the IceCube Collaboration. The left panel corresponds to the event called Bert that ocurred in August 2011, whereas the right panel shows the event in January 2012, called Ernie. Each sphere represents a DOM. Colors represent the arrival times of the photons where red indicates early and blue late times. The size of the spheres is a measure for the recorded number of photo-electrons. Figure courtesy of the IceCube Collaboration.
• Figure 4: Distribution of the deposited energies (left) and declination angles (right) of the IceCube observed events compared to model predictions. Energies plotted are in-detector visible energies, which are lower limits on the neutrino energy. Note that deposited energy spectra are always harder than the spectrum of the neutrinos that produced them due to the neutrino cross-section increasing with energy. The expected rate of atmospheric neutrinos is based on northern hemisphere muon neutrino observations at lower energies. The estimated distribution of the background from atmospheric muons is shown in red. Due to lack of statistics from data far above the cut threshold, the shape of the distributions from muons in this figure has been determined using Monte Carlo simulations with total rate normalized to the estimate obtained from the in-data control sample. Combined statistical and systematic uncertainties on the sum of backgrounds are indicated with a hatched area. The gray line shows the best-fit canonical $E^{-2}$ astrophysical spectrum with all-flavor normalization (1:1:1) of $E_\nu^2 \Phi_\nu^{\rm total} (E_\nu) = 3.6 \times 10^{-8}~{\rm GeV} \, {\rm cm}^{-2} \, {\rm s}^{-1} \, {\rm sr}^{-1}$ and a spectral cutoff of 2 PeV derived in Aartsen:2013bka. (An $E_\nu^{-2}$ spectrum is used here as a reference, as this spectral index is expected for canonical firs-order Fermi shock acceleration. In reality, this index may be somewhat larger or smaller.) From Ref. Aartsen:2013pza.
• Figure 5: IceCube skymap in equatorial coordinates of the Test Statistic value (TS) from the maximum likelihood point-source analysis. The most significant cluster consists of five events---all showers and including the second-highest energy event in the sample---with a final significance of 8%. This is not sufficient to identify any neutrino sources from the clustering study. The galactic plane is shown as a gray line with the galactic center denoted as a filled gray square. Best-fit locations of individual events (listed in Table \ref{['tab:events']}) are indicated with vertical crosses ($+$) for showers and angled crosses ($\times$) for muon tracks. From Ref. Aartsen:2013pza.