Abstract
We review the status of the IceCube observations of cosmic neutrinos. We investigate model-independent constraints on the properties of the sources where they originate. Specifically, we evaluate the multimessenger relations connecting neutrino, gamma ray, and cosmic ray observations and conclude that neutrinos are ubiquitous in the nonthermal universe, suggesting a more significant role than previously anticipated. Subsequently, we study the implications of IceCube’s upper limits on the flux from individual point sources, as well as on the “guaranteed” flux of cosmogenic neutrinos.
Highlights
Multimessenger astronomy With the commissioning of the IceCube [1,2] and Advanced LIGO [3] facilities, we are able to observe the universe for the first time using three distinct astronomical messengers
The first challenge is to select a pure sample of neutrinos, roughly 100,000 per year above a threshold of 0.1 TeV for IceCube, in a background of ten billion cosmic ray muons, while the second is to identify the small fraction of these neutrinos that is astrophysical in origin, observed at the level of tens of events per year
The corresponding electromagnetic emission via neutral pions as well as e± pairs is constrained by the isotropic gamma ray background (IGRB) observed by Fermi LAT [107,108] and limits the neutrino intensity of these proton-dominated scenarios [109,110,111,112,113,114]
Summary
Multimessenger astronomy With the commissioning of the IceCube [1,2] and Advanced LIGO [3] facilities, we are able to observe the universe for the first time using three distinct astronomical messengers. An immediate inference made about the large neutrino flux observed by IceCube, which is predominantly extragalactic in origin, is that the total energy density of neutrinos in the high-energy universe is similar to that of gamma rays. Protons accelerated in regions of high magnetic fields near neutron stars or black holes may interact with the radiation or dust surrounding them to produce pions and kaons that decay into neutrinos.
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