When scientists imagine the sources of cosmic neutrinos, they think big: a supernova, or a merger of binary neutron stars, collapsing to form a black hole, or maybe a black hole feeding off matter around it and releasing jets expanding for hundreds of thousands of light-years, with energies up to a thousand times greater than the energy output of the entire Milky Way. We may find that they are not exactly like this, but the sources of very high energy neutrinos will have to be some of the most extreme environments in the universe.
IceCube already excluded gamma-ray bursts (GRBs), which could be produced in supernova or binary neutron star collapsing environments, as a significant source of astrophysical neutrinos. And the search for extragalactic sources now focuses on blazars, a type of active galactic nuclei with relativistic jets oriented directly towards Earth. Blazars are known to dominate the extragalactic gamma-ray emission, and physicists wonder if they are also what dominates the extragalactic high-energy neutrino emission.
A new study from the IceCube Collaboration searched in three years of IceCube data for directional clustering of neutrinos around gamma-ray sources associated with blazars from the second Fermi-LAT AGN catalog. Although some enhancements in the observed neutrino rate from these blazars were found in the 3–30 TeV region, all of them are compatible with fluctuations of the atmospheric neutrino background. From this analysis, IceCube estimates the contribution of Fermi-LAT detected blazars to the neutrino flux to be less than 27%. Their contribution cannot be any larger than 10% if one assumes a proportionality between the gamma-ray and the neutrino emission. These results, submitted today to The Astrophysical Journal, open several new analyses exploring blazars as very high energy neutrino sources.
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