Exploring blazars as astrophysical neutrino sources

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.

Blazars IceCube
90% CL flux upper limits for all 2LAC blazars in comparison to the observed astrophysical diffuse neutrino flux. Image: IceCube Collaboration

The Large Area Telescope (LAT) is the primary instrument of the Fermi Gamma-ray Space Telescope (Fermi) mission and has been imaging the high-energy gamma-ray sky, covering the energy range from below 20 MeV to above 300 GeV. The Fermi-LAT catalog (2LAC) includes 862 GeV-emitting blazars at galactic latitudes above 10 degrees.

This analysis looks for a cumulative signal from the blazars in the 2LAC catalog. Any search for a cumulative signal needs to make some assumptions about the relative contribution of each source. This is included in the likelihood analysis as a weight for each source.

IceCube researchers have performed searches on five subsets of the 2LAC blazar sample, including one with all sources. Two different weighting techniques were used for each search. The gamma-ray weighting model uses as a weight the gamma-ray energy flux observed for each source by Fermi-LAT. In the equal weighting model, which gives the same weight to all sources, IceCube performs a complementary and model-independent test. This test only relies on the hypothesis that the source count distributions of gamma rays and neutrinos have comparable shapes, i.e., that the evolution of the neutrino and gamma-ray luminosity of the blazar population throughout the history of the universe are similar.

Nine of these ten searches observed some overfluctuations in the neutrino signal. Unfortunately, the statistical power is too weak to determine a genuine signal from a fluctuation of the atmospheric background. Additional observational data is needed for this. “By now we have four more years of IceCube data and a new LAT catalog that resolves almost twice as many blazars,” says Thorsten Glüsenkamp, one of the leading authors of this search and then a graduate student at DESY. “If the gamma-ray blazars indeed contribute to a significant fraction of the astrophysical neutrino flux, we should be able to confirm this over the next few years. However, already with this study we know that they cannot be responsible for all of the cosmic neutrinos observed by IceCube. So there will be another puzzle to solve,” adds Glüsenkamp.

+ info “The contribution of Fermi-2LAC blazars to the diffuse TeV-PeV neutrino flux,” The IceCube Collaboration: M. G. Aartsen et al. The Astrophysical Journal 835 (2017) 1, iopscience.iop.org arxiv.org/abs/1611.03874