University of Wisconsin-Madison

Research Highlights: Neutrino Astronomy and Multimessenger Astrophysics

IceCube’s detection of the first high-energy astrophysical neutrino flux points to cosmic neutrinos as the key messengers to reveal an unobstructed view of the universe at wavelengths where it is opaque to light.


Energy and wavelength spectra vs distance of the visible universe. About a fifth of the universe cannot be explored using photon-based telescopes.
Energy and wavelength spectra vs distance of the visible universe. About a fifth of the universe cannot be explored using photon-based telescopes.

Six years after its completion, IceCube has isolated more than 80 high-energy cosmic neutrinos, with energies between 100 TeV and 10 PeV, from more than a million atmospheric neutrinos and hundreds of billions of cosmic-ray muons.

Distribution of the median expected neutrino energy assuming the best-fit spectral index of 2.13. The black crosses correspond to experimental data and blue/red to the conventional atmospheric/astrophysical expectation weighted to the best-fit spectrum.
Distribution of the median expected neutrino energy assuming the best-fit spectral index of 2.13. The black crosses correspond to experimental data and blue/red to the conventional atmospheric/astrophysical expectation weighted to the best-fit spectrum.

In order to filter out this huge atmospheric background, our searches for astrophysical neutrinos focus on high-energy events that start in the detector or that originate in the Northern Hemisphere. The sky map below shows the highest energy events for both searches.

Sky map in equatorial coordinates of the arrival direction of the highest energy events from the starting track search (HESE, green) and the search for muon neutrinos from the Northern Hemisphere (red dotted circles). In the HESE sample, cascade events are indicated together with their median angular uncertainty (thin circles). The angular uncertainty for all tracks, i.e., all red and X-circles is smaller than one degree. The energy in TeV is indicated for each event, either the deposited energy (HESE) or the muon energy proxy. HESE events have a lower energy cut since the atmospheric background in this search is smaller. One event (*) appears in the event sample of both analyses. The grey-shaded region indicates the zenith angle range where Earth absorption of 100 TeV neutrinos is larger than 90%.
Sky map in equatorial coordinates of the arrival direction of the highest energy events from the starting track search (HESE, green) and the search for muon neutrinos from the Northern Hemisphere (red dotted circles). In the HESE sample, cascade events are indicated together with their median angular uncertainty (thin circles). The angular uncertainty for all tracks, i.e., all red and X-circles is smaller than one degree. The energy in TeV is indicated for each event, either the deposited energy (HESE) or the muon energy proxy. HESE events have a lower energy cut since the atmospheric background in this search is smaller. One event (*) appears in the event sample of both analyses. The grey-shaded region indicates the zenith angle range where Earth absorption of 100 TeV neutrinos is larger than 90%.

We have also conducted searches for cosmogenic neutrinos produced in the interactions of cosmic rays with microwave photons. Their energies typically exceed 100 PeV, but so far we have not observed any neutrino above 10 PeV. IceCube currently has the world’s best limit on the flux of cosmogenic neutrinos, which places very strong constraints on the sources of ultra-high-energy cosmic rays (UHECR). Proton-dominated sources are already disfavored.

The PeV neutrinos observed in IceCube, the highest energy neutrinos to date, have a thousand times the energy of the highest energy neutrinos produced with earthbound accelerators and a billion times the energy of the neutrinos detected from supernova SN1987 in the Large Magellanic Cloud, the only neutrinos that had been detected on Earth from outside the solar system prior to IceCube’s breakthrough. However, the most surprising property of these cosmic neutrinos is their large flux rather than their high energy or their origination outside our galaxy.


IceCube detects high-energy neutrinos using the Cherenkov light produced by relativistic charged particles that result from the interaction of these neutrinos with a nucleus of Antarctic ice. The highest energy neutrinos detected to date are included in this video, which also shows a simulated event and the blue Cherenkov cone.

The large neutrino flux observed implies that the total energy density of neutrinos in the high-energy universe is similar to that of gamma rays, but no gamma ray has ever been observed above 10 TeV. The explanation for this nonobservation is revealing. Since the universe is not transparent to the highest energy photons, primary PeV gamma rays are expected to produce lower energy photons after their interaction with the microwave background, resulting in a photon flux in the GeV-TeV energy range. Data from the Fermi satellite is consistent with this expectation, suggesting that neutrinos and gamma rays may originate in common sources.

The figure shows that the astrophysical neutrino flux (black line) observed by IceCube matches the corresponding gamma-ray flux (red line) observed by Fermi. The black data points are combined IceCube results, showing the flux of cosmic neutrinos interacting inside the detector. Also shown, shaded in blue, is the best fit to the flux of cosmic muon neutrinos penetrating the Earth. (see <a href="http://inspirehep.net/record/1492165"target="_blank">paper</a>)
The figure shows that the astrophysical neutrino flux (black line) observed by IceCube matches the corresponding gamma-ray flux (red line) observed by Fermi. The black data points are combined IceCube results, showing the flux of cosmic neutrinos interacting inside the detector. Also shown, shaded in blue, is the best fit to the flux of cosmic muon neutrinos penetrating the Earth. (see paper)

This large neutrino flux also implies that a significant fraction, possibly all, of the energy in the nonthermal universe is generated in hadronic accelerators. The nonthermal universe contains collapsed objects such as black holes or neutron stars. High-energy neutrinos, which are unique fingerprints of hadron acceleration, therefore represent a discovery potential for either revealing new sources or unveiling new insight into the energy generation of known sources.

We have performed all-sky surveys looking for extended regions of neutrino emission as well as bright and faint point sources. These searches use both throughgoing muon tracks and events starting in the detector along with new techniques to search for point sources with neutrino emission below 100 TeV and in the southern sky. No evidence of neutrino emission has been found, but some ideas on the origin of cosmic rays have been challenged.

Discovery potential (5σ, solid red) and sensitivity (dashed red) for an unbroken power-law astrophysical neutrino flux shown against declination. The dashed gray line shows the results by ANTARES. Upper limits of source candidates from gamma-ray catalogs are depicted by red crosses. The blue line represents the upper limit of the flux level of the most significant spots observed in each half of the sky for all declinations; the actual declination position of the spots is indicated by a star.
Discovery potential (5σ, solid red) and sensitivity (dashed red) for an unbroken power-law astrophysical neutrino flux shown against declination. The dashed gray line shows the results by ANTARES. Upper limits of source candidates from gamma-ray catalogs are depicted by red crosses. The blue line represents the upper limit of the flux level of the most significant spots observed in each half of the sky for all declinations; the actual declination position of the spots is indicated by a star.

GRBs were initially the most promising candidate sources of ultra-high-energy cosmic rays (UHECRs). However, IceCube’s searches for neutrino emission in coincidence with GRBs proved that the GRB contribution to the observed astrophysical neutrino flux cannot be larger than about 1%. Recently, we have also expanded these searches to the Southern Hemisphere.

The search for sources of astrophysical neutrinos now focuses on active galactic nuclei (AGNs), especially in blazars, which are known to dominate the extragalactic gamma-ray emission. We have searched for neutrino emission associated with blazars listed in the second Fermi-LAT AGN catalog and did not find any significant correlation. We estimated that the contribution of these blazars to the neutrino flux is less than 27%, possibly even less than 10%, if one assumes proportionality between the gamma-ray and the neutrino flux level of the source.

Flux upper limits at 90% CL for all blazars in the second Fermi-LAT AGN catalog in comparison to the observed astrophysical diffuse neutrino flux.
Flux upper limits at 90% CL for all blazars in the second Fermi-LAT AGN catalog in comparison to the observed astrophysical diffuse neutrino flux.

Neutrinos are not the only new messengers challenging our understanding of the universe. The first gravitational wave (GW) detection by LIGO has pointed to the existence of thirty-solar mass black holes, providing again a glimpse into the most energetic and enigmatic objects in the cosmos. IceCube and LIGO, together with ANTARES and other observatories, are working hand in hand to piece together this new knowledge about our universe, building a new view of the extreme cosmos that anticipates discovery. So far, the first joint neutrino-GW searches have not yet identified a common source, but the analysis techniques are now in place to study an increasing number of GW and high-energy neutrino events.

Sky map for the second gravitational wave (GW) detected by LIGO and the reconstructed directions for high-energy neutrino candidates detected by IceCube (green crosses) and ANTARES (blue cross) within ±500 s around the GW signals. The GW sky map shows the reconstructed probability density contours of the GW event at 90% CL.The neutrino directional uncertainties are below 1◦ for most of the candidates and in any case are too small to be shown. Map in equatorial coordinates.
Sky map for the second gravitational wave (GW) detected by LIGO and the reconstructed directions for high-energy neutrino candidates detected by IceCube (green crosses) and ANTARES (blue cross) within ±500 s around the GW signals. The GW sky map shows the reconstructed probability density contours of the GW event at 90% CL.The neutrino directional uncertainties are below 1◦ for most of the candidates and in any case are too small to be shown. Map in equatorial coordinates.

IceCube has developed a powerful real-time follow-up program that targets the detection of transient sources. This multimessenger program sends alerts of single and clusters of high-energy neutrino events (multiplets), typically within one minute of the event detection. In collaboration with other observatories, we aim at identifying the electromagnetic counterpart of a rapidly fading source or coincident gravitational waves. Single event alerts are distributed publicly as GCN alerts, while multiplet alerts are distributed through individual agreements with optical, X-ray, and gamma-ray observatories. Searches for bursts of low-energy neutrinos from nearby supernovas are performed, and above threshold detection is announced rapidly within of the SNEWS network.


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