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.
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.
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.
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.
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.
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.
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.
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.
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.