Research Highlights: Neutrino Astronomy and Multimessenger Astrophysics
Neutrino astronomy has emerged to reveal the sources of the highest energy particles in our universe, the so-called ultra-high energy cosmic rays (UHECR). IceCube’s detection of the first high-energy astrophysical neutrino flux confirmed cosmic neutrinos as the key messengers to reveal an unobstructed view of the universe at wavelengths where it is opaque to light.
The large neutrino flux measured by IceCube also implies that a significant fraction, possibly all, of the energy in the nonthermal universe is generated in powerful hadronic accelerators powered by objects such as black holes or neutron stars.
In July 2018, IceCube, gamma-ray telescopes Fermi and MAGIC, and several other experiments announced the detection of neutrinos and photons from blazar TXS 0506+056. These results constitute the first-ever identification of a likely source of extragalactic neutrinos and of high-energy cosmic rays.
This breakthrough detection is the outcome of a multimessenger collaboration with detectors and scientists across the globe and in space. Follow-up observations by gamma-ray, X-ray and optical telescopes were triggered by a real-time neutrino alert from IceCube on September 22, 2017.
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 to identify the electromagnetic counterpart of a rapidly fading source or coincident gravitational waves. Single event alerts are distributed publicly as GCN alerts, while multiple 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 the SNEWS network.
Seven years after its completion, IceCube has isolated more than 100 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.
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. The observation of neutrino and gamma-ray emission from TXS0506+06 is the first evidence that blazars, and possibly other sources of gamma rays, are the sources of both gamma-ray and neutrino emission at the highest energies.