Excess neutrinos and missing gamma rays? Coronae of supermassive black holes may be the hidden sources of mysterious cosmic neutrinos seen on Earth.
The origin of high-energy cosmic neutrinos observed by the IceCube Neutrino Observatory, whose detector is buried deep in the Antarctic ice, is an enigma that has perplexed physicists and astronomers. A new model could help explain the unexpectedly large flux of some of these neutrinos inferred by recent neutrino and gamma-ray data. A paper by Penn State researchers describing the model, which points to the supermassive black holes found at the cores of active galaxies as the sources of these mysterious neutrinos, appears June 30, 2020 in the journal Physical Review Letters.
“Neutrinos are subatomic particles so tiny that their mass is nearly zero and they rarely interact with other matter,” said Kohta Murase, assistant professor of physics and of astronomy and astrophysics at Penn State and a member of Center for Multimessenger Astrophysics in the Institute for Gravitation and the Cosmos (IGC), who led the research. “High-energy cosmic neutrinos are created by energetic cosmic-ray accelerators in the universe, which may be extreme astrophysical objects such as black holes and neutron stars. They must be accompanied by gamma rays or electromagnetic waves at lower energies, and even sometimes gravitational waves. So, we expect the levels of these various ‘cosmic messengers’ that we observe to be related. Interestingly, the IceCube data have indicated an excess emission of neutrinos with energies below 100 teraelectron volt (TeV), compared to the level of corresponding high-energy gamma rays seen by the Fermi Gamma-ray Space Telescope.”
Scientists combine information from all of these cosmic messengers to learn about events in the universe and to reconstruct its evolution in the burgeoning field of “multimessenger astrophysics.” For extreme cosmic events, like massive stellar explosions and jets from supermassive black holes, that create neutrinos, this approach has helped astronomers pinpoint the distant sources and each additional messenger provides additional clues about the details of the phenomena.
For cosmic neutrinos above 100 TeV, previous research by the Penn State group showed that it is possible to have concordance with high-energy gamma rays and ultra-high-energy cosmic rays which fits with a multimessenger picture. However, there is growing evidence for an excess of neutrinos below 100 TeV, which cannot simply be explained. Very recently, the IceCube Neutrino Observatory reported another excess of high-energy neutrinos in the direction of one of the brightest active galaxies, known as NGC 1068, in the northern sky.
Reference: “Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical Tests with the MeV Gamma-Ray Connection” by Kohta Murase, Shigeo S. Kimura and Peter Mészáros, 30 June 2020, Physical Review Letters.