Supermassive black hole crowns may be the hidden sources of mysterious cosmic neutrinos seen on Earth

The origin of the high-energy cosmic neutrinos observed by the IceCube Neutrino Observatory, whose detector is buried deep in the Antarctic ice, is a puzzle that has puzzled physicists and astronomers. A new model could help explain the unexpectedly large flux of some of these neutrinos inferred by recent gamma ray and neutrino data. A paper by Penn State researchers describing the model, which points to supermassive black holes found in the nuclei of active galaxies as the sources of these mysterious neutrinos, appears in the journal June 30, 2020. Physical Review Letters.

“Neutrinos are subatomic particles so small that their mass is almost zero and they rarely interact with other matter,” said Kohta Murase, an assistant professor of physics and astronomy and astronomy at Penn State and a member of the Center for Astrophysics Multimessenger at the Gravitation Institute. and the Cosmos (IGC), who led the investigation. “High-energy cosmic neutrinos are created by energetic cosmic ray accelerators in the universe, which can 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 has indicated excess neutrino emission with energies below 100 teraelectron volts (TeV) , compared to the level of corresponding high-energy gamma rays seen by the Fermi gamma-ray space telescope. “

Scientists combine information from all these cosmic messengers to learn about events in the universe and reconstruct their evolution in the burgeoning field of “multimessenger astrophysics.” For extreme cosmic events, such as massive stellar explosions and supermassive black hole jets, which create neutrinos, this approach has helped astronomers identify distant sources, and each additional messenger provides additional clues to 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 a match for high energy gamma rays and ultra high energy cosmic rays that fit a multimessenger image. However, there is increasing evidence of an excess of neutrinos below 100 TeV, which cannot be explained simply. 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.

“We know that high-energy neutrino sources must also create gamma rays, so the question is: where are these missing gamma rays?” Murase said. “The sources are somewhat hidden from our view in high-energy gamma rays, and the energy budget of neutrinos released into the universe is surprisingly large. The best candidates for this type of source have dense environments, where gamma rays they would be blocked by their interactions with radiation and matter, but neutrinos can easily escape. Our new model shows that supermassive black hole systems are promising sites, and the model can explain neutrinos below 100 TeV with modest energy requirements. “

The new model suggests that the corona, the super-hot plasma aura that surrounds stars and other celestial bodies, around the supermassive black holes at the core of galaxies, could be a source. Analogous to the corona seen in an image of the Sun during a solar eclipse, astrophysicists believe that black holes have a corona over the rotating disk of material, known as an accretion disk, that forms around the black hole through its influence gravitational. This corona is extremely hot (with a temperature of about a billion Kelvin), magnetized and turbulent. In this environment, particles can accelerate, leading to particle collisions that would create neutrinos and gamma rays, but the environment is dense enough to prevent high-energy gamma rays from escaping.

“The model also predicts electromagnetic counterparts of neutrino sources in ‘soft’ gamma rays rather than high energy gamma rays,” said Murase. “High-energy gamma rays would block, but this is not the end of the story. Over time, they would cascade to lower energies and be released as ‘soft’ gamma rays in the megaelectron volt range, but Most existing gamma-ray detectors, like the Fermi gamma-ray space telescope, are not tuned to detect them. “

There are projects in development that are specifically designed to explore such gentle gamma-ray emission from space. In addition, the next and next generation neutrino detectors, KM3Net in the Mediterranean Sea and IceCube-Gen2 in Antarctica will be more sensitive to sources. Promising targets include NGC 1068 in the northern sky, for which excess neutrino emission was reported, and several of the brightest active galaxies in the southern sky.

“These new gamma ray and neutrino detectors will allow deeper searches for multimessenger emission from supermassive black hole crowns,” said Murase. “This will allow us to critically examine whether these sources are responsible for the large mid-energy level neutrino flux observed by IceCube as our model predicts.”