Hidden sources of mysterious cosmic neutrinos seen on Earth

Date:
July 1, 2020
Source:
Penn State
Summary:
A new model points to the coronoe of supermassive black holes at
the cores of active galaxies to help explain the excess neutrinos
observed by the IceCube Neutrino Observatory.



FULL STORY ==========================================================================
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 [DATE] in the journal
Physical Reviews 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.

"We know that the sources of high-energy neutrinos must also create
gamma rays, so the question is: Where are these missing gamma rays?" said Murase. "The sources are somehow 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 would be blocked by their interactions with radiation
and matter but neutrinos can readily escape. Our new model shows that supermassive black hole systems are promising sites and the model can
explain the neutrinos below 100 TeV with modest energetics requirements."
The new model suggests that the corona -- the aura of superhot plasma that surrounds stars and other celestial bodies -- around supermassive black
holes found at the core of galaxies, could be such a source. Analogous
to the corona seen in a picture of the Sun during a solar eclipse, astrophysicists believe that black holes have a corona above the rotating
disk of material, known as an accretion disk, that forms around the black
hole through its gravitational influence. This corona is extremely hot
(with a temperature of about one billion degrees kelvin), magnetized,
and turbulent. In this environment, particles can be accelerated, which
leads to particle collisions that would create neutrinos and gamma rays,
but the environment is dense enough to prevent the escape of high-energy
gamma rays.

"The model also predicts electromagnetic counterparts of the neutrino
sources in `soft' gamma-rays instead of high-energy gamma rays," said
Murase. "High- energy gamma rays would be blocked but this is not the end
of the story. They would eventually be cascaded down to lower energies
and released as `soft' gamma rays in the megaelectron volt range, but
most of the existing gamma-ray detectors, like the Fermi Gamma-ray
Space Telescope, are not tuned to detect them." There are projects
under development that are designed specifically to explore such soft
gamma-ray emission from space. Furthermore, upcoming and next- generation neutrino detectors, KM3Net in the Mediterranean Sea and IceCube-Gen2 in Antarctica will be more sensitive to the sources. The promising targets
include NGC 1068 in the northern sky, for which the excess neutrino
emission was reported, and several of the brightest active galaxies in
the southern sky.

"These new gamma-ray and neutrino detectors will enable deeper searches
for multimessenger emission from supermassive black hole coronae,"
said Murase.

"This will make it possible to critically examine if these sources are responsible for the large flux of mid-energy level neutrinos observed
by IceCube as our model predicts."

========================================================================== Story Source: Materials provided by Penn_State. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Kohta Murase, Shigeo S. Kimura, Peter Me'sza'ros. Hidden Cores
of Active
Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical
Tests with the MeV Gamma-Ray Connection. Physical Review Letters,
2020; 125 (1) DOI: 10.1103/PhysRevLett.125.011101 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200701084750.htm

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