Science + Technology

Astronomers identify source of neutrinos, a step toward resolving a century-old riddle

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IceCube Lab
IceCube/National Science Foundation

In this artist’s rendering, based on a real image of the IceCube Lab at the South Pole, a distant source emits neutrinos that are detected below the ice by IceCube sensors.

An international team of scientists has found the first evidence of a source of high-energy cosmic neutrinos — ghostly subatomic particles that can travel unhindered for billions of light-years from the most extreme environments in the universe to Earth.

The observations are a step toward resolving a century-old riddle about what sends subatomic particles such as neutrinos and cosmic rays speeding through the universe.

Cosmic rays are charged particles whose paths cannot be traced directly back to their sources due to the powerful magnetic fields that fill space and warp their trajectories. But the powerful cosmic accelerators that produce them also produce neutrinos, which are uncharged particles, unaffected by even the most powerful magnetic field. Because they rarely interact with matter and have almost no mass — they are sometimes called “ghost particles” — neutrinos travel nearly undisturbed from their accelerators, giving scientists an almost direct location of their source.

Jamie Yang, Savannah Guthrie/IceCube/NSF
The July 2018 cover of Science features neutrinos

Two research papers published July 13 in the journal Science have, for the first time, provided evidence for a giant elliptical galaxy with a massive, rapidly spinning black hole at its core (known as a “blazar”) as a source of high-energy neutrinos.

“This is one of the first concrete steps in a century in solving the mystery of the origin of ultra-high-energy cosmic rays,” said Nathan Whitehorn, UCLA assistant professor of physics and astronomy and a co-author of both Science papers.

The observations were made by the National Science Foundation-supported IceCube Neutrino Observatory at the Amundsen–Scott South Pole Station and confirmed by telescopes around the globe and in Earth’s orbit.

This blazar is in the constellation Orion, is about 4 billion light-years from Earth, and was first singled out by IceCube on Sept. 22, 2017. It is designated by astronomers as TXS 0506+056.

Since they were first detected more than 100 years ago, cosmic rays — highly energetic particles that continuously rain down on Earth from space — have posed an enduring mystery: What creates and launches these particles across such vast distances, and where do they originate?

Equipped with a nearly real-time alert system, triggered when a very high-energy neutrino collides with an atomic nucleus in the Antarctic ice in or near the IceCube detector, the observatory broadcast coordinates of the Sept. 22 neutrino alert to telescopes worldwide for follow-up observations. Two gamma-ray observatories, NASA’s orbiting Fermi Gamma-ray Space Telescope and the Major Atmospheric Gamma Imaging Cherenkov Telescope, or MAGIC, in the Canary Islands, detected a flare of high-energy gamma rays associated with TXS 0506+056, a convergence of observations that convincingly implicated the blazar as the most likely source. TXS 0506+056 is among the 100 most luminous sources in the known universe, Whitehorn said.

Bolstering the observations are measurements from other scientific instruments, including optical, radio and X-ray telescopes.

Cosmic rays are the highest energy particles ever observed, with energies up to 100 million times the energies of particles in the Large Hadron Collider at CERN in Switzerland, the most powerful human-made particle accelerator. These extremely high-energy cosmic rays can be created only outside our galaxy and their sources have remained a mystery until now.

Following the Sept. 22 incident, the IceCube team quickly scoured the detector’s archival data and found a flare, or dramatic brightening, of more than a dozen astrophysical neutrinos discovered in late 2014 and early 2015, coincident with the same blazar, TXS 0506+056. This independent observation greatly strengthens the initial detection of a single high-energy neutrino and adds to a growing body of data that indicates TXS 0506+056 is likely the first identified accelerator of the highest energy neutrinos and cosmic rays, Whitehorn said.

Detecting the highest energy neutrinos requires a massive particle detector, and IceCube fits the bill. Encompassing a cubic kilometer of deep, pristine ice a mile beneath the surface at the South Pole, the detector is composed of more than 5,000 light sensors arranged in a grid. When a neutrino interacts with the nucleus of an atom, it creates a secondary charged particle, which, in turn, produces a characteristic cone of blue light that is detected by IceCube and mapped.

IceCube continuously monitors the sky, in all directions, and detects a neutrino every few minutes.

UCLA is among 49 institutions in 12 countries that are IceCube collaborating institutions.

The lead scientist for the IceCube Neutrino Observatory is Francis Halzen, a University of Wisconsin–Madison professor of physics.

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