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Ghostly Visitors

Ghostly Visitors

Published July 12, 2018


An international team of scientists, including two researchers from Georgia Tech, 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, made by the IceCube Neutrino Observatory at the Amundsen–Scott South Pole Station and in coordination with telescopes around the globe and in Earth’s orbit, help resolve a more than a century-old riddle about what sends subatomic particles such as neutrinos and cosmic rays speeding through the universe.

Since they were first detected over one hundred 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? Where do they come from?

Illustration of the IceCube detector shows the interaction of a neutrino with a molecule of ice

What is a neutrino?

Neutrinos are uncharged particles, unaffected by even the most powerful magnetic field. Because they rarely interact with matter and have almost no mass, they travel nearly undisturbed, giving scientists an almost direct pointer to their source.

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

Evidence of a Blazar as Neutrino Source

Two papers published July 13 in the journal Science have for the first time provided evidence for a known blazar as a source of high-energy neutrinos detected by the National Science Foundation-supported IceCube observatory. This blazar, designated by astronomers as TXS 0506+056, was first singled out following a neutrino alert sent by IceCube on September 22, 2017.

“The evidence for the observation of the first known source of high-energy neutrinos and cosmic rays is compelling,” said Francis Halzen, a University of Wisconsin–Madison professor of physics and principal investigator for the IceCube Neutrino Observatory.

Artist's concept of a blazar

What is a blazar?

A blazar is a galaxy with a super-massive black hole at its core. A signature feature of blazars is twin jets of light and elementary particles emitted from the poles along the axis of the black hole’s rotation. In this blazar, one of the jets points toward Earth.

“The era of multi-messenger astrophysics is here. Each messenger gives us a more complete understanding of the universe and important new insights into the most powerful objects and events in the sky,” said NSF Director France Córdova. “Such breakthroughs are only possible through a long-term commitment to fundamental research and investment in superb research facilities.”

A blazar is a galaxy with a super-massive black hole at its core. A signature feature of blazars is twin jets of light and elementary particles emitted from the poles along the axis of the black hole’s rotation. In this blazar, one of the jets points toward Earth. This blazar is situated in the night sky just off the left shoulder of the constellation Orion and is about four billion light years from Earth.

“Scientifically, this is very good news,” said Ignacio Taboada, an associate professor in Georgia Tech’s School of Physics and member of the Center for Relativistic Astrophysics also at Georgia Tech. As leader of the “Transients Science Working Group” within IceCube, he oversaw all the studies that inquired on the correlation TXS 0506+056’s gamma ray flare and the neutrino alert of September 22, 2017. “For years, we’ve had a long list of potential sources for high-energy neutrinos. Now we have a specific source – blazars – that we can look at very carefully.”

Georgia Tech Ph.D. student Chun Fai (Chris) Tung contributed to the publications by reconstructing archival IceCube data searching for very-high energy neutrinos that might be correlated with blazars other than TXS 0506+056.

Portrait of Ignacio Taboada

Georgia Tech Associate Professor Ignacio Taboada with a digital optical module of the kind used in the IceCube Neutrino Observatory. The observatory has 5,000 of the modules suspended in clear ice to detect blue light indicating passage of a neutrino.

“For years, we’ve had a long list of potential sources for high-energy neutrinos. Now we have a specific source – blazars – that we can look at very carefully.” — Ignacio Taboada

Taboada is also a team member of the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, a very-high-energy detector operating in central Mexico. HAWC studies transient sources, such as Active Galactic Nuclei (AGNs) and Gamma-Ray Bursts (GRBs). HAWC did not see the blazar flare, likely because it was too far away.

“At the highest energies, the universe is essentially opaque to very high energy gamma rays, and the farther away you are, the more opaque the universe is,” Taboada said. “If the blazar had been closer we likely would have seen it with HAWC.”

Illustration of station on the surface with sensors below the ice

In this artistic composition, 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, called DOMs. (IceCube/NSF)

Confirming the IceCube Observation

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 September 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.

Fermi was the first telescope to identify enhanced gamma-ray activity from TXS 0506+056 within 0.06 degrees of the IceCube neutrino direction. In a decade of Fermi observations of this source, this was the strongest flare in gamma rays. A later follow-up by MAGIC detected gamma rays of even higher energies.

A series of colored spheres on a black background

This image shows signals produced by a high-energy neutrino detected by IceCube on Sept. 22, 2017. With an estimated energy of 290 TeV, this was the tenth alert of this type sent by IceCube to the international astronomy community and launched a series of multi-messenger observations that allowed the identification of the first source of high-energy neutrinos and cosmic rays.

These observations prove that TXS 056+056 is one of the most luminous sources in the known universe and, thus, add support to a multimessenger observation of a cosmic engine powerful enough to accelerate high-energy cosmic rays and produce the associated neutrinos. Because neutrinos interact so weakly with matter, only one out of many millions that sailed through Antarctica’s ice, was detected by IceCube on September 22.

Bolstering these observations are coincident measurements from other instruments, including optical, radio and X-ray telescopes. “The ability to globally marshal telescopes to make a discovery using a variety of wavelengths in cooperation with a neutrino detector like IceCube marks a milestone in what scientists call multi-messenger astronomy,” said Halzen.

“These intriguing results also represent the remarkable culmination of thousands of human years of intensive activities by the IceCube Collaboration to bring the dream of neutrino astronomy to reality,” said Darren Grant, a professor of physics at the University of Alberta and the spokesperson of the IceCube Collaboration, an international team with over 300 scientists in 12 countries.

Solving a Century-Old Mystery

Austrian physicist Victor Hess showed, in 1912, that ionizing particles detected in the atmosphere arrive from space. These cosmic rays are the highest energy particles ever observed, with energies up to a hundred 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 only be created outside our galaxy and their sources have remained a mystery until now. Scientists had speculated that the most violent objects in the cosmos, like the mysterious gamma ray bursts, colliding galaxies, and the energetic black hole cores of galaxies known as active galactic nuclei, such as blazars, could be the sources.

Illustration with symbols of particles being directed by a blazar toward earth

In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. Neutrinos are always the result of a hadronic reaction such as the one displayed here. Gamma rays can be produced in both hadronic and electromagnetic interactions.

“Fermi has been monitoring some 2,000 blazars for a decade, which is how we were able to identify this blazar as the neutrino source,” said Regina Caputo, the analysis coordinator for the Fermi Large Area Telescope collaboration. “High-energy gamma rays can be produced either by accelerated electrons or protons. The observation of a neutrino, which is a hallmark of proton interactions, is the first definitive evidence of proton acceleration by black holes.”

“Now, we have identified at least one source of cosmic rays because it produces cosmic neutrinos. Neutrinos are the decay products of pions. In order to produce them, you need a proton accelerator,” said Halzen.

Cosmic rays are mostly protons and are sent speeding across the universe because the places where they are created act in the same way as particle accelerators on Earth, only they are far more powerful. “Theories predict that the emission of neutrinos will be accompanied by the release of gamma rays,” explained Razmik Mirzoyan, the spokesperson of the MAGIC Collaboration. But there are still a lot of questions on how blazars could accelerate particles to the highest energies. “Gamma rays provide information on how the ‘power plants’ in supermassive black holes work,” added Mirzoyan.

Neutrinos as New Astrophysical Messenger

As the latest astrophysical messenger to enter the game, neutrinos bring crucial new information to uncovering the inner workings of these cosmic ray accelerators. In particular, measurements of neutrinos can reveal the mechanisms for particle acceleration of the proton beam in the densest environments that even high-energy gamma rays may not escape.

Following the September 22 detection, the IceCube team quickly scoured the detector’s archival data and discovered a flare of over a dozen astrophysical neutrinos detected 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 the first known accelerator of the highest energy neutrinos and cosmic rays.

Detecting high-entry astrophysical neutrinos – particles from outside our galaxy – is no easy task. These particles pass through the Earth as if it were glass and can only be detected when they interact with atomic protons and neutrons that are massive enough to stop them. “For the most part, neutrinos go through everything and hardly ever stop to interact,” said Taboada.

The IceCube station on the surface at sunset

The IceCube Lab at sunset in 2017. (Martin Wolf, IceCube/NSF)

IceCube: the World’s Largest Particle Detector

IceCube is by volume the world’s largest particle detector. 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 through the detector’s grid of light sensors. Because the charged particle and light it creates stay essentially true to the neutrino’s direction, they give scientists a path to follow back to the source.

The South Pole was chosen for IceCube because the Antarctic ice is extremely transparent, more transparent than the best ice that can be made in the laboratory, Taboada said. “The more transparent the detector material, the better, because it allows us to see light generated by the particles from farther away.”

IceCube continuously monitors the sky, including through the Earth to the Northern Hemisphere, and detects a neutrino every few minutes. Most of the neutrinos it detects, however, are low energy, created by more common phenomena, such as the showers of subatomic particles stemming from cosmic ray particles crashing into atomic nuclei in the Earth’s atmosphere.

Particles of particular interest to the IceCube team pack a more energetic punch. The neutrino that alerted telescopes around the world had an energy of approximately 290 TeV. (The energy of the protons circulating in the 26.7-kilometer ring of the Large Hadron Collider is 6.5 TeV.)

IceCube was built specifically to identify and track high-energy neutrinos. In 2013, the collaboration announced the detection of the first neutrinos from beyond our galaxy and since has made numerous fundamental measurements in the emerging field of neutrino astronomy. The IceCube team also analyzes lower energy neutrinos, with outstanding results that are helping scientists make sense of matter in its most elementary forms.

IceCube Funded Primarily by the NSF

The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is operated by a team headquartered at the University of Wisconsin–Madison. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin–Madison Research Fund in the U.S.

The IceCube Collaboration, with over 300 scientists in 49 institutions from around the world, runs an extensive scientific program that has established the foundations of neutrino astronomy. Their research efforts, including critical contributions to the detector operation, are funded by funding agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the U.S.

About 20 observatories on Earth and in space have participated in the identification of what scientists deem to be a source of very high energy neutrinos and, thus, of cosmic rays. Several follow-up observations are detailed in a few other papers that are also being published.

-- Adapted from a news release provided by the IceCube Collaboration

Media Relations Contact: John Toon (404-894-6986) (

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