IceCube Finds Evidence of the First High Energy Neutrino Source

IceCube Neutrinos Point to Long-Sought Cosmic Ray Accelerator

The observations, made by the IceCube Neutrino Observatory at the Amundsen–Scott South Pole Station — confirmed by telescopes around the globe and in orbit around Earth — help resolve a century-old riddle about the source of extremely energetic subatomic particles such as neutrinos and cosmic rays speeding through the Universe.

Since they were first detected over a hundred years ago, cosmic rays — highly energetic particles that continuously rain down on Earth from space — have posed an enduring mystery: Where do they come from? How are they accelerated to energies comparable to that of a penalty kick in football, but concentrated in a single sub-atomic particle?

Because cosmic rays are charged particles, their paths are bent in the magnetic fields that fill space, bending their trajectories so that they do not point back to their source. The powerful cosmic accelerators that produce these cosmic rays will also produce neutrinos: uncharged particles, unaffected by even the most intense magnetic field. Because they rarely interact with matter and have almost no mass — earning them their sobriquet “ghost particle”— neutrinos travel undisturbed from their accelerators, giving scientists an almost direct pointer back to their source.

Two papers published on July 13, 2018 in the journal Science have, for the first time, provided detailed evidence for a known blazar as a source of high-energy neutrinos detected by the IceCube observatory. This blazar, designated by astronomers as TXS 0506+056, was first singled out following a neutrino alert sent by IceCube on 22 September 2017.

“When NBI joined the IceCube collaboration in 2013 we could hardly have foreseen that within a few months we would make the first detection of high energy cosmic neutrinos … and within another five years identify an astrophysical source of these particles” says Subir Sarkar, DNRF Niels Bohr Professor at NBI. “All of us involved with IceCube are thrilled to be part of such a major step forwards in astroparticle physics”.

Blazars are a type active galactic nuclei, a class of giant elliptical galaxies with a supermassive, rapidly spinning black hole at its core shooting out twin jets of light and elementary particles in opposite directions. Blazars have one of these jets points directly at the Earth. This blazar is situated in the night sky just off the left shoulder of the constellation Orion and is about 4 billion light years distant from Earth.

 

Equipped with a nearly real-time alert system — triggered when a very high-energy neutrino collides with an atomic nucleus in the polar ice in or near the IceCube detector — the observatory broadcast coordinates of the 22/9/17 neutrino alert to telescopes worldwide for follow-up observations. Two gamma-ray observatories — the Fermi space telescope and the MAGIC telescope 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.  

These observations show that TXS 0506+056 is one of the most luminous sources in the known Universe, and thus add support to a multi-messenger observation of a cosmic engine powerful enough to accelerate high-energy cosmic rays and produce the associated neutrinos. Only one of these neutrinos, out of many millions that sailed through Antarctica’s ice, was detected by IceCube on 22 September 2017.

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,” says Prof. Francis Halzen, Principal Investigator of the IceCube neutrino telescope.

 

Austrian physicist Victor Hess showed in 1912 that the ionizing particles scientists were detecting in the atmosphere were coming from space. Cosmic rays are the highest energy particles ever observed, with energies going up to a hundred million times the energies achievable at the Large Hadron Collider at CERN in Geneva, 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 remnants of exploding stars (supernovae), colliding galaxies, and the energetic black holes in the cores of galaxies (active galactic nuclei) such as blazars, could be the sources.

Cosmic rays are mainly protons and as they emerge from their cosmic sources they will interact with ambient matter and radiation. “From our understanding of high energy particle interactions we can predict that this will generate both neutrinos and gamma rays” explains Markus Ahlers, Asst. Professor at NBI. But there are many open questions on how blazars could accelerate particles to such high energies. “So far we have had insufficient data to answer these questions” adds Markus.

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 acceleration of the proton beam in the densest environments from which even high-energy gamma rays may not escape.

Following the 22 Sept. 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 cosmic rays which create neutrinos.

Detecting the highest energy neutrinos requires a massive particle detector, and IceCube is by volume the world’s largest. 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’s grid of photomultiplier tubes. 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.

IceCube continuously monitors the sky, including watching 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 astronomy team pack a more energetic punch. The neutrino that alerted telescopes around the world had an energy of approximately 300 TeV. (The energy of the protons circulating in the 27 km ring of the Large Hadron Collider is 6.5 TeV.) The alert was made possible by work from Mohamed Rameez, a postdoc at NBI. “Watching what I thought at first was a routine alert turn  into a series of detections across the electromagnetic spectrum from around the world has been an immensely satisfying experience.” remarks Rameez, who as part of his PhD work searched for cosmic sources of neutrinos with IceCube data and is pleased to have contributed to the alert system that finally made it possible.

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.

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.A. (https://icecube.wisc.edu/collaboration/institutions).

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 also detailed in additional papers released today.

Links to the Science Papers:

Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert

Scientific Contacts:

• D. Jason Koskinen, Assistant professor and NBI-IceCube group leader, koskinen@nbi.ku.dk
• Subir Sarkar, Niels Bohr Professor, sarkar@nbi.ku.dk
• Markus Ahlers, Assistant Professor, ahlers@nbi.ku.dk

Mohamed Rameez, Postdoctoral Researcher, mohamed.rameez@nbi.ku.dk

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