Neutrinos from IceCube detector Point to Long-Sought Cosmic Ray Accelerator

In this artistic 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. Image credit: IceCube/NSF

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

The observations, initially made by the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station and confirmed by ground and space-based telescopes around the globe, help resolve a century-old riddle about what sends subatomic particles such as neutrinos and protons speeding through the universe. The coordinated effort relied on an alert system that UMD researchers played a lead role in developing. 

Two papers published July 13 in the journal Science provide evidence that energetic particles may originate in jets spewing forth from supermassive black holes — astrophysical objects dubbed blazars because of the blazingly bright light they emit toward Earth. One blazar, designated TXS 0506+056 by astronomers, was singled out as a possible source following an automated alert sent around the world by IceCube on September 22, 2017. 

Equipped with a nearly real-time alert system — developed in part by researchers at UMD and triggered by a single, very high-energy neutrino colliding with an atomic nucleus in the Antarctic ice in or near IceCube’s detectors — the IceCube experiment broadcast the coordinates from which the Sept. 22 neutrino likely came. 

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 that also appeared to come from TXS 0506+056, a convergence of observations that convincingly implicated the blazar as the most likely source. 

“This result really highlights the importance of taking a multimessenger approach to these searches,” said Erik Blaufuss, a research scientist in the UMD Department of Physics who led the effort over the past several years to create and deploy IceCube’s high-energy event alert system. “Any one observation made alone would likely not have let us piece together what is actually going on inside this 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, the highest-energy photons. A later follow-up by MAGIC detected gamma rays of even higher energies, and observations from other instruments — including optical, radio, and X-ray telescopes — bolstered the case for TXS 0506+056 as the source of the September 22 neutrino. 

The result demonstrates the advantage of combining the signals from different cosmic messengers, like neutrinos and photons. 

“The era of multimessenger astrophysics is here,” said National Science Foundation Director France Córdova. “Each messenger — from electromagnetic radiation, gravitational waves and now neutrinos — gives us a more complete understanding of the universe, and important new insights into the most powerful objects and events in the sky. Such breakthroughs are only possible through a long-term commitment to fundamental research and investment in superb research facilities.” 

Blazar TXS 0506+056: A tale of cosmic messengers. Credit: Weronika Racz and Igor Rams

Since they were first detected more than a hundred years ago, cosmic rays — highly energetic particles that continuously rain down on Earth from space — have posed enduring questions: What creates and launches these particles across such vast distances? Where do they come from? 

Because cosmic rays are charged particles, their paths are bent by the magnetic fields that fill space. But the powerful cosmic accelerators that produce them also produce neutrinos, which are uncharged and thus unaffected by even the most powerful magnetic fields. Because they rarely interact with matter and have almost no mass, neutrinos travel nearly undisturbed from their creation, giving scientists an almost direct pointer to their source. 

Detecting the highest energy neutrinos requires a massive particle detector, and IceCube is the world’s largest by volume. 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 sensitive cameras. Because a charged particle and the light it creates stay essentially true to the neutrino’s direction, they give scientists a path to follow back to the source. 

Blazer neutrino detection over Antarctica. IceCube/NSF

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 300 TeV. (The energy of the protons circulating in the 26.7-kilometer ring of the Large Hadron Collider is 6.5 TeV.) 

Following the Septiembre 22 detection, the IceCube team quickly scoured the detector’s archival data and discovered a flare of more than 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. 

Blazar TXS 0506+056: A tale of cosmic messengers. Credit: Max Planck Institute for Physics

Sources: University of Maryland, Wikipedia, Max Planck Institute for Physics, MAGIC,