12 July 2018
An international team of scientists, including a number from the UK supported by STFC, 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 international team of 300 scientists included UK researchers from Queen Mary University of London and University of Oxford and the observations, made by the IceCube Neutrino Observatory at the Amundsen–Scott South Pole Station and confirmed by 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?
Cosmic rays are charged particles but their paths cannot be traced directly back to their sources due to the powerful magnetic fields that fill space and warp their trajectories.
However, the powerful cosmic accelerators that produce cosmic rays will also produce neutrinos. Neutrinos are uncharged particles, unaffected by even the most powerful magnetic field. They rarely interact with matter and have almost no mass, so travel nearly undisturbed from their accelerators, giving scientists an almost direct pointer to their source.
For the first time evidence is pointing to a blazar, a giant elliptical galaxy with a massive, rapidly spinning black hole at its core, as the source of high-energy neutrinos.
This blazar, designated by astronomers as TXS 0506+056, was detected by the National Science Foundation-supported IceCube observatory. Situated in the night sky just off the left shoulder of the constellation Orion this blazar is about 4 billion light years from Earth.
The aurora australis shines above NSF's IceCube Neutrino Observatory at NSF's Amundsen-Scott South Pole Station.
(Credit: Sven Lidstrom / NSF)
Dr Teppei Katori, IceCube member from Queen Mary University of London, said: "Multi messenger astronomy and neutrino astronomy are at the next stage. There are two pieces of evidence that high energy astrophysical neutrino signals are related to the distant known blazar, TXS 0506+056, which is about 4 billion light years away. Further studies will shed light on the long-term question, ‘where are high energy cosmic rays coming from?
“Neutrinos have three types, or ‘flavours’. This is new information we can use to observe the universe. Now, we can see (light), listen (gravitational wave), and taste (neutrino) the universe, and multi-messenger astronomy is indeed in the new era! In the UK, we are working on the future extension of the IceCube, IceCube-Gen2, which has a dramatically higher observation capability with these events."
Scientists had speculated that the most violent objects in the cosmos, things like supernova remnants, colliding galaxies, and the energetic black hole cores of galaxies known as active galactic nuclei, such as blazars, could be the sources.
The IceCube team scoured the detector’s archival data and discovered a flare of over a dozen astrophysical neutrinos detected in late 2014 and early 2015, with the same blazar, TXS 0506+056.
This independent observation greatly strengthens the initial detection of a single high-energy neutrino and indicates that TXS 0506+056 is the first known accelerator of the highest energy neutrinos and cosmic rays.
“The evidence for the observation of the first known source of high-energy neutrinos and cosmic rays is compelling,” says Francis Halzen, a University of Wisconsin–Madison professor of physics and the lead scientist for the IceCube Neutrino Observatory.
STFC Media Manager
The IceCube Neutrino Observatory is the first detector of its kind, designed to observe the cosmos from deep within the South Pole ice. An international group of scientists responsible for the scientific research makes up the IceCube Collaboration.
Encompassing a cubic kilometer of ice, IceCube searches for nearly massless subatomic particles called neutrinos. These high-energy astronomical messengers provide information to probe the most violent astrophysical sources: events like exploding stars, gamma-ray bursts, and cataclysmic phenomena involving black holes and neutron stars.
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 and mapped through the detector’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.
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. Gamma-ray observatories, including 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, the highest-energy photons. A later follow-up by MAGIC detected gamma rays of even higher energies.
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. Only one of these neutrinos, out of many millions that sailed through Antarctica’s ice, was detected by IceCube on Sept. 22.