About 3.7 billion years ago, when life was just beginning to take hold on this planet, a neutrino was thrown out of a blazar and launched towards Earth – and it just landed. The ghostly particle was detected by the IceCube Neutrino Observatory in Antarctica and traced back to its distant point of origin with NASA's Fermi Gamma-Ray Space Telescope, helping to confirm the most distant source of neutrinos ever identified.
Neutrinos are very common – in fact, there are billions of them streaming through your body right this second – but they're difficult to detect. With no electric charge and almost no mass, they rarely interact with normal matter, earning them the nickname "ghost particles." Exactly where they come from has been a long-standing cosmological mystery, but scientists suspect they're formed in cataclysmic events like galaxy mergers or supermassive black holes devouring matter.
Because neutrinos aren't affected by magnetic fields of objects, their path through space is more or less perfectly straight. Using that knowledge, an international team of scientists has worked backwards from a neutrino detection to determine where it came from.
The incident kicked off on September 22 last year, when a high-energy neutrino was picked up by the IceCube observatory near the South Pole. The facility detects neutrinos as they collide with water molecules in the Antarctic ice, giving off a flash of light. This particular neutrino struck with the energy of about 300 trillion electron volts, indicating that it has been accelerating for an exceptionally long distance before arriving here, and most likely originated outside the Milky Way galaxy.
Using IceCube, the scientists backtracked to locate the patch of sky where the neutrino came from, and sent out an alert to observatories around the world to search the spot for flares and outbursts of energy and light that might have accompanied it.
Fermi's Large Area Telescope (LAT) observed a large gamma ray emission from that region just when the neutrino arrived. The culprit seems to have been a blazar, an active galaxy with a supermassive black hole at its center that sends a bright jet of particles hurtling towards Earth. This particular blazar, known as TXS 0506, was at a long-term peak of activity at that time.
"Fermi's LAT monitors the entire sky in gamma rays and keeps tabs on the activity of some 2,000 blazars, yet TXS 0506 really stood out," says Sara Buson, one of the NASA scientists who performed the data analysis. "This blazar is located near the center of the sky position determined by IceCube and, at the time of the neutrino detection, was the most active Fermi had seen it in a decade."
After this latest observation, the IceCube team went back through archival data and found more than a dozen neutrino detections in 2014 and 2015 that also point to the same blazar. With that further evidence, TXS 0506 is the first known accelerator of high-energy neutrinos and cosmic rays.
The detection is also significant as another example of different types of cosmic signals coming together to paint more detailed pictures of the cosmos. In October last year, the collision of two neutron stars was detected in the form of gravitational waves, light, radio, X-rays and gamma rays at the same time.
The era of multimessenger astrophysics is here," says France Córdova, Director of the National Science Foundation. "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."
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