For the first time, scientists have discovered the source of a high-energy neutrino, shedding light on the century-long mystery surrounding this elusive particle.
Neutrinos are nearly massless subatomic particles that carry no electric charge, making them extremely rare in their interactions with the surrounding environment. In fact, these particles, often referred to as “ghost particles“, pass through our bodies by the billions every second without us even noticing.
Most neutrinos on Earth originate from the Sun, but there is a small fraction that comes from the most distant reaches of the universe. Their elusive nature has prevented scientists from tracing their origins until now.
The IceCube Neutrino Observatory in Antarctica, along with several other observatories around the globe, has tracked a neutrino emanating from a distant source: a massive elliptical galaxy hosting a rapidly spinning supermassive black hole at its center.
Moreover, these cosmic neutrinos travel alongside cosmic rays—high-energy particles that continuously bombard Earth. As a result, the detectors not only captured a cosmic neutrino but also detected these ultra-energetic cosmic rays.
Back in 1912, astronomers were puzzled by cosmic rays when they were first discovered. At that time, the limitations of human knowledge about particle properties rendered cosmic rays as mysterious as other celestial phenomena in the universe.
“We have been searching for the source of these cosmic rays for a century, and we have finally found it,” said Francis Halzen, the lead scientist of this study at the IceCube Neutrino Observatory and a physics professor at the University of Wisconsin-Madison.
Simulation graphics of the source that emitted neutrinos simultaneously with cosmic rays at incredible speeds. The supermassive black hole at the center of the galaxy’s accretion disk ejects powerful streams of matter into space, perpendicular to the galactic disk. (Photo: DESY, Science Communication Lab).
Achievements from Community Efforts
The journey began with IceCube, a highly sensitive detector buried about 1.5 kilometers beneath the thick ice layers of Antarctica. This system comprises 86 cables, each holding 60 ultra-sensitive optical modules capable of detecting faint light.
This detector is designed to capture the characteristic blue light emitted by a neutrino when it interacts with an atomic nucleus. This light is produced by a secondary particle created during the interaction. The detector is buried deep beneath the ice to block non-neutrino particles from contaminating the measurements.
“To detect a signal from an incredibly weak interaction of neutrinos, physicists had to build an extraordinarily large detector,” said Dr. Susan Cartwright, a particle physicist at the University of Sheffield. For an entire year, the detector recorded only a few hundred neutrinos, as they rarely interacted with their surroundings.
Measuring neutrinos when they interact with their environment is a significant step closer to determining their origin. Scientists compare this challenge to finding a firefly amidst a dazzling display of fireworks.
Before this discovery, IceCube had already recorded neutrinos from outside the Milky Way. At that time, researchers could not accurately pinpoint their sources.
However, on September 22, 2017, one of the distant neutrinos from the universe was detected by the device. It possessed an extraordinarily high energy of about 300 tera-electron volts, which is 50 times the energy of protons accelerated in the world’s largest particle accelerator.
Within a minute of its detection, the detector sent alerts to astronomers worldwide to focus their telescopes on that region of the sky to search for evidence to identify the source of this neutrino.
Nearly 20 ground-based and space telescopes scanned that area across all electromagnetic spectra, from low-energy radio waves to high-energy gamma rays. This collective observation pinpointed the origin of the mysterious neutrino, traced back to a source known as TXS 0506+056, located approximately 4 billion light-years from Earth.
Upon reviewing archival data, the IceCube research team discovered several other cosmic neutrinos observed in late 2014 and early 2015 that also seemed to originate from the same source.
“By combining all observations from ground-based and space observatories, we have compelling evidence of a neutrino source that is rich in energy, accompanied by high-energy cosmic rays,” stated Albrecht Karle, a scientist at IceCube and a physics professor at the University of Wisconsin-Madison.
Reconstructed simulation graphics based on actual images of IceCube, the neutrino observatory located beneath 1.5 kilometers of thick ice in Antarctica. (Photo: IceCube/NSF).
The Universe’s Diverse Celestial Bodies
The newly discovered source is a particularly bright, active galaxy that emits both light and particles simultaneously, with one of those directed towards Earth. (This explains why we receive such strong signals from it.)
Astronomers have identified thousands of such sources in the universe, yet none have emitted neutrinos like TXS 0506+056. “This is indeed a unique source, and we are responsible for uncovering its secrets,” Halzen added.
Not only have scientists confirmed that this source emits neutrinos, but they also seek to understand the mechanisms that enable its acceleration. How does this source simultaneously eject neutrinos and cosmic rays into space at such astonishing speeds?
Halzen expressed optimism that we will find answers to these questions in the near future. Moving forward, we will utilize two different types of signals to explore the universe, as we have done in this study.
The discovery of neutrinos also helps illuminate other mysteries we are investigating. In October 2017, researchers reported analyzing the collision between two supermassive neutron stars by observing the electromagnetic radiation and gravitational waves emitted from them.
“We are in the era of multi-messenger astrophysics. Each messenger, from electromagnetic radiation to gravitational waves and now neutrinos, provides us with a more complete understanding of the universe and new insights into celestial bodies and the most powerful events in the sky,” stated France Cordova, director of the National Science Foundation, the agency overseeing IceCube.
