An international team of scientists has found compelling evidence that some the tiniest, most elusive particles we know about—neutrinos—are produced by one of the brightest, most energetic events in the universe. The key to this evidence? A single neutrino, detected by the IceCube Neutrino Observatory on September 22, 2017.
|The IceCube Neutrino Observatory is buried at depths between 1.5 and 2.5 kilometers below the South Pole. The only visible equipment is the IceCube Lab, also called the ICL, which hosts the computers that collect data from the sensors buried in the ice. In this artistic rendering, based on a real image of the ICL, the September 2017 neutrino event is shown in the ice below Antarctica’s surface.
Image credit: IceCube Collaboration/NSF.
Instead of detecting light, like other observatories, IceCube detects neutrinos. Neutrinos are neutral particles with practically no mass that travel at nearly the speed of light. They don’t interact with electromagnetic radiation, and rarely interact with matter. Enormous numbers of neutrinos are constantly zipping through the Earth, and even your body, entering and exiting from all directions.
Detecting neutrinos requires huge, dense particle detectors, usually buried deep underground or within mountains to screen out other particles that could produce false positives. IceCube is the largest particle detector in the world by volume, coming in at one cubic kilometer. It’s called “IceCube” because it’s built right into the Antarctic ice-sheet.
On the rare occasion that a neutrino does hit an atomic nucleus, the interaction produces characteristic flashes of blue light. IceCube is an array of more than 5,000 light sensors designed to pick up these flashes. By tracking how flashes progresses through the array, IceCube scientists can determine the energy of the instigating neutrino and reconstruct its path.
IceCube has detected lots of neutrinos since it opened in 2008. The majority of them are atmospheric neutrinos produced when high-energy particles called cosmic rays collide with atoms in the Earth’s atmosphere. IceCube has also detected neutrinos produced in astrophysical events—called astrophysical neutrinos—although far fewer. High-energy astrophysical neutrinos are likely produced by energetic phenomena occurring outside of our galaxy.
Astrophysical neutrinos are fascinating mysteries. In theory they could tell us a lot about what’s happening in the universe, but first we need to know how and where they are produced. The sun and supernovae produce some of the astrophysical neutrinos, but IceCube has detected others with energies way too high to be from those sources—these are the especially interesting ones. “While IceCube has seen neutrinos from outside our galaxy before, we have never been able to attribute them to a specific source,” explains Karen Andeen, a team member from Marquette University. Until now.
When IceCube detected the high-energy neutrino last September, it automatically sent alerts with information on the neutrino’s path and energy to several other observatories. Why? By turning many different kinds of telescopes and detectors toward the patch of sky where a high-energy neutrino originates, astronomers hope to converge on a likely source. This time, for the first time, they did—identifying the likely source as a galaxy named TXS 0506+056.
After receiving the alert, two gamma ray telescopes, the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) in the Canary Islands and the space-based Fermi Gamma-ray Space Telescope, turned their attention to TXS 0506+056 and saw a very-high-energy gamma ray signal—just the signal you would expect from a neutrino-producing source. Other telescopes looked too, and together this team of observatories across the world constructed a picture of TXS 0506+056 in wavelengths ranging from gamma rays to radio waves.
The bottom line: It’s bright across the spectrum. TXS 0506+056 is 4 billion light years away so you can’t see it with the naked eye, but it’s actually one of the most luminous objects in the sky. The galaxy is a blazar, a large galaxy with a massive, rotating black hole at its core that emits jets of high-energy particles. Observations show a blazar flare, an especially bright signal across a range of wavelengths, happening right around the time that the neutrino arrived at IceCube.
To make sure that the flare and the neutrino’s arrival weren’t just a coincidence, the scientists looked at the problem from another angle. IceCube scientists went back through all of their data—nearly ten years of it—and examined all high-energy neutrinos originating from the same patch of sky as the September 2017 neutrino. They noticed that between September 2014 and March 2015, IceCube detected significantly more than usual coming from that direction—likely from another flare by the same blazar.
Together, these results provide convincing evidence that blazars are a source of high-energy neutrinos, although not definitive proof. “This is not a slam dunk, but it is certainly compelling,” says Andeen. Andeen credits the collaboration of multinational experiments, known as the multimessenger astronomy community, for this achievement. “Science on this scale is a fantastic example of something that is a truly a global undertaking,” she says. “[W]e need each other to make the next big leaps in understanding. We are entering a new era!”
This research was published today in two companion papers in Science, a journal published by AAAS, the American Association for Advancement of the Sciences. You can access the papers for free at sciencemag.org.