Gravitational waves have deservedly held the astronomical spotlight for the last two years, opening up a new way to see the cosmos. But thanks to a groundbreaking discovery, it might well be time for the humble neutrino to take the stage.
That’s because, for the first time ever, a global team of astronomers has found the source of some of these high-energy particles coming from the distant universe. Neutrinos are hard to spot, and we've never found the source of any at such a distance before.
But in two papers in the journal Science, scientists describe how they located a source of neutrinos 4 billion light-years from Earth. It’s an energetic galaxy known as a blazar, called TXS 0506+056, which has a giant spinning supermassive black hole at its core and fires out twin jets of particles.
On September 22, 2017, the IceCube observatory at the South Pole detected an incoming high-energy neutrino. This advanced detector has a real-time alert system, and broadcasted the coordinates of the detection to astronomers around the world just 43 seconds after its discovery.
About 20 observatories including NASA's orbiting Fermi Gamma-ray Space Telescope responded to the alert, and trained their views on the skies to try to work out where it was coming from. What they found was this blazar, energetically flaring and sending out gamma rays. And, as luck would have it, it also sent neutrinos in our direction, and we were able to detect one.
“These results are a remarkable chain of events,” Darren Grant from the University of Alberta, and spokesperson for IceCube, told IFLScience. “Taken all together, these results provide an incredibly intriguing picture for the first identified cosmic ray source.”
Looking through the archives of IceCube’s data, scientists found a further dozen events associated with this object in late 2014 and early 2015. That helped them confirm that the single high-energy neutrino spotted in 2017 almost certainly came from the blazar.
And that’s important for a number of reasons. For one, it’s the first time we’ve ever found the source of a high-energy neutrino. For another, it’s the most distant detection of a neutrino in the universe we’ve ever made. And it tells us a lot more about cosmic rays.
For more than a century the source of cosmic rays has been a mystery. We know they continuously rain down on Earth from space, but we’ve never been quite sure where they’re coming from. As cosmic rays are charged particles, their trajectories get altered by magnetic fields in space, making it hard to see where they’re coming from.
Neutrinos, however, act as “ghost particles” – as they have almost no mass and rarely interact with matter. So neutrinos from this blazar traveled in almost a straight line directly towards Earth, allowing their origin to be worked out.
On two previous occasions we have detected sources of low-energy particles associated with cosmic rays, namely the Sun and a nearby supernova, called SN 1987A. High-energy neutrinos, however, can tell us a whole lot more about how fascinating objects like blazars actually work.
“We are at the beginning of understanding what sources and mechanisms can accelerate these tiny particles to such high energies,” Azadeh Keivani from Penn State University told IFLScience. “The discovery of high-energy neutrino sources could tell us about the origins of cosmic rays that produce them in particle interactions at the source.”
The discovery of gravitational waves meant that we could study some extreme events in the universe, like merging black holes and neutron stars, which are impossible to see with regular telescopes. In a similar manner, high-energy neutrinos allow us to see another hidden side of the universe.
By observing events in both light and neutrinos, this opens up a new type of multimessenger astronomy. This can tell us more about how distant galaxies form and evolve, and probe some of the processes taking place in things like supermassive black holes.
“Blazars dominate the high-energy sky and therefore they have long been proposed as potential neutrino sources,” Dr Marcos Santander from the University of Alabama told IFLScience. “We need to understand what could make TXS 0506+056 a neutrino source in order to find more like it among the thousands of blazars known to emit gamma rays.”
To make more detections like this in future, scientists are already working to upgrade IceCube, tentatively calling it IceCube-Gen2, to increase its volume by 10 times. Coupled with upcoming gamma-ray observatories like the Cherenkov Telescope Array, scientists hope to be able to pinpoint even more neutrino sources.
Along with the discovery of gravitational waves, it heralds an exciting new era of astronomy where we can study objects not just in electromagnetic radiation, but in the other particles they emit too. Before we could see the universe – now we can hear it, too.