On Dec. 4, 2012, physicists at the Antarctic IceCube Observatory bagged the highest energy neutrino ever detected. The experiment, which hunts for neutrino events within a subsurface cubic-kilometer of clear glacial ice, has registered around 100 high-energy neutrino observations, but the particle nicknamed "Big Bird" remains the most energetic. Now, years later, NASA astronomers equipped with the Fermi Gamma-ray Space Telescope can say with some assurance where (and when) that single particle came from: the gamma-ray blazer known as PKS B1424-418.
The group's work is described this week in Nature Physics.
Neutrinos are unlike most of the particles we're more used to. For one thing, they tend to ignore most of those particles—neutrinos only very weakly experience the electromagnetic force, which means that they're invisible to electricity, light, and matter itself in all but the most rare occasions. Detecting a neutrino on one of these occasions is unlikely enough to require enormous detectors (like a cubic-kilometer of Antarctic ice) laden with vast arrays of photomultiplier tubes and sensors ready to catch the barest flicker of light as a neutrino interacts with the detector in such a way as to produce an electron or muon, which are particles we can observe.
Because neutrinos are so ambivalent when it comes to matter, they make good probes for observing the distant (and very old) universe. That is, it's reasonable to imagine a single particle produced billions of light years away arriving here on Earth intact and uninterfered with: a particle postcard of a very different time and place. Neutrinos also happen to be extremely powerful, with energies on the order of petaelectronvolts (or a quadrillion or a 1 with 15 zeros after it). As explained in the video above, that's about a million million times greater than the energy of a dental X-ray packed into a single particle with a mass of less than one-millionth that of a single electron. It's probably a good thing neutrinos don't interact with most matter.
In the summer of 2012, the Fermi satellite's Large Area Telescope (LAT) registered an abrupt brightening ("flaring") of the galaxy PKS B1424-418, which lives about 9.12 billion light-years from Earth (which means that we're seeing the galaxy as it was 8 billion years ago, give or take). The galaxy lit up to nearly 30 times that of its originally observed brightness. That's wild, but PKS B1424-418 was an intense place to begin with: a variety of active galaxy known as a blazer featuring a supermassive black hole at its core. It's here that the extra luminescence comes from: the intense outward jets of energized particles that result from a whole lot of cosmic stuff trying to collapse into a black hole all at once. Blazers are among the most energetic things in space.
Being able to correlate a single particle, however energetic, with a galaxy billions of light years away is kinda nuts. "Tentative associations of high-energy neutrinos with flaring blazars have been suggested before, but it remained questionable whether a high-enough neutrino flux could be produced in the candidate flares," the current paper explains. "Here, we have identified for the first time a single source that has emitted a sufficiently high fluence during a major outburst to explain an observed coinciding petaelectronvolt-neutrino event."
And that's what we're talking about here: correlation. The IceCube experiment was only able to narrow the likely source of the neutrino down to a still fairly large slice of sky, but one limited enough that astronomers could reasonably look for their neutrino's source. The flaring PKS B1424-418 seemed to be the only candidate and the Fermi scientists are able to say that it's the source with only a 5 percent margin of error. In a statement from NASA's Goddard Space Flight Center, study co-author Felicia Krauss recalled that happening upon the blazer was a "moment of wonder and awe."
There's a lot left we still don't know about neutrinos and the galactic origins of cosmic rays in general. As the paper notes, better statistics and monitoring at IceCube will continue to improve our still-limited understanding of this hidden but unfathomably powerful realm.