For most of astronomy's history, understanding the heavens was limited to what we could see: the narrow band of the electromagnetic spectrum that constitutes visible light. Only over the last century or so have we expanded beyond that, into the infrared and microwaves and up into the higher energies of X-rays and gamma-rays. The past few years have brought an even more fundamental change: we've started detecting astronomical events without photons at all. This was done most famously by LIGO, the hardware that detected gravitational waves. But LIGO was actually late to the game, as the South Pole's IceCube detector had started listening in on cosmic neutrinos a few years earlier.

But in one critical aspect, LIGO beat IceCube to the punch: it spotted an event where the gravitational wave signal was paired with an optical signal, a burst of gamma rays. This marked the first instance of what's being termed "multimessenger" astronomy, where a single event is observed using physically distinct signals.

While IceCube has spotted some phenomenally energetic neutrinos, we've not been able to match those with a specific photon source. As of today, that has changed with the announcement that an energetic neutrino was likely to have been sent our way by a blazar, a supermassive black hole with a jet pointed in Earth's direction.

Hunting neutrinos

High energy neutrinos sent here from space are tied in to one of the biggest mysteries of cosmic rays: what produces particles with such phenomenal energy? These particles can carry more than 1,000,000 times the energy we can generate using our most powerful particle accelerators. We have some ideas as to the processes that can produce them, but we have yet to definitively identify a cosmic ray source.

These particles, however, are likely to be the source of highly energetic neutrinos. As neutrinos don't have a charge, they can't be accelerated on their own. Instead, they have to be produced by the decay of another highly energetic particle, which allows them to inherit some of that particle's energy. So the detection of a Peta-electronVolt neutrino implies the prior existence of a particle with even more energy—one of the cosmic rays we've been trying to understand.

IceCube is designed specifically to identify neutrinos from space. It's a cube of Antarctic ice (IceCube, get it?), a kilometer on a side, laced with photodetectors. Neutrinos will sometimes interact with the atoms that make up the ice; if they're muon neutrinos, the interactions will produce a muon. If the neutrino is energetic enough, the resulting muon will have enough energy to travel faster than the speed of light in the ice. Since that's frowned upon by physics, the muon will rapidly lose energy by radiating away photons (these photons are called "Cherenkov Radiation"). The photodetectors of IceCube are designed to pick up these photons, allowing us to reconstruct the neutrino's energy and where it came from.

So, while we have some detectors that are designed to track cosmic rays as they plow through our atmosphere, IceCube can also track their sources by the neutrinos that are produced along with them.

(The cosmic rays that slam into the atmosphere are actually a problem for IceCube, since those collisions also produce muons that enter the detector. But these will be apparent at entry as signals on the detector surface. By excluding those and only focusing on muons that appear within the detector volume, it's possible to limit the events to those caused by a neutrino arriving from a great distance.)

Alert!

So far, while IceCube has spotted dozens of extremely high-energy neutrinos and figured out roughly where in the sky they came from, there wasn't anything interesting emitting photons from those areas. Nevertheless, the IceCube team developed a system where it could send out alerts shortly after the arrival of a neutrino, providing other astronomers with a rough sense of where it came from. Follow up calculations with more precise directional information would then be prioritized.

This was all in place last September when neutrino 170922A arrived, causing IceCube to send out its 10th alert. The neutrino deposited about 24 Tera-electronVolts into the detector alone, about double the Large Hadron Collider's 13TeV collisions. The IceCube team estimates that the neutrino showed up with a total energy of 290TeV. This, in turn, implies that the particle that generated the neutrino had energies in the Peta-electronVolt range, which places it well in the range of energetic cosmic rays.

But more significantly, the neutrino came from the same area as a known source of high-energy photons, the blazar TXS 0506+056. Blazars are versions of a quasar, a supermassive black hole at the center of a galaxy that's actually feeding on matter. The feeding process creates jets of particles and photons, powered by the magnetic field of the black hole and its environment. Blazars are just rare cases when those jets are lined up to point at Earth, allowing us to view a quasar as if we were looking down the barrel of one of its jets.

Blazars tend to vary their output over time, as wobbles in the jets and changes in the amount of infalling matter alter the energy sent in the direction of Earth. Followup observation with orbiting gamma- and X-ray telescopes showed that TXS 0506+056 had entered a period of higher activity at the time the neutrino was detected.

The researchers compared past observations of TXS 0506+056 and compared its positional information to the data obtained from IceCube. They estimate that, for any models where the elevated output of the blazar was associated with neutrino production, the possibility of their close association arising by chance is ruled out at the level of three standard deviations. Put differently, the coincident location of the blazar and neutrino source is unlikely to be chance but not so solid that we can announce a definitive discovery.

Not the first?

In a separate paper, the IceCube team goes back and revisits all the data it has collected in the past, trying to figure out whether this is the first time they've seen something from TXS 0506+056, combing through seven years of observations. The analysis determined the general background of neutrino detections and then determined whether there were any periods where neutrinos from the same direction as TXS 0506+056 were present above the background levels. During a period between 2012 and 2015, there was an excess of 13 neutrinos from that direction. That may not sound like much, but for neutrinos, where detecting any of them is a serious challenge, that's enough to make for a 3.5 standard deviation excess. Again, it's not a definitive discovery, but it's extremely suggestive.

Finally, the apparent association of the neutrino and the blazar inspired a large number of telescopes to examine the area across a wide range of the spectrum. It turns out that, when all the wavelengths are considered, TXS 0506+056 is among the 50 brightest objects and the most luminous object at that distance from Earth, "more than an order of magnitude more luminous than nearby blazars."

Those observations helped constrain the physical properties of the blazar, allowing a team of researchers to compare different models of neutrino production: one where the neutrinos are mostly produced by the interactions of accelerated particles like protons and one where they're produced by interactions among electrons. In the end, if the jets of the blazar were dominated by the interactions of protons and other composite particles, the odds of detecting a neutrino were bad, at about two percent. But if electrons dominated, then the odds are truly awful. As a result, it looks like TXS 0506+056's jets are primarily composed of protons and similar particles.

There will undoubtedly be further observations of TXS 0506+056 that will give us a clearer idea of the blazar's properties. And IceCube is proposing to expand the volume of ice strung with photodetectors, increasing its sensitivity to the rare cases where neutrinos bump into the Antarctic ice. So we can hopefully expect a gradually expanding picture of how high-energy particles get produced by the Universe and the conditions that generate such extreme energies. It may not be quite as dramatic as the first discovery of gravitational waves, but this area of "multimessenger" astronomy is likely to contribute a lot to our understanding of the most extreme conditions in the Universe.

Science, 2018. DOI: 10.1126/science.aat1378, 10.1126/science.aat2890 (About DOIs).

Astrophysical Journal DOI not yet available.