In a far corner of the cosmos 4 billion years ago, a tiny particle called a neutrino started its journey to Earth.
It zipped along at near the speed of light towards our neck of the universe, never deviating from its course.
Being the smallest, lightest particle known to physics, the neutrino flew through matter like it was nothing.
Without an electric charge, it remained immune to the swaying influence of magnetic fields.
Soon after the neutrino set off, life emerged and flourished on Earth. Continents shuffled around in their tectonic dance. Plants, animals and fungi spread to all corners of the globe and mass extinctions repeatedly killed them off again.
And the neutrino continued on its way, until the evening of September 22, 2017 when it plunged through the Antarctic ice and crashed into an atom.
More than a kilometre beneath the surface, this collision created another particle called a muon.
The muon continued through the ice in the same direction but threw off a blue glow as it went — light that was sensed by basketball-shaped detectors suspended in the ice nearby.
Within a minute, the neutrino’s trajectory was calculated from the light pattern picked up by the detectors and sent to telescopes around the world.
They trained their eyes on that patch of the sky, scouring it for signs of the cosmic accelerator that spat out the neutrino all those aeons ago.
And, for the first time, they found one.
In the journal Science today, astrophysicists pinpoint the first confirmed birthplace of a high-energy neutrino: a massive galaxy housing a supermassive black hole, spewing jets of matter towards us.
The black hole is called a blazar. Such objects were known to be a likely source of these baffling particles — but had never before been caught in the act.
And that confirmation is a big deal, said Nicole Bell, an astroparticle physicist at the University of Melbourne who was not involved with the work.
“This step of identifying a source is absolutely essential in order to genuinely do astronomy with neutrinos,” she said.
Neutrinos, Dr Bell explained, will now allow us to “see” otherwise invisible parts of the universe.
We’re bathed in neutrinos, with trillions flying through our body every second, but you’d never know.
They mostly sail straight through the entire Earth unimpeded, earning their nickname “ghost particles”.
There are a few ways to make a neutrino. For instance, they’re a by-product of nuclear fusion, created when hydrogen mashes together to form helium in the Sun.
And energetic particles — mostly protons — from outer space, called cosmic rays, bombard our atmosphere to produce showers of neutrinos (along with other particles).
Neutrinos from the Sun and atmosphere are mostly low-energy particles.
But it’s the high-energy neutrinos created outside our galaxy, by the most cataclysmic events in the universe, that many astroparticle physicists are interested in.
The IceCube observatory near the South Pole has been picking up high-energy neutrinos since 2013. But to find out precisely where they came from, it needed a little help from other telescopes.
If something is violent enough to spit out high-energy neutrinos, the reasoning goes, it must also throw off light, such as gamma-rays, radio-waves and X-rays.
So when IceCube picked up the tell-tale signs of a high-energy neutrino on September 22 last year, it sent the neutrino’s cosmic coordinates to the astronomical community: a line of sight just off the left shoulder of the Orion constellation.
In the first of two papers published today, the Fermi and MAGIC telescopes found a flare of high-energy gamma-rays which seemed to emanate from a known blazar called TXS 0506+056.
Other instruments, such as optical and radio telescopes, also measured activity from TXS 0506+056 in the days and weeks that followed.
Getting as many eyes on the sky as possible helps physicists lock onto and understand a source, according to James Miller-Jones, an astrophysicist at the International Centre for Radio Astronomy Research at Curtin University.
“Certain frequencies of light are produced by different processes, so each region of the spectrum is giving us a different window into what’s going on,” said Dr Miller-Jones, who is also a member of the Very Large Array radio telescope team, which also observed the blazar about a fortnight after the IceCube alert.
All the additional telescopes collected compelling evidence that TXS 0506+056 fired the neutrino off to Earth, said Gary Hill, an astroparticle physicist at the University of Adelaide and member of the IceCube collaboration.
But to bolster their conclusions, the team looked back through IceCube’s archives and found a flurry of about 15 more high-energy neutrinos, detected in 2014 and 2015, which also appear to originate from TXS 0506+056.
They reported these findings in a second paper.
What the blazes happens in a blazar?
To fire off a high-energy neutrino, you need a powerful particle accelerator — as powerful as, say, a supermassive black hole.
Astrophysicists think most massive galaxies have a supermassive black hole in their centre. Our galaxy, the Milky Way, has one.
It’s being fed at a very low rate, Dr Miller-Jones said, so it’s relatively dim.
While ours is quiet, some supermassive black holes, such as the one in TXS 0506+056, can be extremely active.
In their case, matter accumulates in a disc around the black hole and heats to hundreds of thousands of degrees.
The black hole produces powerful jets that act as particle accelerators, spraying material from the disc into space.
If a jet points towards Earth, the cosmic beacon is called a blazar.
So how are high-energy neutrinos born? No-one knows for sure, but the prevailing theory begins with cosmic rays such as protons, Dr Hill said.
As a proton whizzes along a supermassive black hole’s jet, it bounces back and forth across a shock wave, gaining an energy boost with each bump and accelerating it to near the speed of light.
But if they interact with matter or radiation, they make a particle called a pion.
“When the pions decay, they make neutrinos and gamma-rays,” Dr Hill said.
Blazars aren’t the only source of neutrinos, Dr Hill added. For instance, another neutrino-producer group might be “starburst” galaxies, which are in the midst of churning out lots of stars.
“There’s evidence they make cosmic rays too. If they do, that’s a similar sort of mechanism [to a blazar] — cosmic rays interact and make pions, which make neutrinos.”
New kind of astronomy
Knowing more about the origin of high-energy neutrinos is vital to develop a new way of observing the universe: neutrino astronomy.
Astronomy traditionally relies on light. But the problem with light is that it can get absorbed along the way, Dr Bell said.
Cosmic rays aren’t ideal either. Being charged particles, they’re deflected by magnetic fields.
But the slippery qualities that make neutrinos so hard to detect also mean they’re ideal to look at otherwise invisible or even entirely new objects.
“You might see a source where all the light is trapped, it doesn’t get out, but neutrinos do get out,” Dr Bell said.
In the meantime, Dr Hill hopes the IceCube detector will be upgraded.
“The general idea is we put more strings in the ice, to make detection volume bigger, and also build detectors on the surface to allow us to screen out background atmospheric cosmic rays,” he said.