Before Earth was born, a supermassive black hole far, far away shot a catastrophic jet of radiation into space. It was so powerful that one piece of that radiation was able to travel for 4.6 billion years through the universe and reach Earth, where it terminated in a quiet ping inside a detector buried under Earth’s south pole last September. Just like that, a potentially major finding was on humanity’s cards.
Meanwhile on Earth, scientists working at the same detector, called IceCube, reported a curious finding in 2013. They had identified 28 high-energy variants of particles called neutrinos between 2010 and 2012 coming from a source outside the Solar System. They were of such high energy that they couldn’t have come from the only two extraterrestrial objects we knew were capable of emitting them: the Sun and a supernova known as 1987A, about 168,000 lightyears away.
The higher the neutrino’s energy is, the higher the energy of the natural phenomenon that produced it. At very high energies (upwards of ~10,000 GeV/c2, where 1 GeV/c2 is about the mass of a proton at rest), we’re looking at natural and colossal particle accelerators whose energy efficiency makes the Large Hadron Collider look like a spinning keychain. What do these behemoths look like and how do they work?
Neutrinos are commonly called “ghostly” but “snooty” might be more apt. They are particles that flood space but very rarely interact with normal matter, as if they refuse to acknowledge matter’s proletarian presence. There are 100 billion neutrinos passing through your body every second and one of them will interact with you in your lifetime, two if you’re lucky.
They are emitted by many high-energy events. A nuclear reactor generates trillions of neutrinos per second, and the Sun trillions upon trillions. They are also born when cosmic rays from outer space collide with Earth’s upper atmosphere, showering neutrinos towards the ground.
We already know of some candidates capable of emitting very-high-energy neutrinos; black holes and supernovae are two of them. However, although “IceCube has been detecting neutrinos of astrophysical origins for the last six or seven years, none of those events have been associated with any known source,” Debanjan Bose, a physicist at IIT Kharagpur, told The Wire. “Follow-up observations for those events by other telescopes found nothing either.”
We’re also yet to unravel the precise mechanism of their production. In 2013, when IceCube didn’t have the wherewithal to say whether the 28 neutrinos were from a common source or where they where coming from, it did know these neutrinos were much more powerful than those from the Sun or 1987A.
A part of the answer lay in that quiet September ping, which scientists have announced today with great fanfare.
II. Two sides of a mystery coin
According to announcements on July 12, the black hole that produced the ancient flare is possibly a blazar. A giant galaxy with an active supermassive black hole at its heart – guzzling interstellar gas and belching heat and light – can sometimes focus these emissions in a beam in Earth’s general direction. These systems are called blazars.
“As sources of high-energy neutrinos and cosmic rays, blazars have always been among the most promising candidates from the theoretical side,” Chad Finley, a physicist at Stockholm University, told The Wire. “However, recently it had started to seem that we should be seeing evidence of neutrinos from blazars by now, if they were the main sources.” That evidence, at least the promise of it, is finally here.
The neutrino detected in September has an estimated energy of 290,000 GeV.
A blazar is defined by the laser-like beam of high-energy radiation that it sometimes shoots out along its poles, travelling at near lightspeed towards Earth. The supermassive black hole at the centre of a blazar is thought to be the source of these beams, called relativistic jets. When a blazar emits a relativistic jet, it is said to be flaring. Scientists don’t fully understand how these jets are emitted, although they are thought to arise from super-hot matter falling into the black hole.
The blazar currently in the limelight, designated TXS 0506+056, is located 4.6 billion lightyears away from Earth. The discovery is exciting for multiple reasons. One is that this blazar produced relativistic jets so powerful that the neutrinos in them had enough energy to travel 40,000 billion billion km and reach Earth to give the precocious field of neutrino astronomy its first pièce de résistance.
Yet another is that this blazar may have been one of the sources of those 28 high-energy neutrinos spotted at IceCube before 2012. However, Finley clarified, “We don’t yet know what fraction of the total high energy neutrino flux” at IceCube or at other detectors “might be due to blazars”. This is an important unanswered question at the moment, he added.
Most of all, we now know of a source of high-energy neutrinos outside the Milky Way galaxy that can be studied by neutrinos. According to Finley, who is a member of the IceCube research collaboration and was involved in processing the new finding, “This has been the goal of neutrino astronomy for decades.”
However, Roger Romani, a physicist at the Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, struck a more cautious note: “I’d rate it as interesting enough to deserve some careful thought and study of the implications, but not secure enough to bet one’s career on.”
The first step towards producing neutrinos is to accelerate protons to a high speed, giving them more and more energy. Supermassive black holes can do this when they consume matter, belching radiation that energises the matter around it.
The energised protons then decay to neutral and charged pions. Each neutral pion further decays to photons (electromagnetic radiation) that ‘conventional’ telescopes can detect. Each charged pion decays to a muon and a muon neutrino. The muon finally decays into a muon neutrino, an electron and an electron neutrino. So for each proton, there are three neutrinos: two muon neutrinos and one electron neutrino. The more powerful the process that accelerated the protons, the more energetic the neutrinos will be.
Axiomatically, that the blazar TXS 0506+056 emitted neutrinos is a sign that it accelerated protons. An important implication of this “is that at least some blazar jets accelerate protons or other baryons,” Romani said. “This is a big deal.” Bose agreed, and called this implication what made him go “wow” about the discovery.
Energetic protons and atomic nuclei are the primary components of cosmic rays – radiation streaming in from outside the Solar System and, since their discovery over a century ago, of unknown origin. As with high-energy neutrinos, TXS 0506+056 might be able to resolve this conundrum as well.
“Most studies of light from blazars find that they are adequately explained by jets whose particles are primarily electrons and positrons,” Romani explained – and this explanation provides a sense of the amount of energy in the jets. However, protons are over 1,800-times heavier than electrons, which means the jets will have to be that much more energetic to accelerate them. He believes most blazars still emit jets dominated by electrons and positrons and that among a few others, “a proton component” is present in some flares. But “until we gather more such neutrinos from several sources, it’s very difficult to say how widespread the phenomenon is.”
For now, we think we know where some cosmic rays come from, and “we can study this blazar in detail,” Bose said, alluding to TXS 0506+056 as a natural laboratory of “physics under extreme conditions”.
III. The multi-messenger way ahead
Being able to study an object using the neutrinos it emits is a privilege. If a body emits charged particles like protons or electrons, their trajectories through space become warped by numerous magnetic fields in their path. Neutrinos, on the other hand, don’t interact with electric or magnetic fields, not even with gravity, enabling them to shoot out in straight lines and travel unhindered for billions of years, especially if they’re high-energy neutrinos. If a neutrino streaks through IceCube in a particular direction, physicists simply have to look back along the same line to find the direction of its source.
This is why, as Bose says, “neutrinos are the only tool to probe our universe beyond a certain energy.” Bose worked with the IceCube detector from 2009 until last year. And thanks to their abundance and longevity, they could help us understand how our universe was when it was very young and why it is the way it is today.
Astrophysicists were alerted to the existence of the blazar source when, on September 22, 2017, they received an alert from an automated program running inside IceCube data looking for the signatures of high-energy neutrinos. The strength of the signal corresponding to this neutrino was too weak to count as evidence.
Fortunately, there was a way out. On the day the program alerted physicists to the presence of a high-energy neutrino in IceCube’s midst, astronomers using various ground- and sky-based telescopes had observed a gamma-ray emission from the same patch of the sky. The odds of a coincidence were small enough to suggest that the physicists and the astronomers were looking at a common source of the gamma rays as well as the high-energy neutrinos. Blazars are expected to release energetic gamma rays as well.
This corroboration, Finley said, gave physicists the confidence they needed to go back through archival data and search for signs of its activity in the past.
And there it was: between 2012 and 2015, they found stronger signs of high-energy neutrinos impinging within the IceCube detector. The data was good enough to breach the statistical significance required to claim evidence (not discovery), and they have claimed it. To paraphrase Finley, it was a sign that “there was something here rather than nothing”.
This phrasing is closer to how Romani put it: “As with any discovery hinged on a single event” – the one in September 2017 – “or even the mild excess” – between 2012 and 2015 – “one should be somewhat cautious. There have been statistical claims of neutrino-blazar associations before that have fallen by the wayside. This result looks better, but it is not of overwhelming significance.”
Finley agreed. “There are large uncertainties given the data we have so far,” he said, “which means it will be possible to extrapolate in many different directions.”
A question automatically arises: why wasn’t the 2012-2015 data flagged earlier?
Even though neutrinos almost never acknowledge the presence of matter, detectors like IceCube designed to log their interactions have recorded over “half a million” events. “A very tiny fraction of these come from space,” Finley continued, “and the rest are created in the atmosphere when cosmic rays arrive at Earth. Only a few neutrinos per year are so high energy that they stand out from this background on their own.”
The September 2017 alert was one such case, and it was made possible by physicists knowing where to look. “Otherwise it, is hard to identify the rare neutrinos from space within this large background of neutrinos from our atmosphere.”
That the blazar was tracked down with both neutrino and electromagnetic data has focused attention on a big takeaway from this affair: blazars are now the subject of multi-messenger astronomy. In this form of astronomy, astrophysicists study cosmic objects in more than one channel simultaneously. The two channels here are radiation and neutrinos. This can yield more information about a cosmic object than just one channel would – as we all found out with the discovery of the neutron-star merger.
“It has long been hoped that neutrinos could join the panoply of astrophysical messengers,” Romani said, “and the hard work by IceCube and the other neutrino teams place us at the threshold of this era.” Bose in turn called it “a tremendous boost for astroparticle physics”.
After the September 2017 alert was shared among a wider circle of observers, multiple telescopes on ground and in space sprung into action and began observing this patch, quickly establishing that the blazar was the most probable source as well as further characterising it. These included the Fermi Large Area and AGILE telescopes (both in low-Earth orbit), MAGIC (Canary Islands), HESS (Namibia), HAWC (Mexico), Subaru (Hawaii) and VERITAS (Arizona), among others.
“Five-ten years from now we may look back and say ‘they really caught the first wisps of the cosmic neutrino sources’ or we may look back and say ‘Too bad, another statistical fluke’,” Romani said. “But I hope for the former, particularly since with several more years’ observation, we could collect enough signal to probe how these blazar jets might get” protons and atomic nuclei “into the act.”