One of the highest energy neutrinos ever detected probably came from a distant galaxy with a jet of plasma spewing directly at us, an international team of scientists from 16 observatories has announced in the journal Science.
Neutrinos are the smallest fundamental particle – more than a million times lighter than an electron – and are very difficult to detect. Although copious numbers are created in violent cosmic events and reach the Earth, most pass straight through detectors – and indeed the entire Earth.
Nonetheless IceCube, an array of detectors buried in the ice in Antarctica, has detected several hundred high-energy neutrinos since it began operating in 2010, but none of them could be pinned to a specific source.
However, in 2017 a new alert system was set up at facility, so when it clocked a neutrino in September with an energy of nearly 300 tera-electronvolts – the equivalent energy of 20 trillion car batteries connected together – a message was broadcast to observatories around the globe that a high-energy particle had been detected.
As telescopes swiveled in the direction indicated by IceCube, astronomers realised the source was an ultra-compact type of quasar, or blazar, called TXS 0506+056: a violent galaxy four billion light years away, with a powerful black hole at its heart that was sucking in matter and shooting out jets of plasma, one of which is aimed straight at the Earth.
Astronomers had seen gamma rays emitted by TXS 0506+056 before, but now it was flaring violently, emitting rays more intensely than in the past. As they studied the blazar more closely they realised that despite its brightness, it was further away than they had thought, says Gary Hill, a member of the IceCube collaboration based at University of Adelaide in Australia.
“We didn’t know initially how far away it was,” he explains. “Four billion light years is pretty distant, so it is intrinsically extremely luminous, probably one of the most luminous blazars ever measured.”
“So this neutrino could have been travelling to us since around the formation of the Earth.”
The team theorise that TXS 0506+056’s jet of plasma accelerated protons to extreme energies by bouncing the particles back and forth like two tennis players whacking a ball.
For example, the flare up in energy could have been caused as some dense matter fell into the black hole, sending a shockwave out through the blazar’s jet.
The fast-moving pulse behind the shockwave shoots protons ahead of it – like a tennis serve over the shockwave net. Magnetic fields in the slower-moving plasma in front of the jet return the serve through the oncoming shockwave, only to have the protons volleyed back by the magnetic fields behind it, with an extra dose of energy.
Unlike terrestrial tennis racquets the magnetic plasma reflections cause no loss of energy, so the protons move faster and faster.
The process ends when the proton escapes, or collides with dust, gas, or photons and forms a neutrino via the production of a short-lived particle called a pion. The reaction has been often recorded at the Large Hadron Collider, although at about one hundredth the energy measured by IceCube.
The IceCube team then decided to go through their previous eight years of data to see if other high-energy neutrinos had originated from the same part of the sky. With careful sifting, they found that in the period September 2014 to March 2015 there were about 15 more detections than would be expected from a random distribution.
The team also published these findings in Science at the same time as the single 2017 neutrino detection.
The combination of the gamma rays and the observation of a burst of neutrinos in the recent past seems to add up to a link between the particle and TXS 0506+056, but Hill hopes lots of observatories will catch the blazar’s next flare and put the issue beyond doubt.
“We’ll be scrutinising this thing forever, now,” he says.
