Blazar glory
For the first time ever, a high- energy neutrino has been tracked to its source – a galaxy four billion light years away.
Every second 100 trillion neutrinos pass through your body. These ghostly particles are among the most abundant in the universe, but apart from the Sun and a supernova in 1987, we’ve never known precisely where they are coming from.
Now Icecube, a neutrino detector buried under the Antarctic ice, has pinpointed a source: a violent galaxy four billion light years away that is spewing a jet of plasma in our direction, according to a paper published in Science by astronomers from the Icecube collaboration. Neutrinos are the smallest fundamental particles – more than a million times lighter than an electron – and very difficult to detect. Icecube has detected only several hundred highenergy neutrinos since it began operating in 2010.
Until now, none of those neutrinos could be traced back to a specific source. However, when Icecube clocked a neutrino last September with the whopping energy of nearly 300 teraelectronvolts, a message went out to observatories around the globe.
As telescopes swivelled in the direction indicated by Icecube, astronomers realised the source was an ultra-compact type of quasar, or “blazar”, called TXS 0506+056, in a galaxy four billion light years away. Among the brightest objects in the universe, quasars are jets of plasma released as galactic matter is sucked into a black hole. In this case, one of the jets was aimed straight at the Earth.
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 the University of Adelaide. “This neutrino could have been travelling to us since around the formation of the Earth.”
The Icecube team also went back through their data to see if other highenergy neutrinos had come from the same part of the sky. With careful sifting, they found some likely candidates between September 2014 and March 2015.
The evidence seems to add up to a link between high-energy neutrinos and TXS 0506+056, but Hill says we will need to see more flares from the blazar to put the issue beyond doubt.