Researchers searching for the source of cosmic rays are going back the drawing board after ruling out gamma ray bursts as the most likely source.
A report in the journal Nature by scientists from the IceCube collaboration, concluded there wasn't enough evidence to link gamma ray bursts and cosmic rays.
Cosmic rays are highly energetic charged particles such as protons and heavier atomic nuclei, that strike the Earth from deep space with 100,000,000 times more energy than can be created in particle accelerators like CERN's Large Hadron Collider.
Various models predict gamma ray bursts are the only events capable of producing enough energy to accelerate cosmic rays.
One of the paper's authors, Associate Professor Jenny Adams from New Zealand's University of Canterbury, says tracing cosmic rays back to their source is impossible because they're deflected by magnetic fields and galaxies.
"The solution was finding extremely weakly interacting subatomic particles called neutrinos which aren't effected by magnetic fields," says Adams. "Wherever cosmic rays are produced, neutrinos should also be produced. Because they're neutral they'll point back to their source."
"That's why the IceCube neutrino detector was built."
Buried up to two kilometres under the Antarctic ice at the South Pole, IceCube is the only instrument big enough to detect neutrinos generated by gamma ray bursts.
"The problem is, we found no neutrinos coming from gamma ray bursts," says Adams. "And if there aren't neutrinos, there can't be cosmic rays."
"We built IceCube to detect Gamma Ray Burst neutrinos, we didn't expect not to find any. I guess in some ways it's disappointing."
"But it's also deepened the mystery, because we now have to look for new possible sources to solve the riddle."
Adams says active galactic nuclei such as quasars and blazars, powerful energy beams produced by supermassive black holes, may be possible candidates.
"The IceCube data we published in Nature came when the detector was only between half and three quarters complete," says Adams.
Now with IceCube finished and up to full capacity, Adams believes the search can focus on active galactic nuclei.
"Hopefully we'll find neutrinos or rule out some more models as to where cosmic rays can be produced," says Adams.
But one of the papers other authors, Dr Gary Hill from the University of Adelaide, isn't so confident.
"The statistical significance between cosmic rays and active galactic nuclei hasn't grown over time despite more observations," says Hill.
"Gamma ray bursts were our big shot. They were a good candidate."
"It really is a mystery because we can't figure out where cosmic rays could come from."