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First discovery of high-energy neutrino source

For the first time, scientists have traced back a high-energy neutrino to its source in space. It was produced by a so-called blazar—a supermassive black hole. Researchers from neutrino detector IceCube report this in Science. ‘This is a milestone for neutrino science,’ says physicist Dorothea Samtleben from Leiden University and Nikhef, who performed control measurements with neutrino detector ANTARES.

One of the big questions in science is where cosmic rays come from. For decades, we have been detecting high-energetic particles that are bombarding Earth from space. During their long journey through the Universe of sometimes billions of lightyears, they are deflected within all kinds of magnetic fields. So it remains a mystery which direction they actually come from. Fortunately, ultra-lightweight particles called neutrinos might come to the rescue. According to particle physics theory, high-energy neutrinos are produced simultaneously with cosmic radiation, and because they barely interact with the world around them, they fly right through the Universe in a straight line. If one accidentally strikes Earth, we can derive from its direction where cosmic radiation comes from.

Supermassive black hole

On September 22nd 2017, such a neutrino hit IceCube, on Antarctica. Through a dragnet of telescopes on Earth and in space, scientists subsequently found the source: a supermassive black hole at about four billion lightyears, in the Orion constellation, which serves as a natural particle accelerator. Scientists from the eighteen involved observatories present their results in Science. A second analysis, also in Science, shows that neutrinos previously detected by IceCube originate from the same source.

ANTARES

Researchers from ANTARES, the neutrino detector in the Mediterranean Sea, helped out in the quest. Dorothea Samtleben coordinates the search for cosmic sources at ANTARES. She is pleased with the discovery: ‘This is an important milestone for neutrino science. It is the first time that a cosmic neutrino has been traced back to its source in a distant galaxy. This is also promising news for KM3NeT (successor of ANTARES, ed.), because we expect these signals to be evenly distributed across the sky.’

Review

Samtleben and her colleagues reviewed their ANTARES data for September 22nd 2017. ANTARES did not find a neutrino signal from the direction of the blazar that day. ‘IceCube is on the South Pole, we are on the Northern Hemisphere. That is a disadvantage for this source. ANTARES is less susceptible to the part of the sky where the blazar can be found.’ The team also did a targeted search through the data of the period 2007-2017. ‘From that period we found one neutrino coming from the direction of the blazar, but that is too little to draw conclusions on,’ says Samtleben. The analysis of ANTARES has been submitted to the journal Astrophysical Journal Letters and appears on arXiv.

Header image credit: DESY, Science Communication Lab

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