Four billion years ago, our solar system and Earth still in their infancy, a neutrino was emitted in our direction by a huge galaxy with a supermassive black hole in its center. It eventually arrived at Earth on Sept. 22 last year and was recorded by our IceCube neutrino telescope at the South Pole. The energy of the neutrino was extremely high – 45 times higher than that of protons in CERN’s Large Hadron Collider. Coincident with that event, gamma-ray astronomers observed that this galaxy (named TXS 0506+056) was in a high state of activity. The probability that this coincidence was just accidental came out as only 1:1000. A clear proof that this galaxy is a neutrino source? Not yet: sometimes even things with small probability must happen! The really thrilling story followed when we scanned through all the data taken since 2009. For a period of few months in late 2014/early 2015, we observed a clear excess of a dozen neutrino events from the direction of TXS 0506+056. And the chance probability for such an excess is only 1:5000!
In my life, I have seen several walls falling, politically and scientifically. The fall of the Berlin wall in 1989 was clearly the most overwhelming event for me personally. It happened just one year after I had started working in the field of neutrino astronomy. Next came the discovery of a diffuse flux of high-energy extraterrestrial neutrinos with IceCube in 2013, named the “breakthrough of the year” by the journal Physics World. “Diffuse” means that the arrival directions seemed to be spread uniformly over the whole sky, with no preferred direction indicating an individual source. And now the first evidence for exactly such an individual source!
Is this another “falling wall”? We don’t know yet for sure. The chance for such an observation just by accident is tiny, but it’s not below a millionth yet. This is the limit from where on the notoriously skeptical physicists call an observation confidently a “discovery”.
Neutrino physicists need stamina. The next activity outbreak of a galaxy might be observed very soon – and possibly be recorded not only by IceCube but also by neutrino telescopes just under construction. And only then, eventually, we will know that a new branch of astronomy has been born! September 22, 2017 was just the exciting start.