With the help of ten years of measurements at the one cubic kilometer “IceCube” detector system in the Antarctic ice, an international team of researchers has succeeded for the first time in detecting high-energy neutrinos from our Milky Way. Previously, “IceCube” had only registered high-energy neutrinos from distant galaxies. Although theoretical considerations had predicted similar particle radiation from the Milky Way, astronomers had so far searched in vain for it. Only the use of modern methods of machine learning has made the signal visible in the data collected by the detector, the scientists report in the journal Science.
Neutrinos are quite shy fellows: They rarely interact with ordinary matter. In order to detect the volatile particles, large amounts of matter are required, which consist of substances that are as pure as possible and that can react with neutrinos. Such a substance is, for example, water – and in the ice of the Antarctic it is present in large quantities in a sufficiently pure form. If a neutrino reacts – which rarely happens – with a water molecule, electrically charged particles are created that race through the ice at almost the speed of light and emit light, the so-called Cherenkov radiation.
Where do the neutrinos come from?
The researchers are looking for this light with “IceCube”. This is – the name says it all – an ice cube. A huge ice cube: Its edge length is one kilometer. The physicists of the “IceCube” project have sunk a total of 5160 light amplifiers to a depth of up to 2.5 kilometers in one cubic kilometer of Antarctic ice. In this way, they can not only capture the Cherenkov light, but also determine the direction from which it comes – and thus also the direction of origin of the neutrinos.
Neutrinos play an important role in nuclear physics, for example in nuclear fusion inside the sun. But the neutrinos that “IceCube” is searching for are millions to billions of times more energetic and are produced in stellar explosions and in the vicinity of supermassive black holes in distant galaxies. But also in our Milky Way, the interaction of cosmic rays with gas and dust should produce high-energy neutrinos, together with gamma rays. But while this gamma radiation could be detected from satellite observatories, the search for the galactic neutrinos has so far been unsuccessful.
Filtered out the noise
The problem: Cosmic radiation also produces neutrinos in the Earth’s atmosphere – and this noise is superimposed on the signal from the Milky Way that is being sought. However, by improving their methods, the “IceCube” researchers have now succeeded in making the neutrinos from the Milky Way visible. On the one hand, the scientists filtered out those events that come from the southern sky and thus from the direction of the center of the Milky Way.
And to determine the exact origin of the registered neutrinos, a machine-learning-based method developed primarily at the TU Dortmund was used. “These improved methods meant that we were able to use about ten times more neutrinos for the evaluation than before, and with better directional resolution,” explained Mirco Hünnefeld from the TU Dortmund. “All in all, our analysis was three times more sensitive than previous search methods.”
The “IceCube” data evaluated in this way provides for the first time an image of the Milky Way as it would appear with “neutrino eyes”. “And this image confirms what we know so far about the Milky Way and cosmic rays,” said IceCube researcher Steve Sclafani. But that’s just the beginning. “IceCube” continues to collect data and the methods are to be further improved. “So we get an image with better and better resolution,” explained Denise Caldwell of the “IceCube” project. In this way, the scientists want to find out exactly where the neutrinos originate. “And of course we also hope to discover previously unknown, never-before-seen structures in our Milky Way.”
