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The neutrino sky takes shape

An artist

An artist's impression of neutrino emission from the galactic plane. Picture: IceCube/NSF. Original photo taken by Martin Wolf

Neutrinos from the galactic disc of the Milky Way were recently detected. Now researchers are setting their sights on new goals.

Neutrinos are considered the most elusive of elementary particles, only interacting extremely rarely with normal matter, meaning they are immensely difficult to detect. Most of the time they whiz across the earth without any interaction. They also fly right through galactic gas and dust clouds that are opaque to other particles. They can even escape from the center of a star and thus provide information about the firing cycle of our sun or about stellar explosions known as supernovae.

Marek Kowalski Senior scientist in the IceCube group at DESY in Zeuthen. Photo: DESY/ Gesine Born

But what makes working with them so difficult also opens up exciting possibilities for research. "Neutrinos come to us from places that nothing else can tell us about," says Marek Kowalski from the German Electron Synchrotron DESY. The astrophysicist heads the IceCube group at DESY, which plays a leading role in the IceCube experiment.

IceCube consists of more than 5,000 highly sensitive photodetectors distributed over a kilometer deep in the ice of Antarctica. At this depth, most types of cosmic radiation are very well shielded. The detectors here register the tiny flashes of light produced by the rare neutrino reactions in the transparent ice as a clear medium is required to measure light so weak. That is why the technique is employed in Antarctica. There are, however, similar, smaller neutrino detectors which are located underwater. IceCube, with a volume of around one cubic kilometer, is by far the largest neutrino detector and specializes in particularly high-energy neutrinos.

The optical modules have been sunk to a depth of 1,450 to 2,450 m in 80 boreholes so far. Picture: DESY/Bernhard Voigt

Only under extreme astrophysical conditions such as supernovae or in the jets of supermassive black holes in galaxy centers can such energetic particles be created. "In recent years, we have been able to detect some extremely energetic neutrinos with IceCube whose origin lies in particularly strong active galactic nuclei," says Kowalski. These neutrinos are produced as by-products after a proton has first been accelerated to enormous energies in the jet of the giant black hole at the center of these galaxies. The high-energy proton then slams into an atomic nucleus of interstellar matter, whereupon, among other things, extremely high-energy neutrinos are formed.

The origin of such a neutrino event even points to a violent stellar death: During a flyby of a black hole, a star was completely torn apart and then swallowed up. In addition to electromagnetic radiation, high-energy neutrinos were also released, one of which IceCube was able to detect. The electromagnetic component was picked up by X-ray and gamma-ray detectors, among others. The event could be reconstructed based on this data. The simultaneous detection of high-energy neutrinos provides scientists with important insights into which processes are at work in such extreme cosmic environments.

"Since neutrinos are electrically neutral and almost don’t interact at all with matter, they can reach us from huge distances across the universe," the researcher explains. Charged high-energy particles such as protons or electrons are deflected by the intergalactic magnetic fields. Their origin,therefore, cannot be easily determined. Only uncharged particles like gamma rays or neutrinos fly on a direct path from their source to us. "But very energetic gamma radiation is absorbed over cosmic distances, so at very large distances only neutrinos remain as messengers of the highest-energy processes," Kowalski elaborates.

Nevertheless, the exact determination of the direction from which the neutrinos originate is not easy: Neutrino detectors do not reach the angular resolution of optical or radio telescopes by far. Nevertheless, it has already been possible to identify some extragalactic sources with the Active Galactic Nuclei.

Recently, the IceCube scientists were also able to detect neutrino emission from our own galaxy, the Milky Way. "To do this, we used completely new machine learning methods and used them to evaluate around 60,000 neutrino events from ten years of measurements," says Kowalski. This revealed an excess of neutrinos from the galactic disc. Their origin presumably lies in the remnants of supernovae or in the jets of stellar black holes that pull matter from a companion star and eject part of the infalling gas in almost light-speed particle jets.

Neutrinos from the Milky Way: Light areas symbolize measurements with high significance, dark ones those with low significance. Picture: Science Communication Lab für DESY/Milchstraßenpanorama: ESO/S. Brunier CC BY 4.0

No individual sources within the Milky Way can yet be identified in the neutrino sky map. Only a few extragalactic sources can be found in the panorama. But this should change in the future. On the one hand, with longer observation times, the angular information will become sharper. "And on the other hand, IceCube will be further expanded," says Kowalski. In the center of the facility, new, highly sensitive photodetectors will ensure that IceCube can detect more low-energy neutrinos. In a few years, the instrumented volume in the ice is also to be significantly increased, thereby increasing the overall data rate several times over.

This will help solve some of the mysteries of high-energy astrophysics. "We don't yet know exactly how the extremely energetic cosmic rays are produced," admits Kowalski. Among other things, it is not clear how protons, for example, can be accelerated to such enormous energies before they produce neutrinos through nuclear processes. There are some plausible models. But only with better data will it be possible to better narrow down the astrophysical processes behind them. "You need some patience while the neutrino sky map is pieced together piece by piece from years of observations," Kowalski concludes. "But it's worth the wait, because what we learn about the cosmos from neutrinos is something no other cosmic messenger can tell us."

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