It is still largely unclear from which sources the high-energy cosmic rays come. However, neutrinos from deep space could help identify these sources. Physicists at the IceCube neutrino observatory at the South Pole have now discovered an exciting peculiarity in the influx of these cosmic particles: By analyzing a good ten years of detector data, they identified a significant intensity peak in the energy spectrum of the neutrinos. This bend in the curve at around 30 teraelectron volts provides the first indications that these high-energy neutrinos are not created by a uniform process or just one class of cosmic sources, but are due to different mechanisms. This helps narrow down the models.
Neutrinos are also considered “ghost particles” because these almost massless elementary particles hardly interact with matter and are therefore difficult to detect. This is particularly true for high-energy cosmic neutrinos that come from deep space. Because they are hardly distracted or interact, they can cover long distances almost unhindered. This makes such neutrinos valuable witnesses of high-energy processes in space and thus possible sources of cosmic radiation. To observe them, however, physicists need large detectors like the IceCube Neutrino Observatory at the South Pole. This includes thousands of photodetectors embedded in one cubic kilometer of Antarctic ice. When neutrinos interact with atoms in the ice, cascades of secondary particles and longer particle trails are created, each producing characteristic photoemissions. Using this Cherenkov radiation, physicists can understand how energetic the neutrino was and from which direction it came.
Four hypotheses put to the test
Physicists from the IceCube collaboration have now evaluated data from the neutrino observatory from the last ten years. To do this, they analyzed the energy spectrum of the particle tracks and cascades of the secondary particles – the curve that reveals how many neutrinos of what energy arrive. They compared this spectrum with four different hypotheses about its course. Accordingly, the energy distribution of the cosmic neutrinos could correspond to a uniform power function, a power function with a “truncated” upper limit, a so-called log-parabolic function or a fractional power function – i.e. a curve that follows different power functions in its course.
The evaluations showed: Neither a uniform power function nor a log-parabolic function fits the observed energy spectrum of the cosmic neutrinos. Instead, there was a significant intensity peak at around 30 teraelectron volts. “For the first time we actually see a structure in the energy spectrum,” explains co-author Markus Ackermann from the German Electron Synchrotron (DESY) in Hamburg. “We see the intensity of the neutrinos increasing to a point at an energy of about 30 teraelectron volts and then decreasing again. This means that we have found the energy at which neutrino emission is strongest in the universe.” Previous studies had already suggested such a “knee” at this point in the neutrino energy spectrum. “We have now demonstrated this break in the spectrum to be statistically significant for the first time,” writes the team of physicists.
Help in finding the sources
The shape of the energy spectrum and the location of the intensity peak now enable researchers to exclude many of the sources of neutrinos and thus also of high-energy cosmic rays predicted by theoretical models. “The data allows us to improve our models for producing these neutrinos and to narrow down the production processes and source classes much better than before,” says Ackermann. “Now that we see the peak in intensity, many things become much more concrete – how the neutrino sources work, what processes produce the neutrinos and how particles in these sources are accelerated.” This will allow astrophysicists to focus in the future on the potential particle sources and processes that have the appropriate environmental conditions and physical mechanisms to produce neutrinos with the observed energies.
“The results are an important first step,” says co-author Marek Kowalski from DESY and the Humboldt University of Berlin. “In the future, we want to combine spectral and compositional measurements to gain insights into the magnetic fields and other properties of the cosmic accelerators. However, to really understand how these accelerators work, we need IceCube-Gen2.” The planned IceCube-Gen2 observatory is intended to significantly expand the existing IceCube detector at the South Pole and significantly increase its sensitivity.
Source: Abbasi et al. (IceCube Collaboration), Physical Review Letters, doi: 10.1103/2gh9-d4q7