The IceCube laboratory is located near the Amundsen-Scott South Pole Station in Antarctica. This is, for example, where the computers that record the data are located.
Astrophysics
Source Desperately Sought
Every few weeks, something special happens in the Antarctic: Deep below the surface, a trace of light passes through the ice. It only lasts fractions of a second, but will be preoccupying researchers for years.
At the South Pole, the IceCube neutrino detector extends up to 2.5 kilometers deep into the ice layer. Since 2009, scientists from Ruhr University Bochum and elsewhere have been using IceCube to search for the sources of cosmic radiation. This radiation is constantly hitting Earth in the form of various particles like electrons, protons, and neutrinos. The radiation is everywhere, but its origin is uncertain. Neutrinos, which are also known as ghost particles, are being examined to trace the source of these rays. They are able to penetrate space and matter across vast distances without interacting, making them ideal candidates for finding the sources of cosmic radiation, no matter what obstacles are in their path, because they travel from their origin to Earth in a more or less straight trajectory.
Their ghostly nature is thus a major benefit, but also a significant challenge. Neutrinos can only be measured on Earth if they happen to interact with another particle. The IceCube team has been waiting patiently for these events for many years. If a neutrino interacts in the ice, it can create a new particle called a muon. As the muon passes through the ice, it leaves a faint, bluish light in its wake, and it is this light that the photosensitive IceCube detectors pick up.
This illustration shows what the inside of the IceCube detector looks like: 5,160 sensors, called digital optical modules, capture the light signals from the muons.
The IceCube sensors, known as DOMs (digital optical modules), are arranged in strings, one of which can be seen here in the ice.
Troublemakers from our own atmosphere
“To be precise, we’re seeing this light thousands of times every second,” says Professor Anna Franckowiak, lead of the Research Group for Multi-wavelength and Multi-messenger Astronomy and member of the Cosmic Interacting Matter Collaborative Research Center in Bochum. “But most particles we detect in this manner come from our own atmosphere and aren’t the ones we’re looking for,” she explains. In fact, the particles that form in our atmosphere are making it more difficult for the researchers to find the distant origins of cosmic radiation. They have to be filtered out of the measurement results.
More about the detector
More about the detector
The digital optical modules, or DOMs for short, which are the individual detector units of IceCube, are arranged in 86 strings. On each string (gray), many DOMs are attached beneath each other. When a neutrino interacts in the ice, a muon is created that is followed by a blue cone of light. This light is registered by DOMs from different strings (colored spheres). Red represents an early arrival time, blue a late one. The size of the colored spheres represents the intensity of the detected light.
The IceCube team is looking for high-energy neutrinos that most likely do not come from Earth’s atmosphere, but from space. When the researchers detect a trace of light in the ice, they calculate three things: the energy of the particle that caused the interaction, the direction it came from, and the probability that it is a neutrino from space.
Analyses in real time
Anna Franckowiak’s team has refined the algorithm for this over the years, and made it faster. “We need 30 seconds to calculate the energy and direction of a neutrino,” she explains. “Then we immediately disseminate the information worldwide.” With special apps like Astro-COLIBRI, developed in cooperation between RUB and Université Paris-Saclay, anyone can find out when a neutrino event has occurred and turn their telescope in the corresponding direction. Because the signal that Franckowiak’s team sends out is machine-readable, suitable telescopes can even be automatically aligned when the IceCube team’s alarm goes off. Researchers use these to scour the region of the sky the neutrino came from for a particularly high-energy object that may have emitted the neutrino.
Eyes Always on the Sky
Eyes Always on the Sky
“These celestial objects might only illuminate very briefly, so it’s crucial that our system works in real time,” Franckowiak emphasizes. For example, supernovae – exploding stars with great mass – could be a source of cosmic rays. Yet as energetic as such an event is, it is just as quickly over.
Precisely determining neutrino trajectories
In addition to their particularly fast algorithm, the researchers in Bochum have a second calculation method to determine direction: It is about two hours slower, but a factor of four to five times more precise than previous methods. “Once the more precise data is available, we update our original neutrino alert,” explains Angela Zegarelli, postdoctoral researcher in Franckowiak’s chair and lead of IceCube’s Reconstruction Research Group.
The Bochum researchers Anna Franckowiak, Nora Valtonen-Mattila, Giacomo Sommani and Angela Zegarelli (from left) are members of the IceCube projekt.
The calculations resume when a potential source of the neutrino has been found. “Then we determine how likely it is that, when we look in the direction the neutrino came from, we see such a celestial object light up that has nothing to do with the neutrino.” The value represents the probability that the coincidence of the neutrino and the source happened by chance.
The astrophysicists have strict parameters. They only claim to have discovered a source of cosmic radiation when the probability is 1 in 1.7 million. This means if they were to observe the sky 1.7 million times, they would only find a potential neutrino source once, which in reality has nothing to do with the neutrino they measured. In technical language, the team refers to the likelihood of 1 in 1.7 million as 5 Sigma.
The likeliest neutrino sources
IceCube has not yet identified any neutrino source with this likelihood, but the team has come close a few times. In 2017, the research consortium discovered a neutrino that – with a likelihood of 3 Sigma, or 1 in 1,000 – came from a blazar. This is a galaxy with an active black hole at its center that swallows matter, a portion of which is emitted toward Earth as a jet.
IceCube got even closer to the target value in 2022 when Giacomo Sommani, a doctoral researcher in Franckowiak’s research group, identified two neutrinos that appeared to come from an active galactic nucleus. Unlike the case from 2017, the possible source was not a blazar but rather a black hole without a prominent matter jet in the galaxy NGC 7469. The likelihood: 3.2 Sigma. In 2023, IceCube identified 80 neutrinos with somewhat lower energies that the team associated with the active galactic nucleus of NGC 1068 with a probability of 4.2 Sigma. “This is close, but we want 5,” says Franckowiak.
The galaxy NGC 7469 could be a source of cosmic rays, or more precisely, the black hole at its center, known as the active galactic nucleus. The object is 220 million light-years away from Earth. In 2022 and 2023, IceCube detected two high-energy neutrinos that may have originated from NGC 7469.
The IceCube team continues to search, not just for galaxies but also for smaller potential sources. “In the meantime, we’ve come to consider tidal disruption events as potential sources,” Franckowiak explains. “These occur when a star comes too close to an inactive black hole that is not swallowing up any matter, but stretches the star because of its strong gravity. The side of the star facing the black hole is pulled further than the other side, which can tear the star apart.”
Supernovae and tidal disruption events
Over the years, IceCube had discovered three neutrino events that may have been attributable to tidal disruption events. However, “After we improved our algorithm for trajectory reconstruction, we analyzed the events again and the neutrino paths don’t match the positions where the tidal disruption events occurred,” says Franckowiak.
In addition to tidal disruption events, the IceCube team is considering another phenomenon as a possible source of cosmic radiation: supernovae, massive explosions that occur at the end of the lives of stars ten times the mass of the sun. It is known that supernovae produce neutrinos with low energies. However, these extreme events could potentially also produce high-energy neutrinos that can traverse the vastness of space. While no one has yet been able to prove this, the IceCube team has found initial evidence. A supernova could be attributed with a probability of 3 Sigma to a high-energy neutrino identified by the detector.
Awaiting the explosion in the Milky Way
Secretly, Anna Franckowiak is waiting for something else: an event right on Earth’s doorstep, a supernova in our own Milky Way. “We could then use IceCube to track this even if the neutrinos don’t achieve the highest energy levels,” explains Nora Valtonen-Mattila, postdoctoral researcher in Franckowiak’s chair and head of the Low Energy research group for IceCube. “Because then so many neutrinos would come from this direction that there wouldn’t just be a trace of light, but rather our entire detector would light up at once. That would be sensational.” A sensation that only occurs one to three times every century in the Milky Way. “It would be amazing to experience that,” says Franckowiak.
Anna Franckowiak heads the Multi-wavelength and Multi-messenger Astronomy working group at Ruhr University Bochum.
Because the IceCube team can’t rely on this, they’re constantly working to improve their detection methods. Scientists are continually optimizing the algorithms for determining the direction of neutrinos. Ever-improving telescopes are increasing the chances of finding potential sources. IceCube will also soon be upgraded with new detector strings at its center, allowing it to capture neutrinos with slightly lower energies. At some point, a ghost particle has to appear that unequivocally reveals its origin.
What is Happening inside the Neutrino Sources
Original publications
Original publications
