The IceCube neutrino detector in Antarctica released the most energetic neutrinos in our Milky Way galaxy.

The South Pole IceCube Neutrino Observatory – the world’s largest and most spectacular telescope – has detected the first emission of neutrinos from within the Milky Way, an achievement that will change the way astronomers view our galaxy.

Neutrinos are tiny, electrically charged particles that pass through matter undetected. They are formed in extremely complex environments like those around supermassive black holes, and travel unimpeded through space and matter in a straight line.

Because black holes and exploding stars are too far away to visit, and too extreme to reproduce in the laboratory, scientists rely on cosmic messengers — such as visible light from stars — to study them. Neutrinos are another type of cosmic messenger, but they are too small to be seen with our eyes, or most types of telescopes.

This is where IceCube comes in. The observatory, located in Antarctica, contains 1,000,000 billion tons of ice with frozen sensors. The sensors light up when they detect a passing neutrino, and, depending on the arrangement of the sensors, researchers can determine the energy and path of the neutrino that produced the light.

From there, researchers can use energy and direction to try to determine where the neutrino came from in the universe.

As interim director of the Wisconsin IceCube Particle Astrophysics Center, I ensure that we have the people and resources needed to help researchers succeed using the IceCube observatory.

Detecting neutrinos using ice

Detecting light from neutrinos on IceCube’s sensors can be difficult. IceCube records about 2,600 events every second, although most of these events come from large particles called cosmic rays, which also create a steady shower of neutrinos when they hit the atmosphere. Only a few hundred of the hundred thousand neutrinos observed each year come from galaxies or extragalactic sources rather than cosmic rays.

Finding neutrinos from space, rather than light from space, is like trying to see a blurry object in a multi-colored picture – you have to be careful not to blur what you’re trying to reveal.

Surprisingly, the first two sources of neutrinos that the IceCube researchers have already detected came from outside the Milky Way – one of them was a very bright galactic object called a blazar. These neutrinos were distant, but far more powerful than any source from within the Milky Way.

Finding the fragile Milky Way neutrinos required a brilliant project by IceCube collaborators at Drexel University and Dortmund University. Their work on IceCube’s detection of the first Milky Way neutrinos was published in Science on June 29, 2023.

Scientists can use several techniques to filter neutrinos from space from cosmic ray neutrinos and other cosmic ray noise. We can choose by energy, where the more energetic neutrinos are the more likely they are from space. Researchers can also look for clusters of neutrinos, because neutrinos from outside our galaxy tend to cluster together. Finally, researchers can observe neutrinos from transient, cosmic events such as black holes that have already been detected by other telescopes.

In 2013, IceCube published the first evidence of cosmic neutrinos identified based on their energy. These were single neutrinos – so researchers couldn’t tell where they were coming from.

Searching for a neutrino source

Although scientists have determined that the recently discovered neutrinos came from within our own galaxy, they do not have a clear map of the Milky Way to determine where the newly discovered neutrinos came from. Further analysis to determine the exact location of the neutrino emission is the next step.

There are several ways to improve the search for sources. First, the longer scientists observe and the more data they collect, the better they can determine where the neutrino is coming from – but correcting by 10 takes 100 times as much. So being cautious is more valuable than being patient.

Here are some ways to make sense. First, researchers can refine the selection by choosing the best possible astronomical events, so that candidates with neutrinos are in the sample. They can also improve the path of neutrinos – it’s like going back to the museum with new glasses to get a better look. Finally, they can try to find a way to reduce the background, like looking at the area where the image is covered with less paint.

It took all this intelligence to detect the Milky Way’s weak neutrinos. Our team found ways to improve the sample size, and we used machine learning to refine the scenarios. This reduced the background enough to follow our neutrinos back to the Milky Way.

Of the many types of natural light we study, the light from the Milky Way shines the brightest because it is so close. But for neutrinos, that’s not the case – Galaxy NGC1068, which is millions of light years away, emits more powerful neutrinos than the Milky Way. This tells us that not all galaxies have the same ability to emit high-energy particles, and that we need to find and study more galaxies that emit neutrinos in order to understand the universe of the Milky Way.

IceCube is planning a more powerful upgrade that will make the recorder eight times larger. When the upgrade is complete in the 2030s, scientists will be able to continue searching for neutrinos with modern technology.