IceCube neutrino detector in Antarctica detects first high-energy neutrinos emitted in our galaxy, the Milky Way

The South Pole IceCube Neutrino Observatory, the world’s largest and strangest telescope, has detected the first neutrino emissions from within the Milky Way, a result that will shape how astronomers view our galaxy.

Neutrinos are tiny electrically neutral particles that pass through most matter undetected. They are created in extreme environments such as those surrounding massive black holes and travel unhindered through space and matter in a straight path.

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 even most types of telescopes.

This is where IceCube comes into play. The observatory, based in Antarctica, is made up of a billion tons of ice fitted with a frozen sensor grid. The sensors turn on when they detect a passing neutrino, and based on the arrangement of the sensors, researchers can determine the energy and direction of the neutrino that created the flash.

An illustration of a green colored spherical sensor, attached to a cable, frozen in ice.
Sensors embedded in the ice allow researchers to detect neutrinos.

From there, researchers can use the energy and direction to try to figure out where in the universe the neutrino is coming from.

As interim director of the Wisconsin IceCube Particle Astrophysics Center, I make sure we have the people and resources needed to help researchers use the IceCube Observatory successfully.

Detecting neutrinos using ice

Identifying flashes of light from neutrino interactions on IceCubes sensors can be a challenge. IceCube records about 2,600 events every second, although most of these events come from high-energy particles called cosmic rays, which also produce a constant shower of neutrinos when they hit Earth’s atmosphere. Only a few hundred of the hundreds of thousands of neutrinos observed each year come from galactic or extragalactic sources rather than cosmic rays.

A neutrino interacts with ice in the IceCube detector, producing light recorded by the IceCube sensors and indicating its direction and energy. Ice cube.

Finding neutrinos from outer space, rather than those from cosmic rays, is like trying to see a faint element in a portrait covered in many layers of paint: you have to be careful not to remove what you’re trying to find.

Surprisingly, the first two neutrino sources previously identified by the IceCube researchers came from outside the Milky Way, one of which was a very bright galactic object called a blazar. These neutrinos were quite distant, but higher in energy than any source within the Milky Way.

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

Scientists can use some tricks to filter neutrinos from space from cosmic ray neutrinos and other cosmic ray disturbances. We can sort by energy, with the higher energy neutrinos being more likely to come from space. Researchers can also look for neutrino clusters, because neutrinos from outside our galaxy tend to lump together in one place. Finally, researchers can look for neutrinos from transient astrophysical events such as black holes that have already been detected by other telescopes.

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

Looking for a neutrino source

Even though scientists figured out that these more recently discovered neutrinos came from our own galaxy, they don’t have a clear enough map of the Milky Way to identify the single location where the newly discovered neutrinos originated. Improving the analysis to determine the specific location of the neutrino emission is the next step.

There are a few ways to improve your source search. First, the longer scientists watch and the more data they collect, the more likely they are to spot a neutrino source, but to improve by a factor of 10 requires 100 times more data. So being smart has a better payoff than being patient.

Here are some ways to be smarter. First, researchers can improve event selection by choosing which cosmic events to focus on, so that more potential candidate neutrinos are present in the sample. They can also better reconstruct the path of the neutrinos, it’s like revisiting a museum with new glasses to see more clearly. Finally, they can try to find a way to reduce the background, a bit like looking for a region where the portrait is covered by fewer layers of paint.

It took all these tricks to see the Milky Way’s faint neutrinos. Our team found ways to improve sample size and we used machine learning to improve event reconstruction. This reduced the background enough to trace our neutrinos back to the Milky Way.

For most of the forms of cosmic light emission we study, light from sources within the Milky Way shines brightest because they’re closest. But for neutrinos, that’s not the case. The galaxy NGC1068, tens of millions of light-years away, emits more high-energy neutrinos than the Milky Way. This tells us that not all galaxies have the same ability to produce high-energy particles, but also that we need to find and study more neutrino-emitting galaxies to understand the cosmic quirks of the Milky Way.

IceCube is planning a high-energy upgrade that would make the detector array about eight times larger. Once the upgrade is finished in 2030, scientists will be able to continue their search for neutrinos with improved technology.

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