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The scientific question eluded researchers around the world for more than a century.

But one day last fall, a mile below sheets of ice at the Amundsen–Scott South Pole Station in Antarctica, a detector pinged.

“In fact, it was pretty uninteresting,” UW-Madison physics professor Francis Halzen said of the day that sparked weeks of additional observation, months of analysis and the reporting of a major scientific discovery announced Thursday and to be published Friday in the journal Science.

Scientists have long sought to explain what propels high-energy particles, known as neutrinos, through the universe.

An international team of scientists led by Halzen and other researchers at UW-Madison identified a blazar — a technical term for a galaxy with a massive spinning black hole in its center — as the first known cosmic source for a neutrino detected Sept. 22, 2017. Their research found the blazar is four billion light-years away.

The discovery represents the latest astronomical breakthrough for the IceCube Neutrino Observatory, a National Science Foundation-supported endeavor led by UW-Madison.

“This is the beginning of something,” Halzen said. “There’s a whole part of astronomy that was a black box for us. I think we opened that up.”

Operational since 2010, the Antarctic IceCube facility first identified neutrinos beyond the Earth’s galaxy in 2013, a discovery that had Halzen hoping further research would lead to a better understanding of neutrinos’ origins.

Five years later and Halzen’s team has identified the first known cosmic source for neutrinos, a finding that could lead to an even greater understanding of the workings of the universe.

“This is at least as big (as the 2013 discovery),” Halzen said of his team’s findings.

Neutrinos have no charge and little mass, which is where the nickname “ghost particle” is derived. The particles travel billions of light-years through space in a straight line without bending.

On the rare occasion when a neutrino collides with something, it sets off a trail of subatomic debris that casts a blue light. The IceCube detector records these bursts of blue, so scientists can later determine the direction and energy level of the neutrino.

Last September, the roughly 5,000 bowling ball-sized optical sensors that make up the detector recorded a high-energy neutrino.

Within 43 seconds, the detector buried a mile below the South Pole’s surface ice sent a telegram to IceCube’s roughly 300 researchers dispersed across the globe.

Halzen remembers he was in Madison, but can’t pinpoint where he was — his office? his house? — when the detector pinged that day.

He didn’t think much of the alert. The team receives a note about a high-energy neutrino about about once a month.

The energy level of this neutrino, however, stood out as the highest recording for a particle at IceCube— about 46 times greater than the energy of protons in the world’s most powerful particle accelerator, which is located in Europe.

And two other telescopes, one stationed on the Canary Islands and a NASA satellite in space, also detected the same blazar in the following weeks, further confirming the galaxy’s black hole powers neutrinos through galaxies, stars and anything else in its path.

“It was only when we started reading the other telegrams that this became an exciting event,” Halzen said.

Corroborating IceCube’s observations with other telescopes marks a milestone in what scientists call multi-messenger astronomy.

Another part of the team’s research that bolsters their findings: When scouring their data of previous neutrino collisions, IceCube researchers found a series of “spectaular bursts” in late 2014 and early 2015 that came from the same blazar.

It wasn’t detected then because researchers were looking in so many different directions of the sky and making so many observations that it did not seem significant at the time, Halzen said.

In 1912, Austrian physicist Victor Hess proved charged particles in the atmosphere came from space, not Earth. In the 105 years since Hess’ discovery, scientists culled a list of potential sources for these particles.

Supernova remnants. Colliding galaxies. Active galatic nuclei.

Blazars were “far from the top-ranked source in the guessing game of where cosmic rays come from,” mostly because there was no supporting evidence, Halzen said.

The team’s discovery reverses what was a general consensus in the astrophysics community. And it helps advance understanding of multi-messenger astrophysics.

“This is actually very exciting. We’re learning about blazars as we go,” Olga Botner, former IceCube spokesperson and physics professor at Uppsala University in Sweden, said Thursday.

Other scientists have already written their own research based on the IceCube’s data, some of which will also publish Friday.

After the announcement Thursday, Halzen said he and others will not have a luncheon or a traditional celebration.

He said he looks forward to sitting down and reading others’ research.

“This is a whole part of the universe that we are only now trying to understand,” he said.

State Journal reporter Chris Aadland contributed to this report.

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