Deep within the South Pole, spanning one billion tons of ice stands the IceCube Neutrino Observatory — the first detector of its kind. It is equipped with a telescope that can observe the universe’s farthest reaches by tracking subatomic particles called neutrinos. By tracking their energy and direction, researchers can learn about the particles’ origin and begin to piece together the cosmos.
The Wisconsin IceCube Particle Astrophysics Center plays a leading role in maintaining and operating IceCube while also analyzing data as part of an international collaboration. Executive Director of WIPAC Jim Madsen said Wisconsin played an integral part in the development and construction of IceCube.
“You don’t see that animal going through your backyard — what we see are the tracks that it leaves,” Madsen said. “A few times a month, we see a very high energy neutrino that we know came from outer space.”
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Subatomic particles with no electrical charge called neutrinos have a mass of almost zero and are the main component in the research conducted at IceCube. They are unique because they can pass through opaque objects uninterrupted, allowing them to travel to the farthest reaches of the universe.
Though about 100 trillion neutrinos pass through the body every second, the particles are hard to understand because they barely interact with matter. IceCube has about 5,500 sensors 1.5 to 2.5 kilometers below the surface of the ice that can detect light produced when a neutrino does interact with matter.
Madsen said once in a while a neutrino will knock into a proton or neutron just right and create electrically charged particles that produce light. IceCube’s sensors are able to track the direction and energy of the neutrino based on this information to find its source.
Results shared in a globally attended webinar and published on Nov. 4 in Science provide evidence of a new source of high-energy neutrinos from an active galaxy, NGC 1068. According to the results, active galaxies have a black hole at their core, which led researchers to hypothesize they may contribute to a significant fraction of the extragalactic neutrino flux. NGC 1068, in particular, is centered around an obscured black hole, meaning the brightness of neutrinos compared to gamma rays indicates the black hole is obscured from view in light. Neutrinos are able to escape this obscured region.
According to the results, about 80 high-energy neutrinos were detected from NGC 1068, which is well above the average from that direction. IceCube member and University of Wisconsin physics professor Justin Vandenbroucke said this is the largest source of neutrinos observed to date.
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In a webinar, UW physics professor and principal investigator at IceCube Francis Halzen said while one neutrino can single out a source, multiple neutrinos can reveal the core of the most energetic objects in the cosmos.
In 2018, IceCube first observed a galaxy known as TXS 0506+056 as a source of high-energy neutrinos. While a flux in neutrinos has been observed from both TXS 0506 and NGC 1068, the two active galaxies differ in important ways.
“With TXS, we detected two different bursts of neutrinos at two different times, whereas with NGC 1068, it seems to be flowing continuously, which means we can keep studying and keep learning more about it and our detector,” Vandenbroucke said. “We calibrate as well as we can, but it’s challenging to be confident that we get all the calibrations right. Now that we have this source in the sky, we can actually use it to calibrate our detector to measure how precisely we can measure the directions of our neutrinos.”
Aside from the consistency of neutrino flux, the two sources also differ in distance from earth. Vandenbroucke said NGC 1068 is about 47 million lightyears away, which is relatively close on a cosmic scale, while TXS 0506 is about 100 times further away.
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But this discovery is just the beginning. Vandenbroucke said the team hopes IceCube’s research endeavors will lead to the discovery of more sources of high-energy neutrinos and an understanding of other types of particles and signals.
Another aspect of IceCube that makes it unique is the global collaboration involved. Currently, 350 people from 58 institutions across 14 countries make up the team.
“I can’t overemphasize how important the role of everyone who’s been involved in this project is to contribute to [its] success,” Madsen said. “It’s really cool to be able to be a part of the project. We still have about 75 people or so working in Madison on the project as part of WIPAC.”
