RAPID CITY, SD (July 17, 2018) - An international team of scientists, including researchers at the South Dakota School of Mines & Technology, have found the first evidence of a source of high-energy cosmic neutrinos, ghostly subatomic particles that can travel unhindered for billions of light years from the most extreme environments in the universe to Earth.
Detecting high-energy cosmic neutrinos requires a massive particle detector, and IceCube is by volume the world's largest. Encompassing a cubic kilometer of deep, pristine ice a mile beneath the surface at the South Pole, the detector is composed of more than 5,000 light sensors arranged in a grid. When a neutrino interacts with the nucleus of an atom, it creates a secondary charged particle, which in turn produces a characteristic cone of blue light that is detected by IceCube and mapped through the detector's grid of photomultiplier tubes. Because the charged particle along the axis of the light cone stays essentially true to the neutrino's direction, it gives scientists a path to follow back to the source.
The observations, made by the IceCube Neutrino Observatory at the U.S. Amundsen-Scott South Pole Station and confirmed by telescopes around the globe and in Earth's orbit, help resolve a more than a century-old riddle about what sends high-energy cosmic rays speeding through the universe.
Since they were first detected over 100 hundred years ago, cosmic rays--highly energetic subatomic particles that continuously rain down on Earth from space--have posed an enduring mystery: Where do they come from? How do they obtain such high energies that can be orders of magnitude higher than that the most powerful accelerator on Earth can produce?
Because cosmic rays are charged particles, their paths cannot be traced directly back to their sources due to the magnetic fields that permeates space and warp their trajectories. But the powerful "cosmic accelerators" that produce them will also produce neutrinos. Neutrinos are electrically uncharged particles, therefore unaffected by even the most powerful magnetic field. Because they rarely interact with matter and have almost no mass--hence their sobriquet "ghost particle"--neutrinos travel nearly undisturbed from their accelerators, giving scientists an almost direct pointer to their source.
Two papers published this week in the journal Science have for the first time provided evidence for a known blazar as a source of high-energy neutrinos detected by the National Science Foundation-supported IceCube observatory. A blazar is an active galactic nucleus hosted in a giant elliptical galaxy with a massive, rapidly spinning black hole at its core. This blazar, designated by astronomers as TXS 0506+056, was first singled out following a neutrino alert sent by IceCube on Sept. 22, 2017. "The evidence for the observation of the first known source of high-energy neutrinos and cosmic rays is compelling," says Francis Halzen, a University of Wisconsin-Madison professor of physics and IceCube principal investigator.
A signature feature of blazars is that twin jets of light and elementary particles, one of which is pointing to Earth, are emitted from the poles along the axis of the black hole's rotation. This blazar is situated in the night sky just off the left shoulder of the constellation Orion and is about 4 billion light years from Earth.
"IceCube is not only the leading neutrino astronomy instrument that enabled such an exciting observation, it is also a great platform for science education," says Xinhua Bia, Ph.D., associate professor of physics at SD Mines. Bai participated in the research, development and construction of the IceCube Observatory. He also spent one entire year at the South Pole.
Bai's own research on IceCube is funded by the National Science Foundation and includes Mines Ph.D. student Emily Dvorak. "I love being part of this experiment," says Dvorak. "We are a world-wide collaboration of scientists and graduate students, so not only do I learn science, but also about a lot of cultures around the world." Bai and Dvorak are working on a new way to study neutrino and cosmic ray events that land outside of the IceCube array. This method increases the precision of the experiment by including more quantities and the number of events that can be used for scientific studies. "When you have a detector as reliable as IceCube, the more events we can measure the smaller the uncertainties. That is crucial in making discoveries like this one," says Bai.
IceCube has also opened the door for undergraduate research projects that have contributed to the overall success of the experiment. Mines physics major Stefan Aviles became involved in IceCube when he helped debug a problem in event direction reconstruction. Aviles then landed a Research Experiences for Undergraduates (REU) summer internship program in Mainz, Germany.
"IceCube was my first real scientific research experience and I think it was a great place to start," says Aviles. "My internship in Germany last summer gave me the amazing opportunity to contribute to this multinational project and further encouraged my interest and excitement about a career in physics."
The project also involves area high school students who take part in the IceCube annual Master Class. It provides practice that is generally not conveyed in regular classroom curriculum. The 2018 IceCube Master Class offered at Mines included 18 high school students from Rapid City Stevens and Hill City High School. Students listened to Professor Halzen's science lecture on neutrino astronomy and watched a presentation about how scientists work at the South Pole. They also learned what cosmic ray events look like in IceCube. They then were able to work with actual IceCube data on computers and get some practice measuring cosmic ray properties in the data.
"For my students, it is an opportunity to be brought to the forefront of scientific research in physics and gives them a little insight into the true scientific process. Among many other valuable insights, they learn the importance of statistical analysis and error propagation, how experimental data can be used to constrain theories, and how large, interdisciplinary, collaborative teams are required for many modern scientific pursuits," says Andrew Smith, Ph.D., Stevens High School physics teacher. "It's about helping them understand how truly unanswered questions are approached."
"The enthusiasm and curiosity of those young students are very impressive. I remember they continued practicing and discussing while they were eating their lunch," Bai added.
For researchers like Bai this finding is a testament to almost two decades of study. "I am very happy to see the secret we observed in the deep space after nearly 18-year of effort. When the IceCube precedent Antarctic Muon and Neutrino Detector Array (AMANDA) was about to finish its mission in later 90s, there was a debate whether we should build a larger detector. I am glad we decided to build one, larger and better. After all, like all sciences, neutrino astronomy and the study of cosmic rays also relies on observational facts," says Bai.
Since Galileo Galilei started modern observational astronomy around the beginning of the 17th century with a regular telescope on his balcony, astronomy has expanded in to multi-wavelengths and multi-messengers. Neutrinos are unique because they enable us to see deeper and farther than ever before.
"This finding turns a new page in observational astronomy," says Bai.
IceCube will continue to be the world leading neutrino astronomy and high-energy cosmic ray project. But this is not the final chapter--the next generation array IceCube-Gen2 will be 10 times larger than the current experiment. It will help us see clearer and deeper into space once it is built.
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The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is operated by a team headquartered at the University of Wisconsin-Madison. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin-Madison Research Fund in the U.S.
The IceCube Collaboration, with over 300 scientists in 49 institutions from around the world, runs an extensive scientific program that has established the foundations of neutrino astronomy. Their research efforts, including critical contributions to the detector operation, are funded by funding agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom and the U.S.
Journalists: More information, including a copy of the paper, can be found online at the Science press package at http://www.eurekalert.org/jrnls/sci. You will need your user ID and password to access this information.
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Founded in 1885, the South Dakota School of Mines & Technology is a science and engineering research university located in Rapid City, S.D., offering bachelor's, master's and doctoral degrees. The university enrolls 2,778 students with a student-to-faculty ratio of 15:1. The SD School of Mines placement rate for graduates is 97 percent, with an average starting salary of more than $61,300. Find us online at http://www.sdsmt.edu and on Facebook, Twitter, LinkedIn, Instagram and Snapchat.
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