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
Bai, 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 an NSF funded International Research Experiences for Students (IRES) summer internship 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-years 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.
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.
You can find the full release from
the IceCube Collaboration here.