Research, teaching and outreach in Physics at UW–Madison
astrophysics
UW–Madison physicists key in revealing neutrinos emanating from galactic neighbor with a gigantic black hole
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On Earth, billions of subatomic particles called neutrinos pass through us every second, but we never notice because they rarely interact with matter. Because of this, neutrinos can travel straight paths over vast distances unimpeded, carrying information about their cosmic origins.
Although most of these aptly named “ghost” particles detected on Earth originate from the Sun or our own atmosphere, some neutrinos come from the cosmos, far beyond our galaxy. These neutrinos, called astrophysical neutrinos, can provide valuable insight into some of the most powerful objects in the universe.
For the first time, an international team of scientists has found evidence of high-energy astrophysical neutrinos emanating from the galaxy NGC 1068 in the constellation Cetus.
The detection was made by the National Science Foundation-supported IceCube Neutrino Observatory, a 1-billion-ton neutrino telescope made of scientific instruments and ice situated 1.5-2.5 kilometers below the surface at the South Pole.
These new results, to be published tomorrow (Nov. 4, 2022) in Science, were shared in a presentation given today at the Wisconsin Institute for Discovery.
“One neutrino can single out a source. But only an observation with multiple neutrinos will reveal the obscured core of the most energetic cosmic objects,” says Francis Halzen, a University of Wisconsin–Madison professor of physics and principal investigator of the IceCube project. “IceCube has accumulated some 80 neutrinos of teraelectronvolt energy from NGC 1068, which are not yet enough to answer all our questions, but they definitely are the next big step toward the realization of neutrino astronomy.”
Study led by UW–Madison researcher confirms star wreck as source of extreme cosmic particles
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Astronomers have long sought the launch sites for some of the highest energy protons in our galaxy. Now, a study using 12 years of data from NASA’s Fermi Gamma-ray Space Telescope (Fermi) confirms that a remnant of a supernova, or star explosion, is just such a place, solving a decade-long cosmic mystery.
The newly discovered PeVatron (in pink) is hosted by a supernova remnant (in green) called G106.3+2.7. The supernova remnant is believed to have formed together with the pulsar (in magenta) about 10,000 years ago. Particles accelerated by the shock waves of the supernova remnant interact with the gas in the interstellar medium, producing high-energy gamma-ray emission. Credit: Jayanne English, University of Manitoba, NASA/Fermi/Fang et al. 2022, and Canadian Galactic Plane Survey/DRAO.
Previously, Fermi has shown that the shock waves of exploded stars boost particles to speeds comparable to that of light. Called cosmic rays, these particles mostly take the form of protons, but can include atomic nuclei and electrons. Because they all carry an electric charge, their paths become scrambled as they whisk through our galaxy’s magnetic field, which masks their origins. But when these particles collide with interstellar gas near the supernova remnant (SNR), they produce a telltale glow in gamma rays—the highest-energy light there is.
“Theorists think the highest energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Ke Fang, an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center at the University of Wisconsin–Madison. “The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”
Fang, who led the study, performed the data analysis and developed the theory models. The research team identified a few suspected PeVatrons, including one at the center of our galaxy. Naturally, SNR top the list of candidates. Yet out of about 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.
Massive bubbles at center of Milky Way caused by supermassive black hole
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The enormous clouds of material known as the eRosita and Fermi bubbles extend above and below the galactic plane of the Milky Way. NASA/KAREN YANG/MATEUSZ RUSZKOWSKI/ELLEN ZWEIBEL
New research reveals the origins of enormous bubbles of material emanating from the center of the Milky Way.
The related structures — known as the eRosita and Fermi bubbles and the microwave haze — are the result of a powerful jet of activity from the supermassive black hole at the center of the galaxy. The study, published March 7 in Nature Astronomy, also shows the jet began spewing out material about 2.6 million years ago, and lasted about 100,000 years.
Ellen Zweibel
The work was led by Karen Yang of National Tsing Hua University in Taiwan with University of Wisconsin–Madison astronomer Ellen Zweibel and Mateusz Ruszkowski at the University of Michigan.
The black hole origin of these huge bubbles rules out an alternative model that the expansion of the material was driven by exploding stars. Such a nuclear starburst would last about 10 million years, according to Zweibel, a professor of astronomy and physics at UW–Madison.
“On the other hand, our active black hole model accurately predicts the relative sizes of the eRosita X-ray bubbles and the Fermi gamma ray bubbles, provided the energy injection time is about one percent of that, or one-tenth of a million years,” Zweibel says. “Injecting energy over 10 million years would produce bubbles with a completely different appearance. While both the black hole and stellar explosion models were in reasonably good agreement with the gamma ray data, it’s the discovery of the X-ray bubbles, and the opportunity to compare the X-ray and gamma ray bubbles, which provide the crucial, previously missing piece.”
The enormous structures are nearly 36,000 light-years tall, one-third the diameter of the Milky Way. The eRostia and Fermi bubbles were named for the telescopes that discovered them in 2020 and 2010, respectively.
The IceCube Neutrino Observatory, a massive astroparticle physics experiment located at the South Pole, will be featured in two upcoming documentaries about neutrinos produced for the BBC and PBS NOVA.
Sometimes called the world’s biggest and strangest telescope, IceCube comprises over 5,000 light sensors deployed in a cubic kilometer of ice at the South Pole. Despite its inhospitable environment, the South Pole’s abundance of ice makes it an ideal location for detecting neutrinos: tiny fundamental particles that could reveal unseen parts of the universe.
For these documentaries, IceCube staff from the experiment’s headquarters at the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center of the University of Wisconsin–Madison, captured video footage at the South Pole. During the austral summer of 2019, Kael Hanson, John Hardin, Matt Kauer, John Kelley, and Yuya Makino recorded video at the bottom of the world as they conducted annual maintenance and other work on the observatory. The footage was then sent “up north” for use in the two different documentaries.
The BBC documentary, “Neutrino: Hunting the Ghost Particle,” will premiere on BBC Four on Wednesday, September 22 from 9:00 – 10:00 pm BST. It is described as “an astonishing tale of perseverance and ingenuity that reveals how scientists have battled against the odds for almost a century to detect and decode the neutrino, the smallest and strangest particle of matter in the universe.” The documentary will feature footage and interviews from IceCube and will discuss the experiment’s role in neutrino astronomy.
PBS NOVA will feature IceCube and its science in its “Particles Unknown” documentary premiering on Wednesday, October 6 at 9:00 pm CDT. IceCube will appear near the end of the program, which is also about the hunt for neutrinos, “the universe’s most common—yet most elusive and baffling—particle,” and includes an interview with Hanson, who is also IceCube’s director of operations and the director of WIPAC.
Learn more about IceCube and neutrinos at IceCube’s website.
The IceCube Neutrino Observatory is funded primarily by the National Science Foundation (OPP-1600823 and PHY-1913607) and is headquartered at the Wisconsin IceCube Particle Astrophysics Center, a research center of UW–Madison in the United States. IceCube’s research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the United States. The IceCube EPSCoR Initiative (IEI) also receives additional support through NSF-EPSCoR-2019597.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 University of Wisconsin–Madison Research Fund in the U.S.
Balantekin named co-PI on NSF grant to solve cosmic mystery
A team of University of Wisconsin–Madison and New York Institute of Technology physicists has secured a grant from the National Science Foundation (NSF) in an attempt to solve one of science’s greatest mysteries: how the universe formed from stardust.
Many of the universe’s elements, including the calcium found in human bones and iron in skyscrapers, originated from ancient stars. However, scientists have long sought to understand the cosmic processes that formed other elements—those with undetermined origins. Now, UW–Madison professor of physics Baha Balantekin and co-principal investigator Eve Armstrong assistant professor of physics at New York Institute of Technology, will perform the first known research project that uses weather prediction techniques to explain these events. Their revolutionary work will be funded by a two-year $299,998 NSF EAGER grant, an award that supports early-stage exploratory projects on untested but potentially transformative ideas that could be considered “high risk/high payoff.”
While the Big Bang created the first and lightest elements (hydrogen and helium), the next and heavier elements (up to iron on the periodic table) formed later inside ancient, massive stars. When these stars exploded, their matter catapulted into space, seeding that space with elements. Eventually, stardust matter from these supernovae formed the sun and planets, and over billions of years, Earth’s matter coalesced into the first life forms. However, the origins of elements heavier than iron, such as gold and copper, remain unknown. While they may have formed during a supernova explosion, current computational techniques render it difficult to comprehensively study the physics of these events. In addition, supernovae are rare, occurring about once every 50 years, and the only existing data is from the last explosion in 1987.
Large information-rich data sets are obtained from increasingly sophisticated experiments and observations on complicated nonlinear systems. The techniques of Statistical Data Assimilation (SDA) have been developed to handle very nonlinear systems with sparsely sampled data. SDA techniques, akin to the path integral methods commonly used in physics, are used in fields ranging from weather prediction to neurobiology. Armstrong and Balantekin will apply the SDA methods to the vast amount of data accumulated so far in neutrino physics and astrophysics.
With simulated data, in preparation for the next supernova event, the team will use data assimilation to predict whether the supernova environment could have given rise to some heavy elements. If successful, these “forecasts” may allow scientists to determine which elements formed from supernova stardust.
This project will provide an opportunity to the Physics graduate students interested in neutrinos to master an interdisciplinary technique with many other applications.
“Physicists have sought for years to understand how, in seconds, giant stars exploded and created the substances that led to our existence. A technique from another scientific field, meteorology, may help to explain an important piece of this puzzle that traditional tools render difficult to access,” says Armstrong.
The NSF is an independent agency of the U.S. government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health. NSF funding accounts for approximately 27 percent of the total federal budget for basic research conducted at U.S. colleges and universities.
The content is solely the responsibility of the authors and does not necessarily represent the official views of the NSF.
Magnetic fields implicated in the mysterious midlife crisis of stars
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Artist’s impression of the spinning interior of a star, generating the stellar magnetic field. This image combines a dynamo simulation of the Sun’s interior with observations of the Sun’s outer atmosphere, where storms and plasma winds are generated. | Credit: CESSI / IISER Kolkata / NASA-SVS / ESA / SOHO-LASCO
This post was originally published by the Royal Astronomical Society. UW–Madison physics graduate student Bindesh Tripathi is the lead author of the scientific publication.
Middle-aged stars can experience their own kind of midlife crisis, experiencing dramatic breaks in their activity and rotation rates at about the same age as our Sun, according to new research published today in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.
Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like our Sun to gradually slow down their rotation over billions of years.
In turn, the slower rotation leads to altered magnetic fields and less stellar activity – the numbers of sunspots, flares, outbursts, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields.
Bindesh Tripathi
This decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.
However recent observations indicate that this intimate relationship breaks down around middle age. The new work, carried out by Bindesh Tripathi at UW–Madison and the Indian Institute of Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment. Prof. Dibyendu Nandy, and Prof. Soumitro Banerjee of IISER are co-authors.
Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. A middle aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.
Prof. Nandy comments: “This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”
The new work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen. The best known of these is perhaps the Maunder Minimum around 1645 to 1715, when very few sunspots were observed.
The team hope that it will also shed light on recent observations indicating that the Sun is comparatively inactive, with crucial implications for the potential long-term future of our own stellar neighbor.
Celebrating IceCube’s first decade of discovery
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It was the beginning of a grand experiment unlike anything the world had ever seen. Ten years ago today, the IceCube Neutrino Observatory fully opened its eyes for the first time.
Over the course of the previous seven years, dozens of intrepid technicians, engineers, and scientists had traveled to the South Pole—one of the coldest, driest, and most isolated places on Earth—to build the biggest, strangest telescope in the world. Crews drilled 86 holes nearly two-and-a-half kilometers deep and lowered a cable strung with 60 basketball-sized light detectors into each hole. The result was a hexagonal grid of sensors embedded in a cubic kilometer of ice about a mile below the surface of the Antarctic ice sheet. On December 18, 2010, the 5,160th light sensor was deployed in the ice, completing the construction of the IceCube Neutrino Observatory.
The purpose of the unconventional telescope was to detect signals from passing astrophysical neutrinos: mysterious, tiny, extremely lightweight particles created by some of the most energetic and distant phenomena in the cosmos. IceCube’s founders believed that studying these astrophysical neutrinos would reveal hidden parts of the universe. Over the course of the next decade, they would be proven right.
IceCube began full operations on May 13, 2011 — ten years ago today — when the detector took its first set of data as a completed instrument. Since then, IceCube has been watching the cosmos and collecting data continuously for a decade.
During its first few years of operation, IceCube accumulated vast amounts of data, but it wasn’t until 2013 that the observatory yielded its first major results.
Ellen Zweibel elected to the National Academy of Sciences
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Ellen Zweibel
Astronomy and physics Professor Ellen Zweibel has been honored with membership in the National Academy of Sciences.
Zweibel is among 120 new members — and one of 59 women, the largest group ever — elected to the academy, one of the highest honors that can be conferred on an American scientist. Members are chosen “in recognition of their distinguished and continuing achievements in original research.”
Zweibel, the W.L. Kraushaar Professor of Astronomy and Physics, came to UW–Madison in 2003. She studies the way magnetic fields shape the universe, including the physics of plasmas in stars and galaxies and the cosmic rays they throw out into the universe.
A founding member of the Center for Magnetic Self-Organization, a Physics Frontier Center funded by the National Science Foundation and the U.S. Department of Energy, Zweibel won the American Physical Society’s Maxwell Prize for Plasma Physics in 2016.
The National Academy of Sciences — with the National Academy of Engineering and the National Academy of Medicine — provides science, engineering, and health policy advice to the federal government and other organizations. It is a private, nonprofit institution established in 1863 under a congressional charter signed by President Abraham Lincoln.
Highest-energy Cosmic Rays Detected in Star Clusters
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For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova — when they grow too massive to support the fusion occurring at their cores and explode.
Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae — capable of devouring entire solar systems — are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.
And yet cosmic rays have been observed striking Earth’s atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers — including UW–Madison’s Ke Fang — posit that star clusters like the Cygnus Cocoon serve as PeVatrons — PeV accelerators — capable of moving particles across the galaxy at such high energy rates.
Their paradigm-shifting research provides compelling evidence for star forming regions to be PeVatrons and is published in two recent papers in Nature Astronomy and Astrophysical Journal Letters.
New UW–Madison assistant professor of physics Lu Lu’s research program combines the past with the future. Her research looks for sources of ultrahigh energy particles, which is done by analyzing data that has already been collected. As she says, “Maybe data is already talking to us, we just haven’t looked.” But she is also working toward improving future data collection, which will require more technologically-advanced detectors. “My teachers, my great masters, have taught me that the current young generation has the responsibility to look into new techniques to go to the future for younger generations to proceed forward,” she says about her work in sensor R&D.
On January 1, Professor Lu joined the Department of Physics and IceCube. Most recently, she was a postdoctoral fellow at the International Center for Hadron Astrophysics at Chiba University in Japan. To welcome her, we sat down for a (virtual) interview.
What are your research interests?
My prime interest is astroparticle physics, and my ultimate goal is to find the sources of the highest energy particles in the universe. These particles carry energy of about 1020 electronvolts. This is higher energy than what we have from the Large Hadron Collider and human technologies. The real attractiveness here is we don’t know how nature accelerates these particles. And once we identify the sources, we can test new theories beyond the Standard Model using sources crated by nature.
What are one or two main projects you focus your research on?
I’m involved in two experiments. One is IceCube, the other is Pierre Auger Observatory. I was doing cosmic ray analysis, but cosmic rays are usually charged particles and they are deflected in the magnetic field of the galaxy; they would not travel in a straight line. IceCube studies neutrinos which are neutral particles, they travel directly from the source. Pierre Auger detects ultrahigh energy photons, which are also neutral particles. One thing I want to do immediately after I join Madison is to combine these two experiments to do a joint analysis. We have photon candidates but we haven’t really tried to connect them in the multimessenger regime. By combining Pierre Auger photons with IceCube neutrinos, we could possibly find a transient source, a source that doesn’t constantly emit ultrahigh energy photons or neutrinos but all of a sudden there’s a flare. This type of analysis has never been done, but we have data on disks.
The second thing I’m interested in is using new sensor technologies. In IceCube, we have Gen2 being planned right now. Instead of using a single photon sensor, we’d use a more sensitive design and R&D. UW–Madison is taking the lead of designing this future detector. There’s also radio technology. So, to detect the highest energy neutrinos we need to build a large instrument volume. With optical array, it is really hard to scale up because one has to drill holes inside the South Pole, which is really expensive. But radio technology doesn’t have to go so deep, so they can bury their detectors on the surface areas, and the radiowaves can transmit further away than the optical photons in ice. For optical you have to make the detectors very dense, but for radio you can make the antennas further apart, so that means you can have a larger area and detect more events easily. I think radio is the way to go for the future.
You said you have a lot of data collected already and just need to analyze it. How do you analyze the data from these detectors?
We would have to search for photon candidates from the data from Auger, and identify where it comes from and what the time this event happened. Correspondingly, do we see neutrinos from IceCube coming from the same direction and at the same time? Because you can never be sure it’s a photon. It could be a proton. We then want to build a statistical framework to combine different multimessengers together in real time.
What does it mean if you find a photon in coincidence with a neutrino?
Cosmic rays were first detected more than 100 years ago, and there’s a rich history of studying where they come from. The mystery of origins still remains today because our poor knowledge on the galactic/extragalactic magnetic fields and mass composition of cosmic rays. In my opinion, the most probable way to solve this puzzle is to use neutral particles. If we can identify ultrahigh energy photons in coincidence with neutrinos, that is a smoking gun that we are actually looking at a source and we can finally pin down where in the universe is accelerating high energy particles. And therefore, we can study particle physics maybe beyond Standard Model. It’s just like a lab created by the universe to test particle physics.
What is your favorite element and/or elementary particle?
My favorite elementary particle is the electron anti-neutrino. I like muons, too. My favorite element is hydrogen.
What hobbies and interests do you have?
I’m afraid I’ll disappoint you because my hobby is related to my research: Augmented reality. When I heard about something called Microsoft Hololens, I thought, I could make IceCube a hologram. I bought these special glasses, and then made a program on it and used it for some outreach events. But the glasses are very expensive, so people said, “Okay we can’t buy hologram glasses.” So I moved it to mobile phones so that everyone could look at it for fun. It’s called IceCubeAR (note: download it for iPhones or Android phones). I made it with a group of friends in Tokyo.
