Highest-energy Cosmic Rays Detected in Star Clusters

a false-colored and abstract-looking image of star clusters

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.

For the full news story, please visit https://www.mtu.edu/news/stories/2021/march/not-so-fast-supernova-highestenergy-cosmic-rays-detected-in-star-clusters.html.


Welcome, Professor Ke Fang!

profile photo of Ke Fang

By Madeleine O’Keefe, WIPAC

When you think of scientific meccas throughout the world, Madison, Wisconsin might not be the first place that comes to mind. But for astroparticle physicist Ke Fang, Madison is the place to be. That’s because it’s home to the Wisconsin IceCube Particle Astrophysics Center (WIPAC): the “leader of particle astrophysics in the world,” according to Fang. “Throughout the years, there have been all kinds of meetings and workshops that drive people in this field to Madison because it’s the center for particle astrophysics,” she says.

Ke Fang

Originally from Huangshan, China, Fang earned a B.S. in physics from the University of Science and Technology of China. Afterward, Fang moved to the United States for graduate school and earned her PhD in astrophysics from the University of Chicago in 2015. Following that, she went to the University of Maryland and the Goddard Space Flight Center for a Joint Space-Science Institute fellowship. Most recently, Fang was a NASA Einstein Fellow at Stanford University in California.

Now, Fang has joined WIPAC and the UW–Madison Physics Department as an assistant professor. To welcome Fang and learn more about her, we met up on—where else?—Zoom for an interview.

 Can you summarize your research? 

I use both experiments and theory to understand extreme activities of our universe. We receive multiple types of messengers from the universe—all the way from optical light to gamma rays, cosmic rays, neutrinos, and gravitational waves. These messengers can be emitted by a common source, such as a binary neutron star merger. Specifically, I use theoretical models to understand how these astrophysical events produce different messengers, whether theoretical models explain the data, and how the data compare with theoretical models. I also use the HAWC Observatory, the IceCube Neutrino Observatory, and the Fermi Large Area Telescope (Fermi-LAT) to observe or to find sources directly. For example, I jointly analyze the Fermi-LAT and HAWC data to observe gamma-ray sources from 0.1 GeV to 100 TeV—across six orders of magnitude. Studies using multiple messengers and wavelengths are rewarding because they help us get a full picture of what these astrophysical sources look like.

How did you get into your field of research?

When I started graduate school, high-energy astrophysics was rather new; it’s a field that has quickly grown in the past decade or so. High-energy astrophysics traditionally refers to astrophysics with X-ray observations, because X-rays are higher in energy compared to the optical band that astronomers traditionally use. But in the last few years, high-energy astrophysics has had another burst of delving into even higher energies. When we move up in energy, by millions or billions, we see many new sources that were previously not observable in the X-ray band, or different aspects of sources that were previously seen at lower energies. And there are so many unknowns in this field; we can see surprising things at the highest energies, and many of those observations are discoveries. I think that’s really intriguing.

 What attracted you to UW–Madison and WIPAC? 

I think it’s pretty fair to say that WIPAC—with IceCube, CTA, HAWC, Fermi-LAT, ARA, and IceCube-Gen2—is now the leader of particle astrophysics in the world. I think there’s a close match between my expertise and what is currently being done at WIPAC, and I’m excited about joining the department and joining these explorations of higher and higher energy neutrinos and gamma rays.

What’s one thing you hope students who take a class with you will come away with?

The content you learn from a class is limited, but the contexts where you could apply the knowledge are unlimited.

 What is your favorite particle?

Neutrino. If I have to pick one, neutrino is the one that I have in my heart.

What hobbies/other interests do you have?

 Cooking! I like to explore different things. I come from China, so Chinese cuisine is what I started from when I just moved to the United States. But after all these years, I’m getting more exposed to different types of cuisines and starting to explore more, like with Thai and Italian. When I go to nice restaurants, I try to remember the name of the dish and find the recipe online.



Welcome, Professor Lu Lu!

Lu Lu

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.



IceCube Collaboration awarded 2021 Rossi Prize

The 2021 Bruno Rossi Prize was awarded to Francis Halzen and the IceCube Collaboration “for the discovery of a high-energy neutrino flux of astrophysical origin.”

The Bruno Rossi Prize is awarded annually by the High Energy Astrophysics Division of the American Astronomical Society. The 2021 HEAD awards were announced last night at the 237th AAS Meeting, which is being held virtually. Named after Italian experimental physicist Bruno Rossi—who made major contributions to particle physics and the study of cosmic rays, launched the field of X-ray astronomy, and discovered the first X-ray source, SCO X-1—the Rossi Prize is awarded “for a significant contribution to High Energy Astrophysics, with particular emphasis on recent, original work.”

The IceCube Collaboration is made up of over 300 researchers from 12 institutions in 53 countries. Halzen, the Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin–Madison, is the principal investigator of IceCube. The international group maintains and operates the IceCube Neutrino Observatory, a cubic kilometer of ice at the South Pole instrumented with optical sensors that can detect signals from high-energy neutrinos from outer space.

Read the full story at IceCube’s website

Ellen Zweibel elected AAAS Fellow

Congrats to Astronomy and Physics professor Ellen Zweibel on her election as a Fellow of the American Association for the Advancement of Science. She was elected “for distinguished contributions to quantify the role of magnetic fields in shaping the cosmos on all scales.” Read the full story about all six UW–Madison faculty who earned this honor.

New study provides understanding of astrophysical plasma dynamics

plasma from a sun-like star in the upper left corner is coming out like a string that swirls like a whirlpool around a dot in the center of the image

Stars, solar systems, and even entire galaxies form when astrophysical plasma — the flowing, molten mix of ions and electrons that makes up 99% of the universe — orbits around a dense object and attaches, or accretes, on to it. Physicists have developed models to explain the dynamics of this process, but in the absence of sending probes to developing stars, the experimental confirmation has been hard to come by.

In a study published in Physical Review Letters September 25, University of Wisconsin–Madison physicists recreated an astrophysical plasma in the lab, allowing them to investigate the plasma dynamics that explain the accretion disk formation. They found that electrons, not the momentum-carrying ions, dominate the magnetic field dynamics in less dense plasmas, a broad category that includes nearly all laboratory astrophysical plasma experiments.

plasma from a sun-like star in the upper left corner is coming out like a string that swirls like a whirlpool around a dot in the center of the image
An artist’s conception of the accretion disk | Credit: P. Marenfeld/NOAO/AURA/NSF

Like water swirling around and down an open drain, plasma in an accretion disk spins faster nearer the heavy object in the center than further away. As the plasma falls inward, it loses angular momentum. A basic physics principle says that angular momentum needs to be conserved, so the faster rotating plasma must be transferring its momentum away from the center.

“This is an outstanding problem in astrophysics — how does that angular momentum get transported in an accretion disk?” says Ken Flanagan, a postdoctoral researcher with the department of physics at UW–Madison and lead author of the study.

The simplest explanation is friction, but it was ruled out when the corresponding accretion times, in some cases, would be longer than the age of the universe. A model developed by theoretical physicists posits that turbulence, or the chaotic changes in plasma flow speeds, can explain the phenomenon on a more realistic time scale.

“So ad hoc, astrophysicists say, ‘Okay, there’s this much turbulence and that explains it,’” Flanagan says. “Which is good, but you need to call in the plasma physicists to piece together where that turbulence comes from.”

Flanagan and colleagues, including UW­–Madison physics professor Cary Forest, wanted to build off an idea that the turbulence was coming from an intrinsic property of some plasmas known as magnetorotational instability. This instability is seen in plasmas that are flowing fastest near the center and are in the presence of a weak magnetic field.

“And it’s lucky because there are weak magnetic fields all around the universe, and the flow profile in the accretion disks is set by the gravitational force,” Flanagan says. “So, we thought this plasma instability could be responsible for turbulence, and it explains how accretion disks work.”

To investigate if this intrinsic plasma instability explained the observation, the researchers turned to the Big Red Ball (BRB), a three-meter-wide hollow sphere with a 3000 magnets at its inner surface and various probes inside. They activate a plasma by ionizing gas inside the BRB, then applying a current to drive its movement.

a 3-meter-diameter sphere, painted red and with tons of probes all around it
The Big Red Ball is one of several pieces of scientific equipment being used to study the fundamental properties of plasma in order to better understand the universe, where the hot, ionized gas is abundant. | Photo by Jeff Miller / UW–Madison)

Because they had previously been encountering problems in driving very fast flows, they tried a new technique to drive the flow across the entire volume of plasma, as opposed to just the edges. Fortuitously, the BRB had magnetic field probes from a previous experiment still attached, and when they activated the plasma under these conditions, they found that this new flow setup amplified the magnetic field strength with a peak at the center nearly twenty times the baseline strength.

“We didn’t expect to see that at all, because usually in plasma physics the simplest model is to think of plasmas as one fluid with the heavier ions dominating momentum,” Flanagan says. “The results suggested that the plasma is in the Hall regime, which means the electrons and their motion are entirely responsible for the plasma moving around magnetic fields.”

If they were correct in assuming it was the Hall effect that was driving magnetic field amplification, the equations governing magnetic fields and electric currents say that if you drive the current in the opposite direction, the strength of the magnetic field would be canceled out. So, they switched the current and measured the magnetic field strength: it was zero, supporting the Hall regime explanation.

While the results are not directly applicable to the plasma accretion disks around, say, a very dense black hole, they do directly impact the earth-bound experiments that attempt to recreate and study them.

“Nearly all plasma astrophysical experiments operate in the Hall regime, and so this sort of large qualitative effect is something you’re going to have to pay attention to when you make these sorts of flows in laboratory astrophysical plasmas,” Flanagan says. “In that sense, this work has a pretty broad impact for lots of different research areas.”

This research was supported in part by the National Science Foundation (#1518115) and by the U.S. Department of Energy (#DE-SC0018266).

NSF Physics Frontier Center for neutron star modeling to include UW–Madison

A green, egg-shaped density in the middle has two cones of dark blue representing the gravitational waves projecting perpendicularly out either side of the green density

A group of universities, including the University of Wisconsin–Madison, has been named the newest Physics Frontier Center, the National Science Foundation announced Aug. 17. The center expands the reach and depth of existing capabilities in modeling some of the most violent events known in the universe: the mergers of neutron stars and their explosive aftermath.

The Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) is already an established hub of eight institutions, including UW–Madison, that uses the most extreme environments found in astrophysics — the Big Bang, supernovae, and neutron star and black hole mergers — as laboratories for testing fundamental physics under conditions beyond the reach of Earth-based labs. The upgrade to a Physics Frontier Center adds five institutions, provides $10.9 million in funding for postdoctoral fellowships and allows members to cover an expanded scope of research.

“For 20 years, we’ve expected that the growing precision of astrophysical and cosmological measurements would make this field an increasingly important part of fundamental physics. Indeed, four monumental discoveries — neutrino masses, dark matter, the accelerating universe, and gravitational waves — have confirmed this prediction,” says A. Baha Balantekin, a professor of physics at UW–Madison and one of the principal investigators for N3AS.

Read the full story 

Vandenbroucke group plays instrumental role in proving viability of innovative gamma-ray telescope

Scientists in the Cherenkov Telescope Array (CTA) consortium have detected gamma rays from the Crab Nebula using the prototype Schwarzschild-Couder Telescope (pSCT), proving the viability of the novel telescope design for use in gamma-ray astrophysics. The announcement was made today by Justin Vandenbroucke, associate professor at the University of Wisconsin–Madison, on behalf of the CTA Consortium at the virtual 236th meeting of the American Astronomical Society (AAS).

“The Crab Nebula is the brightest steady source of TeV, or very high-energy, gamma rays in the sky, so detecting it is an excellent way of proving the pSCT technology,” says Vandenbroucke, who is also affiliated with the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at UW–Madison. “Very high-energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.”

Vandenbroucke is coleader of a team made up of WIPAC scientists and other collaborators that developed and operate a critical part of the telescope: its high-speed camera. Vandenbroucke has worked on the design, construction, and integration of the camera since 2009.

Read the full story on the WIPAC website. The WIPAC story was adapted from a CTA press release.

Keith Bechtol, Rob Morgan win UW’s Cool Science Image contest

pieced-together photos of space with a helix nebula the most visible

pieced-together photos of space with a helix nebula the most visibleCongrats to Prof. Keith Bechtol and graduate student Rob Morgan for their winning entry in the UW–Madison Cool Science Images contest! Their winning entry — one of 12 selected out of 101 entries — earns them a large-format print which initially will be displayed in a gallery at the McPherson Eye Research Institute’s gallery in the WIMR building.

This snapshot of the sky contains thousands of distant galaxies, each containing billions of stars. Bechtol and Morgan were looking for the flash of the explosion of a single star, the potential source of a sub-atomic particle called a neutrino, spotted zipping through the Earth by the IceCube Neutrino Observatory at the South Pole. The distant galaxies, swirling billions of light years away, are all the harder to see because of nearby objects, like the pictured Helix Nebula. The image was captured with a Dark Energy Camera and Victor M. Blanco telescope.

To learn more about the Cool Science Images contest and to view the other winning images, please visit https://news.wisc.edu/the-winners-cool-science-images-2020/.


Profs Eriksson, McDermott, Vandenbroucke awarded UW2020s

image of research station at south pole plus a purely decorative image on the bottom half

Twelve projects have been chosen for Round 6 of the UW2020: WARF Discovery Initiative, including three from faculty in the Department of Physics (Mark Eriksson, Robert McDermott, and Justin Vandenbroucke). These projects were among 92 proposals submitted from across campus. The initiative is funded by the Office of the Vice Chancellor for Research and Graduate Education and the Wisconsin Alumni Research Foundation.

The projects were reviewed by faculty across the university. The UW2020 Council, a group of 17 faculty from all divisions of the university, evaluated the merits of each project based on the reviews and their potential for making significant contributions to their field of study.

The goal of UW2020 is to stimulate and support cutting-edge, highly innovative and groundbreaking research at UW–Madison and to support acquisition of shared instruments or equipment that will foster significant advances in research.

Acquisition of a cryogen-free Physical Properties Measurement System (PPMS) for characterization of quantum materials and devices

The project addresses a barrier for UW–Madison researchers in measuring electronic, magnetic, and thermal properties of quantum materials at low temperatures, namely the increasing high costs of cryogens (liquid helium) and lack of a convenient means to perform these measurements in a shared facility. Low-temperature electronic, magnetic, and thermal properties of materials are crucial for fundamental materials discovery and for applications in quantum information, nonvolatile memory, and energy conversion devices.

This project will acquire a cryogen-free Physical Properties Measurement System (PPMS) and house it as a shared-user facility instrument within the Wisconsin Centers for Nanotechnology (CNT). This instrument would be open for all UW–Madison users.

Currently, these measurements depend on external collaborations or low-temperature setups in PI labs which either consume large amounts of cryogens or require time-consuming reconfigurations from experiment to experiment. Having a cryogen-free PPMS would allow researchers to spend less time and money in setting up experiments, potentially freeing up resources for scientific investigations that include new superconducting and topological material discoveries and characterizations of materials for advanced microelectronics and magnetic memory systems.

Jason Kawasaki, assistant professor of materials science and engineering

Jerry Hunter, director of the Wisconsin Centers for Nanotechnology

Paul Voyles, professor of materials science and engineering and MRSEC Director

Song Jin, professor of chemistry

Mark Eriksson, professor of physics

Thomas Kuech, professor of chemical and biological engineering

Daniel Rhodes, assistant professor of materials science and engineering

Chang-Beom Eom, professor of materials science and engineering

Paul Evans, professor of materials science and engineering

Michael Arnold, professor of materials science and engineering

Dakotah Thompson, assistant professor of mechanical engineering

Cracking the structure of ice: establishing a cryogenic electron backscatter diffraction and Raman capability at UW–Madison

The structure and physical properties of ice determine the behavior of glaciers, ice sheets, and polar ice caps (both terrestrial and extraterrestrial). Moreover, ice is of interest because of its unique light transmission properties, which are currently being harnessed by one of the world’s largest astrophysical experiments through the UW–led IceCube collaboration.

This project will develop the capability to perform scanning electron microscopy (SEM) of water and CO2 ice in the UW–Madison Geoscience Department, focusing on electron backscatter diffraction (EBSD) analysis for ice microstructure and Raman spectroscopy for ice composition. EBSD of ice is an extremely rare analytical capability worldwide.

Having this highly specialized type of analysis capability for ice will enable advances in glaciology, climate science, physics, materials science and planetary science. This technology can accelerate research on glacial sliding and ice deformation, and inform long-standing questions about the transformation of air bubbles to clathrates in glacial ice and their potential as archives of Earth’s past atmosphere. In addition, understanding the structure of ice is critical, for example, to accurate measurement of cosmic ray interactions in the IceCube Neutrino Observatory.

As the only lab in the U.S. offering combined ice EBSD analysis and ice Raman analysis, UW–Madison will establish itself as a nexus for cryosphere research, attracting many collaborations from outside UW–Madison.

Chloe Bonamici, assistant professor of geoscience

Lucas Zoet, assistant professor of geoscience

Shaun Marcott, associate professor of geoscience

Justin Vandenbroucke, associate professor of physics/WIPAC

John Fournelle, senior scientist of geoscience

Pavana Prabhakar, assistant professor of civil and environmental engineering

Richard Hartel, professor of food engineering

Hiroki Sone, assistant professor of geological engineering

Interdisciplinary engineering of quantum information systems

This project represents a synergistic effort toward engineering practical quantum information systems (QIS). The research unites the experimental superconducting and semiconducting qubit teams on campus with advanced materials characterization and microwave engineering expertise to uncover the underlying sources of decoherence that limit qubit performance and develop next-generation quantum devices for scalable quantum computing and quantum sensing. This effort will build new interdisciplinary connections that nourish the quantum ecosystem at UW–Madison, cutting across departmental and disciplinary lines.

The potential of QIS has been recognized recently by the $1.4 billion federal National Quantum Initiative, and the newly formed Wisconsin Quantum Institute at UW is home to world-leading efforts in the physics of QIS. This project is a next step in expanding these directions to incorporate the engineering effort necessary to develop practical systems capable of solving real-world problems.

Robert McDermott, professor of physics

Mark Eriksson, professor of physics

Susan Hagness, professor of electrical and computer engineering

Paul Voyles, professor of materials science and engineering

Kangwook Lee, professor of electrical and computer engineering