University of Utah Department of Physics & Astronomy
Biannual Newsletter | Fall 2018 | Volume 7, Issue 1
Exploring Dark Matter Page 8
In This Issue
Message from the Chair
A Passion for Teaching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
As the fall semester comes to a close, it’s a perfect time to reflect on the accomplishments of the Physics & Astronomy Department in 2018.
Gamma Rays and the Origins of Lightning. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
I’m very pleased to report that we had a successful hiring season last spring. Two outstanding new faculty members recently joined us: Claudia De Grandi has been appointed our first Assistant Professor of Educational Practice, and Assistant Professor Yue Zhao specializes in high energy theoretical physics. We are delighted to welcome them to the U and to the Department. You can read more about their exciting work in the accompanying pages.
Solving a Cosmic Mystery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Perovskites Show Promise in Spintronics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Exploring Dark Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Physics & Astronomy Awards in 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 U Emeritus Faculty Receive Top Physics Prizes. . . . . . . . . . . . . . . . . . . . . . . . 11 Mountain-Top Observatory Sees Gamma Rays from Exotic Milky Way Object. . . 12
Our faculty members continue to win accolades for their work and collect some of the world’s most prestigious prizes. Emeritus Professor Bill Sutherland was awarded the 2019 Dannie Heineman Prize for Mathematical Physics, one of the nation’s highest awards for physics—seven previous winners have gone on to receive a Nobel Prize. Distinguished Professor Emeritus Alexei Efros received the 2019 Oliver E. Buckley Condensed Matter Prize from the American Physical Society. This prize is the highest honor for theoretical condensed matter physics, with seventeen previous winners going on to receive a Nobel Prize. See more about these prizes, as well as the remarkable work being done by our talented faculty, in the newsletter.
Peter E. Trapa
In October, the Department hosted the annual meeting of the Four Corners Section of the American Physical Society. The Four Corners Section, which includes Arizona, Colorado, New Mexico, and Utah, supports the work of physics students and provides them with opportunities to present their research and meet physicists in the region. More than 300 people attended the event, which featured approximately 250 presentations. We continue to encourage faculty research, recruit new talent, and provide a worldclass educational experience for undergraduate and graduate physics students. To help us advance our work, we hope you’ll consider supporting the Crimson Laureate Society, established by the College of Science. The goal of the Society is to build a community of alumni and friends who are passionate about the advancement of scientific research and education at the U. For more information on how to support the Physics & Astronomy Department through the Society, please contact the College of Science at 801-581-6958, or visit https://science.utah.edu/cls. As always, we are grateful for your support. Sincerely,
The Crocker Science Center is housed in the historic and newly renovated George Thomas Building. The center serves as a world-class facility for science education with state-of-the-art teaching laboratories and flexible classroom spaces. The building also has integrated advising and tutoring centers.
Peter E. Trapa Chair Department of Physics & Astronomy 1
A Passion for Teaching
Claudia De Grandi recently joined the U as an assistant professor lecturer in the Department of Physics & Astronomy. She first fell in love with physics in high school and was thrilled to discover the beauty of the connection between math and nature. Her initial curiosity to know and study everything matured into the desire to master the skills and tools needed to grasp the most advanced topics in condensed matter theory and their connections with the latest, most sophisticated experiments. Throughout her academic career, De Grandi’s research has been in very specialized areas of physics—studying cold atomic gases and superconducting qubits. Her focus has changed, however, and she now devotes her talents and energy to teaching physics—finding the best way to improve the quality of physics education by understanding the cognitive mechanisms that help or hinder learning and by developing pedagogical techniques to improve learning.
Claudia De Grandi
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“My new position at the U is called Professor of Educational Practice,” she said. “What I like most about it is that I will be teaching, which is something that I absolutely need to do to be happy! But I’ll also devote time to broader projects aimed at improving physics and STEM (Science, Technology, Engineering, and Math) education at different levels—from promoting interdisciplinary curriculum developments in the department to fostering the adoption of innovative teaching methods. My goal is to work with other faculty to make STEM an inclusive and welcoming environment for everyone.”
Connecting with Students
Roots in Italy
Part of De Grandi’s motivation for teaching physics is to use it as a way to connect with students. “I want to show each of them how much beauty there is in physics to discover and also empower them with quantitative skills they may not know they have.” De Grandi recognizes that it takes time to understand the needs and backgrounds of her students. “Getting to know them is always a new challenge that keeps me busy and excited. I’m always striving to improve my teaching by using ideas from cognitive sciences and psychology and experimenting with new approaches.”
De Grandi grew up in Milan, Italy, where she received her bachelor’s and master’s degrees in physics from the University of Milan. In 2011, she obtained her Ph.D. in theoretical condensed matter physics from Boston University. She was at Yale first as a research postdoc in Steven Girvin’s group, and she continued as a teaching postdoc through the Yale Center for Teaching and Learning. During this time, she was able to learn about the latest pedagogical methods while teaching Yale undergraduates, training faculty, and being involved in broader STEM projects. “I can’t imagine where I would be now without the training I received at Yale. My new position at the U relies strongly on the skills and expertise that I developed during my time there.”
For example, De Grandi has researched how classroom settings affect student learning. Do students learn differently sitting in groups around a table versus sitting in rows in a lecture hall? Her research shows how simply sitting around a table creates a sense of community and dramatically improves the student learning experience. She believes it’s also important that all students (regardless of gender orientation, non-gender orientation, or other dimensions of diversity) have the support and resources they need to succeed in class. “There’s still a lot of work to be done to make physics, and science in general, more accessible. We need to foster an environment that is more welcoming to women and other under-represented groups in STEM.” One of the ways she hopes to do this is by finding opportunities to partner with and support high schools and community college programs. She plans to become active in the U’s Center for Science and Mathematics Education, which already embraces many of these initiatives.
De Grandi has biked everywhere she’s ever lived, and she looks forward to continuing this tradition in Salt Lake City. She will certainly devote time to hiking and exploring the outdoors in Utah. De Grandi has an engaging hobby: exploring modern architecture as well as old industrial buildings. “I’m very excited to get started as a new faculty member at the U,” she said. “I’m looking forward to meeting the students and getting to know and be part of the U and the Salt Lake community.”
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Solving a Cosmic Mystery Daniel Wik, assistant professor of physics and astronomy, helped conclude a study using data from NASA’s NuSTAR space telescope to confirm that Eta Carinae, the most luminous and massive stellar system within 10,000 lightyears, is accelerating particles to ultra-high energies. Some of the particles could reach Earth as cosmic rays. John Belz
Gamma Rays and the Origins of Lightning John Belz, associate professor of physics & astronomy, first became interested in cosmic rays in the late 1990s. “There was an interesting, unsolved problem at the time,” said Belz. “Cosmic rays were observed with energies greater than predicted— something we hadn’t expected to see. Eventually the problem was resolved by Utah’s High Resolution Fly’s Eye detector.” The High Resolution Fly’s Eye or “HiRes” detector was an ultra-high-energy cosmic ray observatory located in the west desert of Utah from 1997-2006.
TA near Delta, Utah Today the Telescope Array Surface Detector (TA), a 700-square-kilometer observatory in Utah’s west desert, has replaced HiRes and detects high-energy particles that constantly collide with the Earth’s atmosphere from space. Belz serves as principal investigator of the National Science Foundationfunded Telescope Array Lightning Project, which uses data from the TA as well as a set of lightning detection instruments. His research focuses on lightning and gamma rays—the highest energy light waves on the electromagnetic spectrum—and he and his colleagues are trying to understand the mechanism by which the flash in lightning is initiated. Until 1994, satellites in the Earth’s orbit were expected to detect gamma radiation from major celestial events
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such as exploding stars. Then it was discovered that lightning could produce bursts of gamma rays, known as Terrestrial Gamma Flashes (TGFs), lasting about a millisecond and directed up towards space. In 2013, TA physicists detected the downward-directed counterpart of TGFs in which the radiation is beamed towards the Earth’s surface. This discovery has provided new information and additional questions about how, where, and when the lightning flash begins.
New Detectors “As common as atmospheric lightning is,” said Belz, “we don’t completely understand how it works. The practical application of our research is that it may help us better understand how gamma rays are produced and the nature of lightning and the lightning initiation process.” This past summer the team deployed new detectors at the TA site, including a radio interferometer, that will help them see in greater detail how the gamma rays are produced at the beginning of the flash. After obtaining a Ph.D. from Temple University, Belz did postdoctoral work at Rutgers University and held faculty positions at Montana State and the University of Montana. He joined the U’s Physics & Astronomy department in 2005 as a visiting assistant professor and became an associate professor in 2014.
“The key to accurately measuring Eta Carinae’s X-rays and identifying the star system as the gamma ray source—and thus proving that the colliding winds of this binary system are accelerating cosmic rays—was to fully characterize NuSTAR’s background,” said Wik. Wik previously developed a multi-component background model for the NuSTAR mission, but Eta Carinae’s location in the plane of the Milky Way caused the background of NuSTAR’s nine separate observations to be more complicated than usual. He helped identify additional sources of background and how to account for them, allowing the link between Eta Carinae’s X-ray and gamma ray emissions to become clear. Eta Carinae, located about 7,500 light-years away, contains a pair of massive stars whose eccentric orbits bring them unusually close to Earth every 5.5 years. The two stars contain 90- and 30-times the mass of our Sun, and they pass 140 million miles apart at their closest approach—about the average distance between Mars and the Sun. Adapted from an original release written by Francis Reddy of NASA’s Goddard Space Flight Center, available here: https://go.nasa.gov/2knv825.
Daniel Wik
Eta Carinae’s great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble. Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our Sun. Astronomers cannot yet explain what caused this eruption. Photo credit: NASA, ESA, and the Hubble SM4 ERO Team.
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Perovskites Show Promise in Spintronics Sarah Li joined the University of Utah five years ago as an assistant professor of physics. Her research focuses on using optics to help understand spindependent electronic and magnetic phenomena in semiconductor materials. “My research is fundamental physics—to study how electron spins behave in materials,” said Li. “The research is called spintronics, short for spin electronics, and we use the electron spin to carry information.” Electrons are basically tiny magnets that orbit the nucleus of an element, and they have their own spin orientation relative to the nucleus. “The spin goes two ways—spin up or spin down,” said Li, “and a spintronic device is expected to process more data with less energy consumption than traditional electronics.” As Li and her team study how to manipulate electron spins in either magnetic materials or semiconductors, they hope to discover new ways to store and process information. Their work has practical applications in developing things like quantum computers, with faster speeds that consume less energy and have greater functionality.
Understanding Perovskites In the last few years, Li and her team have focused on understanding the spin-dependent properties of a new type of semiconductor, hybrid organicinorganic perovskites, which are crystalized materials with organic molecules in the structure. Perovskites contain excellent semiconductor properties such as high optical absorption and good conductivity, which make them ideal for use in solar cells, LEDs, transistors, sensors, lasers, and other devices. Li and her team are among the first scientists to show the
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potential of organic-inorganic hybrid perovskites in spintronics. Their experiment began with the researchers using chemical precursor solutions to form a hybrid perovskite thin film, which they placed in front of an ultrafast laser capable of shooting short light pulses of 1 over 10 trillion-seconds duration, 80 million times in a second. The laser was then split into two beams: the first beam was aimed at the film to set the electron spinning in a particular direction. The second beam was bent through a series of mirrors, much like a pinball machine, before hitting the perovskite film at increasing intervals to measure how long the electron held its spin orientation.
Applications for Semiconductors “The results for the hybrid organic-inorganic perovskites were significant,” said Li. “The experiment showed the potential application for them based on spin that was different and distinct from using perovskites for solar cells and LEDs. It was exciting because the hybrid perovskite is truly a promising semiconductor candidate for making spin transistors.” Born and raised in China, Li became interested in physics at age nine or ten when she happened to read some books that belonged to her cousins. “I couldn’t stop reading—physics opened up a whole new world for me, and I found explanations for many phenomena.” Before high school, Li had limited access to physics textbooks, but by eighth grade, she was studying physics in school.
Sarah Li
She attended the University of Science and Technology of China because of its reputation for excellence in research science. “When I began college in the Department of Materials Science and Engineering, my specialty area was materials physics; however, I took many courses in both physics and chemistry.” Before Li entered the university, she planned a five-year program; however, the administration decided to shorten the program to four years. The course work was heavy, with few electives. “My college years were very busy,” said Li. “I received a solid foundation in physics and chemistry, but I didn’t have much of a college or social life.” She received her Ph.D. from the University of California, Riverside, and was involved with a research group in building machine parts, setups, testing, and debugging. “I really enjoyed my years at Riverside,” said Li. “I learned a lot about building
infrastructure, which helped me in building my own lab at the U. I also enjoyed the freedom I had at Riverside to choose the kind of work I wanted to do.” When she isn’t in the lab or teaching, Li and her husband and their young son spend time at museums and playgrounds. “I also enjoy crafts and doing things like cooking, sewing, and watercolor painting, but with a young child I don’t have much time for my hobbies. I do make time to hike because it boosts my energy.” As Li moves forward, she wants her research to continue to be systematic and thorough. Her team’s recent results have opened up new opportunities but also brought up questions, too. “This field of research is still young, and my goal is to develop an understanding of it as completely and deeply as I can. I hope to continue working on spin-device applications that will make a difference.”
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Existence of Dark Matter
Exploring Dark Matter Particle physics is a discipline within the field that studies the nature of the smallest detectable particles that make up matter and radiation. The Standard Model is the theory that explains what these particles are and how they interact with each other, and it was developed by scientists during the 1970s. While the Standard Model explains a lot about the laws of physics, it isn’t able to explain all phenomena, which is why it can’t be considered a complete theory of fundamental interactions. For example, the Standard Model can’t account for dark matter.
Yue Zhao
“Roughly 80 percent of the matter of the universe is made up of material that we can’t directly observe and can’t be described by any particles within the framework of the Standard Model,” said Yue Zhao, assistant professor of physics, who recently joined the University of Utah in July. “This mysterious missing component is known as dark matter, which neither interacts strongly with light nor atoms. Exploring the properties of dark matter is one of my research interests.”
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If we can’t actually see dark matter, how do we know it exists? Astrophysicists can calculate the mass distribution in large astrophysical systems in space such as our galaxy by studying the motion of stars. When scientists began studying spiral galaxies nearly 70 years ago, they expected to see stars near the center of a galaxy moving faster than at the outer edges. Instead, they found that stars both near the center and in outer areas traveled approximately at the same speed, which told them that galaxies contained more mass than could be seen. Additionally, massive objects in the universe bend and distort light, allowing them to act as a lens. By observing how galaxy clusters distort light, astronomers have been able to create a dark matter map in galaxy clusters. In the Standard Model, the familiar matter of the universe is composed of protons and neutrons, called baryonic matter. Most scientists think that dark matter is composed of non-baryonic matter, but their weak interactions with normal matter make them difficult to detect. “One of the things I love about studying dark matter is that it requires creativity,” said Zhao. “Some scientists have spent decades looking for a particular type of dark matter particle without much success. If you’re studying dark matter, you have to keep an open mind and find new methods to look for it.”
LIGO Collaboration One method that interests Zhao is the use of a gravitational wave detector, such as the detectors used at the Laser Interferometer GravitationalWave Observatories (LIGO), located in Hanford, Washington, and Livingston, Louisiana. The LIGO Scientific Collaboration is a group of scientists focused on detecting gravitational waves and using them to explore the physics of gravity. The group
seeks to develop gravitational wave science as a tool to understand astronomy. “While a gravitational wave detector is designed to look for gravitational waves, it also serves as a perfect detector to help find a particular type of dark matter,” said Zhao. “We may find dark matter by using LIGO and by implementing a new data analysis strategy.” Zhao’s love of physics began when he read A Brief History of Time: From the Big Bang to Black Holes by Stephen Hawking. “This is such a great book, and it explains so many deep and fantastic physics concepts with language that can easily be understood by a non-scientist,” said Zhao. “The laws of nature are beautifully described in the book, and I must have reread some of these sections at least five times.”
Moving to the U.S. Although Zhao learned some physics as a child, he didn’t begin studying it formally until he was a teenager. Later, he tackled advanced physics at Peking University and moved to Rutgers University to pursue a Ph.D. “When I first arrived at Rutgers, I didn’t really have a clear idea of what I wanted to do,” said Zhao. “I started working with my thesis advisor by accident, and I was lucky that my research interests and his matched perfectly.” As a newcomer to Utah, Zhao is impressed with its natural beauty. He likes hiking and is looking forward to visiting all of Utah’s national parks. He also wants to get better at skiing. He credits his colleagues with making him feel welcome at the U, and he’s taken advantage of the resources available on campus, especially the services for non-native speakers. As Zhao moves forward in his research, he hopes to build his own LIGO team at the U. “Having a world- class dark matter team to collaborate with LIGO and develop new ideas would be a dream come true,” he said.
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Physics & Astronomy Awards in 2018 The following awards were given in May 2018.
Undergraduate Students
Graduate Students
Departmental Scholarship—for excellent academic achievements and high-quality research Joshua Bromley Connor Houghton Maile Marriott Kaitlin McLean
Diversity Scholarship—for students demonstrating exceptional commitment to the advancement of women and underrepresented groups Flo Doval Neda Lotfizadeh
Paul Gilbert Outstanding Undergraduate Research— for outstanding research in the areas of cosmic ray, astronomy and astrophysics, or high energy physics research Teddy Anderson Martin Hiatt Outstanding Undergraduate Research—for outstanding undergraduate research Alisa Mann David and Karen Imig Undergraduate Scholarship—for exceptional women Rylee Cardon Outstanding Undergraduate Sophomore Jonah Barber Outstanding Undergraduate Junior Robert Stahulak Outstanding Undergraduate Senior Caleb Webb Thomas J. Parmley Scholarship—for remarkable undergraduates Jade Aderibigbe Megan Schnedar Tyler Soelberg Memorial—for a senior-level physics major who demonstrates exceptional performance in science coursework and creative ventures Jade Aderibigbe Preston J. and Phyllis R. Taylor Endowed Scholarship—for undergraduates who have overcome obstacles on the way to strong academic achievements Elom Amematsro Walter W. Wada Endowed Scholarship—for exceptional undergraduates demonstrating a clear passion for physics or astronomy Galen Bergsten Diversity Scholarship—for students demonstrating exceptional commitment to the advancement of women and underrepresented groups Elom Amematsro
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J. Irvin and Norma K. Swigart Endowed Graduate Scholarship—for remarkable graduates Megha Agarwal Rajesh Malla Ye Tian Outstanding Graduate Students—for demonstrated success in graduate-level coursework and research Xiaojie Liu Zhuxi Luo Outstanding Teaching Assistants—for exceptional fulfillment of teaching duties Nathan Gundlach Jason May
Postdoctoral Researcher Outstanding Postdoctoral Research—for important contributions to the department’s research efforts Hao-Liang Liu
Faculty Awards Students’ Choice—for best faculty presentation at the Undergraduate Seminar Inese Ivans—Fall 2017 Christoph Boehme—Spring 2018 Richard Ingebretsen—Spring 2018
College of Science Scholarships and Awards Barry M. Goldwater Scholarship and Dean’s Scholarship Cameron Owen Crocker Science House Residential Living Opportunity Mario Homer Benjamin Powell Eugene Loh Fly’s Eye Cosmic Ray Scholarship Kieran Smout
U Emeritus Faculty Receive Top Physics Prizes Oliver E. Buckley Condensed Matter Prize
Dannie Heineman Prize for Mathematical Physics
Distinguished emeritus professor Alexei Efros has received the 2019 Oliver E. Buckley Condensed Matter Prize by the American Physical Society. The award recognizes outstanding theoretical or experimental contributions to condensed matter physics and is named Alexei Efros in honor of Oliver Ellsworth Buckley, a former president of Bell Labs. Efros shares the award with colleague Boris Shklovskii, a theoretical physicist at the William I. Fine Theoretical Physics Institute at the University of Minnesota, and Elihu Abrahams, distinguished adjunct professor of physics and astronomy at UCLA. The prize is the highest honor for theoretical condensed matter physics. Seventeen previous winners have gone one to receive the Nobel Prize.
Bill Sutherland, emeritus professor of physics at the University of Utah, was awarded the 2019 Dannie Heineman Prize for Mathematical Physics, which he will share with Professor Michel Gaudin at the Institute for Advanced Study, Princeton, Bill Sutherland NJ, and Francesco Calogero, professor of physics at Sapienza University of Rome. The prize was established by the Heineman Foundation and is jointly administered by the American Institute of Physics and the American Physical Society. It is one of the nation’s highest awards for physics—seven previous winners have gone on to receive the Nobel Prize.
Born in Leningrad, Russia, now St. Petersburg, Efros obtained a master’s degree from the Leningrad Polytechnic Institute in 1961 and a Ph.D. in physics a year later from the Ioffe Physico-Technical Institute. In 1986, he received the Landau Prize in theoretical physics from the Soviet Academy of Sciences. He emigrated to the U.S. in 1989 as a visiting professor and distinguished scholar at the University of California, Riverside. He moved to the University of Utah in 1991 and became a distinguished professor in 1994. The Buckley award states: “For pioneering research in the physics of disordered materials and hopping conductivity.” The main achievements of the Efros–Shklovskii collaboration are the theory of the hopping conductivity of semiconductors, based upon the percolation approach and the prediction of the Coulomb Gap in the electronic density of states.
The citation for the 2019 prize says: “For profound contributions to the field of exactly solvable models in statistical mechanics and many body physics, in particular the construction of the widely studied Gaudin magnet and the Calogero-Sutherland, ShastrySutherland, and Calogero-Moser models.” Sutherland’s research has focused on the connections between statistical mechanics and quantum mechanics. He was able to build upon Francesco Calogero’s work and find a solution at finite density, known as the CalogeroSutherland model. Born and raised in Marshall, Missouri, Sutherland was the first to attend college in his family. He received his bachelor’s degree from Washington University and a Ph.D. from the State University of New York at Stony Brook in 1968. Sutherland joined the University of Utah in 1971 as an assistant professor and became a professor in 1982. He served on the faculty of the U’s Physics & Astronomy Department until 2004, when he retired and became emeritus professor.
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Mountain-Top Observatory Sees Gamma Rays from Exotic Milky Way Object Space Jets Accelerate Particles and send a High Energy Signal to Earth The night sky seems serene, but telescopes tell us that the universe is filled with collisions and explosions. Distant, violent events signal their presence by spewing light and particles in all directions. When these messengers reach Earth, scientists can use them to map out the actionpacked sky, helping to better understand the volatile processes happening deep within space.
Anushka Udara Abeysekara
For the first time, an international collaboration of scientists, including physicists from the University of Utah, has detected highly energetic light coming from the outermost regions of an unusual star system within our own galaxy. The source is a microquasar—a black hole that gobbles up matter from a nearby companion star and blasts out powerful jets of material. The team’s observations, described in the October 4, 2018, issue of the journal Nature, strongly suggest that electron acceleration and collisions at the ends of the microquasar’s jets produced the powerful gamma rays. Scientists think that studying messengers from this microquasar may offer a glimpse into more extreme events happening at the centers of distant galaxies. The team gathered data from the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), which is a detector designed to look at gamma ray emissions coming from astronomical objects such as pulsar wind nebula, supernova remnants, and blazars. Now, the team has studied one of the most well-known microquasars, named SS 433, which is about 15,000 light years away. “There are a lot of microquasars in our Milky Way galaxy, but this is the first and only one that we have detected emitting very-high-energy gamma rays,” said Anushka Udara Abeysekara, research assistant professor in the Department of Physics
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& Astronomy at the University of Utah. “To understand the system in detail, we used multiple wavelengths of light, such as X-rays and radiowaves, to piece together the bigger picture.” Quasars are massive black holes that suck in material from the centers of galaxies. SS 433 is a micro-version of a quasar, located inside our own galaxy. The system consists of an object, either a black hole or neutron star, sucking in matter from a single unstable star. The system sprays accelerated particles along the opposite sides of the system, creating two jets. The jets smash into the interstellar medium around the two objects and produce the high-energy gamma rays. A separate collaboration focused on gamma rays, called VERITAS, of which Abeysekara and Dave Kieda, dean of the U’s graduate school, are affiliated, made more than 70 hours of observations of SS 433 but did not detect a strong enough signal to make a definitive claim. HAWC’s measurements cleared the threshold for scientists to claim that they had truly observed gamma rays. In July, Kieda, Abeysekara and the VERITAS array published a paper confirming high-energy gamma ray emissions from a supermassive black hole located in a distant galaxy. “SS 433 is right in our neighborhood, and so, using HAWC’s unique wide field of view, we were able to resolve both microquasar particle acceleration sites,” said Jordan Goodman, a distinguished professor at the University of Maryland and United States lead investigator and spokesperson for the HAWC collaboration. Wherever they originate, gamma rays travel in a straight line to their destination. The ones that arrive at Earth collide with molecules in our
The team gathered data from the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), near Puebla, Mexico. The detector is designed to look at gamma ray emissions coming from astronomical objects. Photo credit: Jordan Goodman atmosphere, creating new particles. Each particle then creates more material, resulting in a particle shower as the signal cascades toward the ground. HAWC, located roughly 13,500 feet above sea level near the Sierra Negra volcano in Mexico, is perfectly situated to catch the fast-moving rain of particles. The detector is composed of 300 tanks of water, each of which is about 24 feet in diameter, and collects data 24 hours a day, seven days a week. When the particles strike the water, they produce a shock wave of blue light, called Cherenkov radiation. Special cameras in the tanks detect this light, allowing scientists to compile the origin story of the gamma rays. Kieda, Abeysekara, co-author Robert Wayne Springer, and two graduate students, Ahron Barber and co-author Michael Newbold in the Department of Physics & Astronomy at the U, have been with the HAWC collaboration from its beginnings. Abeysekara designed and developed the GPS timing and central control system of the instrument that ensure all parts of the instrument work nearly simultaneously. “We can count how fast particles in a shower hit the sensors, and based on that timing, we can get the angle, the size, and the energy of the gamma rays. Then, we can trace it back to its origin in the universe,” said Abeysekara. “All of the data take
up several hard disks each day. The fastest way to transfer data is literally by the truckload. One of our collaborators at the National Autonomous University of Mexico packs his truck with crates full of hard disks down the mountain to Mexico City.” The HAWC collaboration examined data taken over 1,017 days and saw evidence that gamma rays were coming from the ends of the microquasar’s jets, rather than the central part of the star system. Based on their analysis, the researchers concluded that electrons in the jets attain energies that are about 1,000 times higher than can be achieved using Earth-bound particle accelerators, such as the city-sized Large Hadron Collider. The jet electrons collide with the low-energy microwave background radiation that permeates space, resulting in gamma ray emission. “The two jets of SS 433 are the most powerful jets ever observed in our galaxy. That makes them a brighter gamma-ray source compared with other microquasars,” said Abeysekara. “We need to keep observing the sky and increasing our sensitivity to find more microquasars similar to SS 433, if they exist, to improve our understanding of the jets.” Adapted from a press release prepared by the University of Maryland.
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