How NU prof Wen-fai Fong solved the universe’s biggest mysteries

Professor Wen-fai Fong stands in her office at Northwestern University’s Evanston campus. Photo courtesy of Christian Elliott.

Sitting in her top-floor office overlooking Lake Michigan in Evanston, Wen-fai Fong pulls out her phone and reads a cryptic text: “(Swift1075565) 13:09:36.41, 48:37:42.24; Err: 0.06 arcmin; Age: 4m; l, b: 115,+68.2.”

Meaningless spam? Negative. To Fong, an assistant professor of astrophysics and astronomy at Northwestern, it’s an information-dense message that requires her to drop everything.

The text alert from NASA’s Swift satellite means the orbital observatory detected a “gamma ray burst” – an explosion of radiation from a distant galaxy.

She quickly scans the text, which includes a “trigger number” that identifies the burst, celestial coordinates that indicate its origin in the sky, the precision of the detection in arc minutes (a unit of measurement for very small angles) and the age of the burst.

“You can’t see [a gamma ray burst] without a telescope, because it’s coming from very far away,” Fong said. “But it’s like a single new star in the sky.” The burst is incredibly bright to instruments that can detect invisible gamma radiation.

“If you had gamma ray eyes, it would actually light up the sky,” Fong said.

Studying gamma rays allows Fong’s lab at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) to answer some of science’s biggest questions about the universe’s origins. As these fleeting bursts of radiation reach Earth, astronomers at the right place and time can glean information about the origin of the universe’s heavy elements.

“It’s a fundamental question for humankind to know where the metals in your watch or car or computer came from,” Fong said. “In astronomy, if we understand when heavy elements are created in the universe, we can start to understand how stars and galaxies are formed over cosmic time and the chemical enrichment of the universe as a whole.”

Just five years ago, gamma ray astronomers made a major discovery: they traced the bursts to neutron star collisions. Neutron stars are the dense, massive cores of dead stars that reached the end of their billion-year lifespans and died in an explosive supernova.

When orbiting neutron stars collide, pressures and concentrations of neutrons are so high that the heaviest elements in the universe can form – silver, gold, platinum and uranium. Neutron star mergers may be the only way those elements are produced in the universe, and gamma ray bursts are a critical way scientists can learn more about them.

Fong is specifically interested in short bursts of highly energetic gamma radiation lasting two seconds or less. When detectable bursts happen – about ten times per year – Fong’s seven-person core team of postdoctoral fellows and graduate students races to a room of computer monitors at CIERA to trigger observations.

That means remotely commandeering mountaintop telescopes like the Multiple Mirror Telescope Observatory in Arizona and directing operators there so they can capture ancient bursts of energy arriving at precisely that moment. It’s a time of what Fong calls “frenzied activity” at the lab.

“It would be really nice if we had an early warning system,” Fong said. “These things have been traveling for billions of years to reach us and then suddenly we have no notice.”

The gamma ray mystery

When Fong began a Ph.D. program in astronomy and astrophysics at Harvard University a decade ago, no one knew what caused gamma ray bursts. That mystery drew her to the study of explosive transients, which are fleeting bursts of radiation from “anything that explodes, or collides, or does crazy things in the universe, and things that are cataclysmic,” Fong said.

“Point me to an explosion, and I want to find out what’s causing it,” she added.

US military nuclear war detection satellites controlled by the Los Alamos National Laboratory in New Mexico first detected the gamma ray bursts in 1960, but it wasn’t until 2017 – when Fong was a Hubble postdoctoral fellow at Northwestern – that a new facility at Caltech solved the mystery of their origin.

It detected gravitational waves, ripples in space and time produced by two neutron stars colliding, in a distant galaxy. Two seconds later, the Fermi Gamma-ray Space Telescope saw a gamma ray burst from its perch in low Earth orbit – conclusive evidence that those bursts result from kilonovae, or neutron star mergers. But mysteries remain.

“We still don’t know much about what causes two neutron stars to merge and what types of environments they come from,” said Fong. “Every time we see a gamma ray burst, we’re learning more about this special type of system.”

After a short gamma ray burst quickly dissipates, Fong’s team observes the burst’s afterglow – other wavelengths of electromagnetic radiation (x-rays, radio waves or visible light) that reach Earth in the following hours or days. They’re given on-demand “target of opportunity” access to distant telescopes through allocation committees that choose who gets access to them when – it’s a scarce resource.

The challenge is getting as much data from as many wavelengths as quickly as possible, depending on what telescopes are available.

The next step: galaxy forensics. Fong and her student detectives mine the data for clues to understand the ages of the neutron stars, their host galaxies and nearby stars. They rely on infrared spectroscopy, a technique that splits incoming radiation into a spectrum of its component wavelengths. When chartered, the wavelengths correspond precisely to different elements. Using the chartered wavelengths, scientists can prove the compositions of heavy elements neutron star collisions produce.

A more diverse future for astronomy

Fong grew up in a science-focused family in Rochester, New York. Her father, a doctor, and her mother, a manager at Xerox, emigrated from Hong Kong in the 1970s. One of her older sisters is a doctor, and the other is a neuroscientist.

“I feel like I’m kind of the dark horse of the family because of astronomy,” Fong said.

Growing up in the 1990s with a mother who worked long hours at a tech company and traveled frequently shaped her childhood aspirations.

In eighth grade, she was assigned an astronomy science project mapping the moon’s phases for a month. She fondly remembers sitting on her driveway each night looking up at the stars and sketching the moon in her notebook.

In high school and in college at the Massachusetts Institute of Technology, she gravitated toward biology and physics, then decided she wanted to pursue a Ph.D. in astronomy. After two postdoctoral positions, she landed her current job at Northwestern.

Among astronomy labs, CIERA’s makeup is rare – Fong’s core researchers are all women.

“Seeing a supportive group of fantastic women researchers is probably exciting,” Fong said to prospective grad students. “It’s a feedback loop I think.”

She notes her first astronomy mentor was also a woman. Still, she acknowledges women remain underrepresented in the field. According to a decade report released last November, at the Ph.D. level, only 30% of recently hired assistant professors are female today, up from 20% in 2003. Asian Americans, like Fong, make up just 5% of all astronomy faculty.

“I try to build this collaborative environment because students aren’t going to want to get up in the middle of the night unless you lead by example, unless you’re there with them,” Fong said.

In college at the University of Connecticut, CIERA graduate student Jillian Rastinejad only had male physics professors.

“My first two years I really struggled,” she said. “A lot of it was imposter syndrome. My junior year, I took my first astronomy class and had my first female professor, and she made me believe that I could do it.”

Now, she’s a third-year Ph.D. student at CIERA.

This year, Rastinejad mentored a female high school senior in Evanston through CIERA’s Research Experiences in Astronomy program. She taught the student coding and astronomy basics – things Rastinejad notes she didn’t get to study at that age. She also launched a virtual conference called Data Science for Public Good for 40 high schoolers from around the world.

Anya Nugent, a fourth-year Ph.D. student at CIERA, remembers being one of just a couple women astronomers and physics students in college. Fong’s team was a major reason why she chose to study at Northwestern.

“Once you have a group of people who understand where you’re coming from and who have probably been through the same struggles you’ve been through, it really helps with the community feel and I feel more confident in my research,” Nugent said .

A new eye in the sky

After years of delays, on Dec. 25, 2021, NASA launched the $10 billion James Webb Space Telescope nearly one million miles from Earth. From its vantage point orbiting the sun, the new 70-foot-long telescope should be able to detect light from the universe’s first stars, some 14 billion years old.

When Fong worked as a postdoctoral fellow at the University of Arizona’s Steward Observatory in 2014, she knew members of the team building instruments for the project.

“As an astronomy community, I’ve never seen any other satellite that everyone is so looking forward to launching,” she said.

The James Webb telescope’s unprecedented sensitivity and near-infrared sensors will allow Fong’s team to see fainter gamma ray bursts for longer and improve infrared spectroscopy of the neutron star mergers, or kilonovae, that cause them.

Right now, identifying the elements kilonovae produce relies heavily on computer modeling. Fong calls it a “holy grail question” that the telescope could better answer for her field.

“James Webb is able to look very, very deep into the sky, to depths that even the Hubble Space Telescope isn’t able to achieve,” Rastinejad said. “It’ll allow us to see parts of kilonovae that we just weren’t able to observe before.”

Next, Fong will turn her detective lens toward a new mystery of fast radio bursts – less-understood repeating low-energy transient bursts on the opposite end of the electromagnetic spectrum from gamma rays. Current theories attribute these mysterious radio signals to single neutron stars, colliding black holes and even alien starship transmissions.

“Fast radio bursts are what gamma ray bursts were a decade ago,” Fong said.

The James Webb telescope will allow Fong’s team to better characterize fast radio bursts’ host galaxies and potentially solve the mystery of their origin.

She was also happy to see her field highlighted as a major direction for astronomy in the decadal survey.

“Time-domain astrophysics, phenomena which change in brightness over time like gamma ray bursts and fast radio bursts, are top priorities,” she said. “They also made recommendations to raise investigator grant funding through the NSF which would be great to support groups like mine. Overall, it’s a fantastic outcome for the field I am in.”

Thumbnail courtesy of Christian Elliott.

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