Viewfinders

When the ultra-dense cores of dead stars collide in some distant galaxy billions of miles away, Wen-fai Fong gets a text message.

Sometimes, the message comes from NASA’s Swift satellite, a telescope launched in 2004 designed specifically to track gamma-ray bursts. Other times, it’s from the Laser Interferometer Gravitational Wave Observatory (or LIGO), where high-powered lasers bounce down miles-long tunnels and collide, creating an “interference pattern” that can detect measurements 1/10,000th the size of a proton.

Whatever the source, it likely means that Fong — and those around the world who study transient phenomena that are included in an auto-generated group text — will be up all night, tracking the events. An astrophysicist and assistant professor at Northwestern, Fong studies these collisions and the gamma-ray bursts they emit to understand the forces that drive these celestial bodies. Those gamma-rays — “the highest energy form of light we can detect,” says Fong — may contain the secrets of the universe, from detecting the gravitational waves that bend and curve spacetime to understanding how dead stars form, feed, and grow.

“It's a very exciting time to start a PhD or to start research in this field,” Fong says.

It may be more exciting for Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), where Fong and her team study this phenomenon. Fong is one of a young group of researchers whose work has helped solidify CIERA as a world-leading hub for astrophysics.

Earlier this year, CIERA astrophysicist and assistant professor Raffaella Margutti presented her research on AT2018cow, otherwise known as “The Cow,” a bright anomaly that flared up 200 million light years away in the Hercules constellation that was captured by two radio telescopes in Hawaii. After compiling various imagining sources, Margutti and her team theorized that the telescopes had caught a star collapsing under its own gravity, transforming either into a neutron star or a black hole.

“There was instant excitement all over the world,” Margutti says. Her team collaborated with researchers across the globe to verify her findings. “Our access to the telescopes
gave us an edge.”

For her efforts, Margutti was named a 2019 Sloan Research Fellow by the Albert P. Sloan Foundation, which recognizes early-career researchers. Like Fong, Margutti tracks the electromagnetic spectrum (from gamma-rays to radio) to study the most violent events in our universe, including stellar explosions, black holes, and neutron star collisions, which generate the gravitational waves LIGO detects.

Margutti also studies supernova explosions, rare transients (like “The Cow”), and massive stellar eruptions. She considers the neutron star mergers that LIGO can now see as a prominent core to her research.

“I helped lead an effort that resulted in the characterization of the first neutron star merger seen with gravitational waves across the spectrum and I published several papers — one being the most cited paper in 2018 — about the event,” Margutti says.

Another CIERA assistant professor, Alexander "Sasha" Tchekhovskoy, focuses on revealing how black holes and neutron stars interact with their environment. They can devour each other, "spaghettify" (stretch) unlucky stars that wander too close, and enrich the Universe with heavy elements like gold and platinum. They manage to shoot out relativistic beams of gamma rays that allow us to peer billions of years into the past and watch the Universe when it was a toddler,  much younger than the young adult it is today.

Or, as he says, “I study how black holes eat their food, and the burp that comes next.”

It’s a little more involved than that. A computational astrophysicist, Tchekhovskoy develops computer simulations that reveal what happens “under the hood” of stellar collisions.
The simulations make the job of his observer colleagues easier by predicting what the telescopes would see. Using the simulations as a template can help observers better locate collapsing stars and interpret the observations.

Like the others, Tchekhovskoy’s work as been recognized. Last December, Tchekhovskoy received the 2019 Innovative and Novel Computational Impact on Theory and Experiment (INCITE) Award for his proposal, “Simulating Neutron Star Binary Merger Remnant Disks and Tilted Thin Disks.” But the award is more than a plaque; it grants him access to 850,000 node-hours with Summit, the world’s fastest supercomputer. Based in Oak Ridge, Tennessee, Summit can perform up to 200,000 trillion calculations per second.

“It is not trivial to convert codes to run black hole simulations on graphics processing units, a type of graphics card that is the heart of Summit. In fact, our group is the only one in the world who can use such graphics cards for black hole simulations right now,” Tchekhovskoy says. “We are no longer limited by computational resources, which allows us to ask bigger questions than ever before and tackle problems previously deemed too difficult to solve.”

Which makes the INCITE award that much more important to both Tchekhovskoy and CIERA. Tchekhovskoy said he plans to use the allocation to carry out high resolutions simulations of the neutron star binary mergers that both Fong and Margutti have observed in hopes of solving two mysteries that have challenged astrophysicists: how neutron star collisions produce heavy elements, and how black holes digest red-hot disks of gas.

Together, the three researchers are taking what’s known as a “multi-messenger” approach to astrophysics, the idea that combining different information from different types of signals will allow scientists to better understand how astrophysical systems evolve and change.

This kind of work can be seen in Margutti’s research on “The Cow.” Instead of studying the luminous phenomenon using optical telescopes, the team used x-rays, hard x-rays, gamma rays, and radio waves, then put them together to create a clearer picture.

Another technique allows researchers to see the previously unseen. “It’s what we call stacking. We add all of the exposures together to create an ultra-sensitive image,” says Fong, who was named a 2018 Kavli Fellow by the National Academy of Sciences. The fellowship is the society’s premiere way to distinguish young scientists. Fong uses the stacking technique in her research on neutron star collisions to “bring out the features in the galaxy and the location of the neutron star merger that we couldn’t see before.”

But that kind of collaboration is part of why Fong says she enjoys working at Northwestern. “One of the major benefits is the synergy with the other groups,” she says. “I’m an observational astronomer. I study these events and it’s amazing to be in the same department as other early career investigators who are world-class in their knowledge of observations and computations. Being able to collaborate with other faculty and their groups has been really fruitful for me as a young professor.”

The other fun thing? The telescope access, Fong says. CIERA recently signed contracts to secure institutional access to the Multiple Mirror Telescope in Arizona and the W. M. Keck Observatory in Hawaii.

Northwestern researchers, including Fong and Margutti, have special access to these observational facilities, while Margutti’s research team has also been successful in applying for time at the SoAR telescope in Chile.

When she can, Fong likes travel to Keck and spend the night staring at galaxies millions of light years away.  That is, when the sulfur and ash from the nearby volcanoes don’t get in the way.

“It can definitely affect the skies,” she says. “But being there and looking up; it’s amazing.”