LSU Researchers Help Explain What Powers the Universe’s Most Extreme Supernovae

Astronomers may have finally detected something scientists have sought for nearly two decades: high-energy gamma rays escaping from one of the brightest stellar explosions in the universe.

The discovery, made using NASA’s Fermi Gamma-ray Space Telescope, could provide scientists with a powerful new way to study superluminous supernovae — rare cosmic explosions capable of outshining ordinary supernovae dozens of times over.

“We still do not have a fully coherent picture of the physical mechanisms powering superluminous supernovae,” said Michela Negro, an assistant professor in LSU’s Department of Physics & Astronomy and member of the Fermi Large Area Telescope (LAT) Collaboration. “Detecting gamma rays from these events could help distinguish between competing theoretical models and clarify how energy is generated and transferred in the most luminous stellar explosions in the universe.”

Negro served as Analysis Coordinator of the Fermi-LAT Collaboration during the period when the study was initiated and carried out, helping coordinate observational and theoretical researchers involved in the project, including LSU astrophysicist Manos Chatzopoulos, a leading expert on theoretical models of superluminous supernovae.

Fermi continues to surprise us even after 18 years of continuous observations

Michela Negro, Assistant Professor of Physics & Astronomy

At the center of the discovery is a supernova known as SN 2017egm, which exploded about 440 million light-years from Earth — relatively nearby in cosmic terms. Scientists believe the explosion may have produced a magnetar, a rapidly spinning neutron star with an extraordinarily powerful magnetic field capable of injecting enormous amounts of energy into the expanding debris from the blast.

For years, theorists predicted that gamma rays generated deep inside these explosions would eventually escape once the expanding cloud of stellar debris thinned out enough to become transparent. Early on, however, that dense ejecta acts almost like a cosmic fog, trapping the radiation inside and converting it into lower-energy optical light before it can escape.

Detecting those gamma rays has proven extraordinarily difficult because the signal weakens dramatically over cosmic distances. 

“An intuitive analogy is to imagine looking at a lightbulb,” Negro explained. “As you move it farther away, the light appears dimmer because fewer photons reach your eyes. In this case, the superluminous supernova is the lightbulb, while Fermi plays the role of the eye, detecting gamma-ray photons instead of visible light.”

“These explosions are extremely rare, and nearby examples are even rarer,” Chatzopoulos said. “Until now, we lacked both a sufficiently nearby event and enough long-term gamma-ray observations to clearly identify such a signal.”

“What excites me most,” Chatzopoulos added, “is that we are finally beginning to directly observe the high-energy ‘engine room’ powering these extraordinary explosions rather than only the visible light they produce.”

We are finally beginning to directly observe the ‘engine room’ powering these extraordinary explosions rather than only the visible light they produce.

Manos Chatzopoulos, Associate Professor of Physics & Astronomy

One of the key questions surrounding these explosions is whether they are powered primarily by magnetars, by circumstellar material, or by some combination of both. “Gamma rays provide a fundamentally new way to probe the high-energy processes occurring deep inside the explosion and distinguish between these scenarios,” said Chatzopoulos.

Gamma rays also allow scientists to study particle acceleration, magnetic fields, shock physics, and other extreme processes that cannot be directly observed at lower energies.

“Detecting gamma rays from a super-luminous supernova opens a new way to study how nature produces some of the most energetic phenomena in the cosmos,” said Negro.

The findings also highlight the long-term scientific importance of gamma-ray astronomy and missions like Fermi, which has continuously monitored the high-energy sky for nearly two decades.

“Fermi continues to surprise us even after 18 years of continuous observations,” Negro said. “This detection opens a powerful new window into the physics of superluminous supernovae and highlights the importance of future high-sensitivity gamma-ray observatories.”

Researchers now hope future observatories, including the Cherenkov Telescope Array Observatory, will determine whether SN 2017egm is unusual — or simply the first known example of a much larger population of gamma-ray-emitting superluminous supernovae hidden throughout the universe.