Not all supernovae are alike. Some stars suddenly implode, while others explode following a core collapse. There are other known triggers, but all previously observed supernova pathways have something in common: they are deterministic, their evolution set in motion by the characteristics of the star alone.

But most massive stars have companions. There are many types of binary systems, including those where one or both of the pair is a neutron star or black hole. These are called compact binaries, and they can form stable orbits that degrade slowly, over millions or even billions of years, before finally merging. But what if that fateful meeting happens much more quickly, over only a few hundred years?

Sometimes, interaction with dense surrounding gas speeds up the orbital decay. In that case, Roger Chevalier (University of Virginia) theorized in 2012 that the compact object could cause its neighboring star to explode prematurely.

Over a relatively short period of time, the compact object spirals inward, its gravitational force causing the star to puff off its outer layers. As dense gases suffuse a large region around the star, they drag on the mutual orbit and accelerate the merger and ensuing supernova.

Now, for the first time, graduate student Dillon Dong and colleagues claim to have observed the phenomenon, publishing their results in the September 3rd Science.

A Slow Supernova?

It all started because Dong was looking for “orphan” gamma-ray bursts (GRBs). Most GRBs are the result of supernovae, when a high-mass star forms a neutron star or a black hole and explodes. The explosion begins with jets that tunnel through the star, usually visible only if pointed precisely at Earth.

But Dong wanted to find GRBs not pointed straight at us. This can be done indirectly; by observing the radio shockwave a GRB creates when it travels through gas surrounding an object. This is what Dong initially thought he saw in 2017, when the Very Large Array (VLA) Sky Survey detected a sudden burst of radio waves. But there was something different about this particular radio burst.

“It was extremely luminous — equal to the most luminous radio supernova ever recorded,” Dong explains. But follow-up observations using the Keck telescope in Hawai‘i showed that the shock wave was surprisingly slow. “This was very puzzling.”

For the shock wave to be traveling that slowly from such a luminous event, there would have to be a massive amount of material in the way — much more than could be transported by stellar winds prior to collapse.

And there’s another wrinkle in the story — the most important and the most controversial.

X-ray Mystery

On the suggestion of fellow graduate student Anna Ho, Dong examined some uncategorized bursts cataloged by MAXI, an X-ray imager on the International Space Station. “To my surprise,” Dong says, “I found an unusual X-ray burst that, after careful analysis, seems to be at the right time, in the right place in the sky. I was not able to explain this burst with any previously known model.”

The team was then faced with a real mystery. They had found what is perhaps the most luminous radio supernova ever observed, surrounded by dense gas, and associated with high-energy X-ray emission. The X-rays point toward the presence of a relativistic jet, which may occur following merger events.

To explain all of the observations, the team settled on Chevalier’s hypothesis. This interpretation of the data is based on assumptions about how much material relativistic jets can eject, and how observable that might be, the physics of which is not fully understood. The result also assumes the X-rays and radio waves come from the same place, which isn’t a guarantee because MAXI doesn’t have the best resolution.

There’s no way to definitively show that the supernova comes from a merger, because it’s transient — the X-ray flash is over, and the glow of radio and visible light are fading.

Chevalier suggests that astronomers must observe more events of this type before we can know exactly what we are looking at. When the sky-monitoring Vera C. Rubin Observatory comes online in a few years, it should aid in characterizing rare and unusual events, like merger-supernovas.

“If these things are out there, then the thing to look for will be evidence of the energetic, compact object driving the supernova,” Chevalier says. “It could be that everything gets buried by the shell, and you won’t see the neutron star or black hole at the center. But in this case, it seems like they did get the evidence.”

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