A Black Hole or Neutron Star Fell Into Another Star and Triggered a Supernova

What happens when you slam a neutron star (or black hole, take your pick) into a companion star? A supernova, that’s what. And for the first time ever, astronomers think they’ve spotted one.

Back in 2014 the MAXI instrument aboard the International Space Station detected a flare of X-rays from a dwarf, star-forming galaxy sitting 480 million light-years away from us. No big deal; it happens all the time.

Around the same time, a radio survey using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) called the Faint Images of the Radio Sky at Twenty centimeters (FIRST) didn’t find anything unusual in that patch of the sky. Also no big deal.

But then a follow-up survey, the Very Large Array Sky Survey (VLASS) which began observations in 2017, did find something: a bright source of radio emissions coming from the same place. Big deal.

The astronomers behind the survey think they’ve spotted something remarkable. A supernova detonation triggered by a massive case of stellar indigestion; a star consuming a companion black hole or neutron star.

“Theorists had predicted that this could happen, but this is the first time we’ve actually seen such an event,” said Dillon Dong, a graduate student at Caltech and lead author on a paper reporting the discovery in the journal Science.

Here’s what went down in that galaxy far, far away. One star in the binary pair ended its life, destroying itself as a supernova and leaving behind a remnant, either a neutron star or a black hole. Then over the ages that dense remnant swirled closer and closer to its surviving sibling. About 300 years ago, it entered the atmosphere of that companion star. Tearing apart the star, it sent material flinging away into space. Once it reached the core, it disrupted the fusion reactions happening there. Without a source of energy to sustain itself, the star went supernova, giving off the flash of X-rays.

“That jet is what produced the X-rays seen by the MAXI instrument aboard the International Space Station, and this confirms the date of this event in 2014,” Dong said.

Now, the material ejected from 2014 event raced outwards, eventually colliding with the older material thrown out when the dense companion first entered the star, creating shock waves that produced the radio emissions.

“All the pieces of this puzzle fit together to tell this amazing story,” said Gregg Hallinan of Caltech. “The remnant of a star that exploded a long time ago plunged into its companion, causing it, too, to explode,” he added.