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A new kind of supernova makes weird black holes

Scientists observed a new kind of supernova: the early death of a massive star triggered when it eats its companion.

Sometimes, as you age naturally, you lose your sense of balance and are prone to what can be catastrophic falls. And as it turns out, the same goes for stars.

And sometimes you invite your neighbor over, consume the neighbor, and the cannibalized neighbor then eats you from the inside out while you’re still in your prime.

... or maybe that’s where the human/star analogy ends.

In a paper published Wednesday in the journal Science, a team at Caltech detailed the serendipitous first observation of a new kind of supernova, the stupendously explosive death throes of a massive star.

“This is a brand new kind of supernova that we're seeing,” Dillon Dong, a Caltech graduate student in astronomy and first author on the paper, tells Inverse. While supernovae are normally the final act of geriatric and massive stars roughly eight times the mass of the Sun or more, he says, “What we're finding is it's possible for massive stars to actually blow up much sooner.”

The new findings mark the first observation of a stellar event that has been theorized about for decades and provide concrete data to help astronomers better understand the possible behaviors of binary star systems, which make up the majority of massive stars in the universe.

“Pretty much on average, every single massive star has a close binary partner,” Dong says.

The Very Large Array caught the supernova in action — we just didn’t know it at the time. NRAO

What did researchers discover? — The researchers found evidence of the first confirmed “merger triggered core-collapse” supernova, a mouthful that means a massive star went nova not because it had grown old and exhausted its supply of nuclear fuel, but because it merged with its binary partner, and this merger kicked off the larger star’s early death. The object was given the name VT J121001+4959647, also a mouthful.

“All stars are basically propped up by the outward force of (something) like trillions of atomic bombs going off from fusion reactions in the core,” Dong says.

But in this case, a massive star swallowed its binary partner — a formerly massive star that had already collapsed into an extremely dense neutron star or black hole — and in the process of digesting it, the still massive star had its legs eaten away from the inside.

“It basically disrupts fusion within the star, or within the core of the star, and the star loses the pressure support that was keeping it from imploding under its own gravity,” Dong says. “Once you kick the legs out, once you stop those fusion reactions, then it's going to collapse.”

If it wasn’t a black hole before being consumed by the massive star, the neutron star collapsed into a black hole by the time it reached the massive star’s core, according to Dong.

Then it began to eat its larger neighbor, one layer at a time.

“There's sort of like a traffic jam trying to get into the black hole, because black holes can only eat so much, given time,” Dong says, the star stuff swirling at tremendous speed around the black hole like water down a drain.

Along with the swirling hot matter, the associated magnetic field lines are twisted and tangled, forming a sort of funnel up and out of the star, Dong says. This leads to the ejection of material flung up and out by the whirling accretion disk. “This jet basically drills its way through the star that's collapsing,” he says, and the ejected material “gets shot out in a jet at nearly the speed of light.”

Dong says that high-speed ejection led to an X-ray burst that was one of two signals that led researchers to the present discovery. The other signal was in the radio spectrum.

As the massive star’s binary companion spiraled into the core of the massive star, it flung star stuff out and away, “like a garden sprinkler,” as Dong describes it. After the high-speed jet, the massive star collapsed and bounced outward in the massive explosion we call a supernova. But the shockwave from that explosion ran into an obstacle — all that star stuff that was flung out into space by the event — at around 10,000 kilometers per second.

“Think of it sort of like a high-speed car with faulty brakes that's running into heavy traffic in front of it,” Dong says. “You can't hit the brakes, and it just crunches into those cars, and that creates a shockwave, and it decelerates really, really quickly,” creating the radio emission.

A graphic from the National Radio Astronomy Observatory detailing the merger triggered core-collapse supernova event. National Radio Astronomy Observatory

How they did it — The Caltech team didn’t go looking for a new kind of supernova. “We found this object, not because we were looking for it, but because we conducted an open-minded search of a large dataset,” the Very Large Array Sky Survey, Dong says. “It was a serendipitous discovery. And we found it sort of by creating the conditions for serendipity.”

One of the motivations behind the Very Large Array Sky Survey was to understand how the sky changes in radio frequencies over time, he adds, changes that could cause by flares from supermassive black holes or exploding stars in near or distant galaxies.

So Dong and his collaborators began by checking the millions of luminous radio sources in the Very Large Array Sky Survey to see if they had been detected in previous radio surveys of the sky, hoping to detect any new, explosive events. Then they filtered down the millions of recent radio sources they found a tiny list of 20 sources associated with nearby galaxies.

Out of those 20 radio sources, they found one, VT J121001+4959647, associated with an X-ray burst from the same location observed in 2014 by a Japanese space agency experiment on the International Space Station.

But initially, this was puzzling, as it wasn’t immediately clear what could cause both the radio emission and the X-ray emission. “We went through a lot of theoretical models that have been proposed,” Dong says, “And we concluded that the only way to do it was really through binary interaction.”

“It was a lot of hard work,” Dong says, “But it paid off in the end.”

Why it matters — Most massive stars in the universe are born in close primaries, Dong says, and at some point in their lives, they end up siphoning mass off their binary partners. But generally, this isn’t related to anything blowing up. “In our case, it was very coincidentally timed with the actual supernova within like, a couple of centuries,” he says. “There are all sorts of rich things that can happen when these massive stars interact with each other. And this is only really beginning to be uncovered by astronomers.”

What’s next — Dong will continue studying this newly identified merger-triggered core-collapse supernova — particularly the shockwave — as it continues to travel outward and interact with the previously ejected star stuff. Data from the explosion could help astronomers better understand the spiraling path of the binary companion that triggered the explosion, which he says is difficult to model.

“I will, of course, be looking for other examples of this kind of supernova as well in the rest of the VLA Sky Survey,” Dong adds.

Abstract: A core-collapse supernova occurs when exothermic fusion ceases in the core of a massive star, typically due to exhaustion of nuclear fuel. Theory predicts that fusion could be interrupted earlier, by merging of the star with a compact binary companion. We report a luminous radio transient, VT J121001+495647, found in the Very Large Array Sky Survey. The radio emission is consistent with supernova ejecta colliding with a dense shell of material, potentially ejected by binary interaction in the centuries prior to explosion. We associate the supernova with an archival X-ray transient, which implies a relativistic jet was launched during the explosion. The combination of an early relativistic jet and late-time dense interaction is consistent with expectations for a merger- driven explosion.
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