Potential “Failed Supernova” Black Hole Discovered

Astronomers may just have watched a star collapse directly into a black hole — minus the supernova. The failed supernova could help us understand how stars die.

Just two weeks ago, Sky & Telescope reported that amateur astronomers had discovered a massive star exploding in the so-called Fireworks Galaxy (NGC 6946), known for its abundance of supernovae.

Yet years before that a quieter, even more extraordinary event had taken place in the same galaxy. Another massive star had brightened briefly in a faint mimicry of a supernova before it disappeared — presumably, into a black hole.

Scott Adams (Caltech and Ohio State University), Christopher Kochanek, Jill Gerke (both at Ohio State University), and colleagues caught the rare event using the Large Binocular Telescope in Arizona. They were using the pair of 8.4-meter telescopes to monitor a million aging stars in nearby galaxies, waiting for the stars to go pop.

Finally, after four years of watching and waiting, the star known as N6946-BH1 did exactly that — but not in the way you might expect.

Failed Supernova: How to Not Explode a Star

An ordinary massive star, one more than eight times as massive as the Sun, ends its life with a bang. Its heavy, inert core gives way to gravity — becoming either a neutron star or black hole — and the shock of that sudden collapse sends the star’s outer layers flying.

Yet theorists actually have a hard time making a simulated star explode. The problem is, the rebound shock tends to stall, halted by the flow of material onto the crushed core. Only recently have computer simulations come close to detonating a star, but even those incredibly detailed simulations were only 10 milliseconds long. No one has yet modeled a supernova shock wave breaking completely out of a star — those kinds of simulations are at least a couple of years away.

From the simulations we have now, though, it appears that some stars, those whose centers are too compact, will never explode. “The ‘explodability’ of a star seems to be dependent on the density of the layers just outside of the iron core,” Adams explains. When too-dense stars run out of elements to fuse to stave off gravity’s pull, they’ll just collapse.

The First “Failed Supernova” Candidate

Supernovae are rare, and disappearing stars may be even rarer. So Kochanek and colleagues started keeping an eye on a million aging stars in 27 nearby galaxies with the hopes of catching just one on the brink of collapse.

And by 2016, they had found seven. Six of these were standard core-collapse supernovae. But one was something else. The star, about 25 times more massive than the Sun and 22 million light-years away in NGC 6946, had simply winked out of existence.

Now, Adams and colleagues are reporting archival and follow-up observations of this disappearing act in the Monthly Notices of the Royal Astronomical Society (full text here). New analysis of archived Large Binocular Telescope observations show that the star’s light stayed roughly constant over a period of ten years before it changed suddenly In 2009, brightening to become 1 million times more luminous than our sun for several months. While bright, this outburst was nowhere near supernova levels. Within several months, the star’s visible light disappeared, and new observations from the Hubble Space Telescope confirm it’s gone.

But Spitzer Space Telescope images reveal a faint infrared glow — there’s still something there. The authors argue that the glow could come from stellar debris falling back onto the newly formed black hole, heating up and radiating before disappearing into the maw.

Still, it’s too early to definitively rule out alternatives. The infrared glow could instead emanate from dust enshrouding a surviving star, perhaps one that just experienced an extreme outburst.

The team now hopes to follow up with more Hubble and Spitzer images to keep an eye on the infrared radiation. If it starts to brighten, then we might be seeing a surviving star emerging from an expanding cocoon of dust. But if the infrared glow continues to fade, then we’re more likely seeing stellar remnants feeding the black hole. If there’s not too much debris in the way, detection with the Chandra X-ray Observatory could confirm the latter explanation.

“The longer we don't see any rebrightening at the location of the progenitor star,” Adams says, “the more implausible alternative explanations become.”

The find, if confirmed, is an important one, says Philipp Mösta (University of California, Berkeley), because it explains how massive black holes, like the ones detected by LIGO last year, came to be. "This will be an exciting target to follow in the next couple of years."