Invisible Heavyweight in Pisces May Be a Record-Breaking Neutron Star

Why is the extra mass a mystery?

The obvious explanation doesn't work here. Neutron stars can bulk up by siphoning gas from a companion star, but this pair's orbit is far too wide for that to have happened.

"Because it's so separated, that's how they know that it has never dumped material onto the neutron star," says Scott Ransom, a staff astronomer at the National Radio Astronomy Observatory who was not involved in the study.

That leaves two broad possibilities, Ransom says. Either neutron stars can grow much larger than physicists suspected — "that would mean we don't understand supernova physics very well," he notes — or this object formed in an unusually rare way: two smaller white dwarf stars in a tight orbit colliding and merging.

"It's possible that there were two white dwarfs and the red giant, and the two white dwarfs were in a really tight orbit, and eventually those two white dwarfs collided with each other, and turned into a massive neutron star," Ransom explains.

There are still stranger options. What looks like one extra-heavy object could actually be two ordinary neutron stars orbiting each other, or a neutron star paired with a white dwarf.

Could it be a black hole instead?

Ransom sees a real chance the object isn't a neutron star at all. It could be a very low-mass black hole — a possibility he finds somewhat more likely because the object's orbit around the red giant appears circular.

"Astrophysically, there's no reason why you can't have black holes that small, it's just that we don't see any of them in our galaxy," he says. "What [the orbit's shape] probably means is that the birth of that neutron star or black hole was not especially violent."

The reasoning: a supernova that births a neutron star often delivers a recoil "kick" that shoves the newborn remnant into a wild, eccentric orbit or flings it into deep space. Black hole formation is thought to be gentler. "So the fact that the orbit is more circular might be evidence that this is a black hole and not a neutron star," Ransom says.

Will we ever know what it is?

Maybe not. One avenue remains open: follow-up observations with a powerful X-ray observatory could directly detect emissions from the object and distinguish the black-hole scenario from the neutron-star one.

Beyond that, the options thin out fast. Radio telescopes — including the world's best — have already looked and found no pulsations, the repeating radio beeps that would make a neutron star identification ironclad.

"There's no radio pulsations that any telescope in the world will find in the next decades, probably, because they've already looked at it with the world's best [radio] telescopes," Ransom says. "That's unfortunate because that would have been an ironclad signal that it was a neutron star. But besides that, I don't know if there's anything else they could do, so we may never know."

Whatever the object turns out to be, solving the puzzle matters beyond one star system. It could sharpen scientists' understanding of how neutron stars form, and of the extreme physics inside objects where a Sun's worth of matter squeezes into a city-sized sphere.