Gravitational wave mystery could be a sign of a new kind of black hole

By Leah Crane

A strange set of gravitational waves have been sent across space by a mysterious object. It could be the smallest black hole ever found or the largest neutron star.

Gravitational waves are ripples in space time that are caused by the motion of massive objects. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected these waves from many pairs of black holes colliding over the last few years, as well as one pair of neutron stars.

Now they have found a truly puzzling collision, said LIGO team member Katerina Chatziioannou at a meeting of the American Astronomical Society in Hawaii on 6 January. A LIGO detector in Louisiana spotted signs of two objects colliding, but nobody is quite sure what one of the objects is.

In this smash-up, one of the objects was definitely a neutron star with a mass between 1.1 and 1.7 times the mass of the sun. While the other object is probably also a neutron star, months of analysis have not been able to prove it is, says LIGO team member Nelson Christensen.

Its mass could be as high as 2.5 times that of the sun, which means it could be massive enough to be a black hole.

“We’ve never seen any neutron star with this large a mass,” says Christensen. “The question is, is it really a neutron star? If it is, then we’ve detected a really strange heavy neutron star, but if it’s a black hole it’s a really light black hole.”

While the idea of such a low-mass black hole is plausible, the lightest one anyone has found thus far is 3.3 times the mass of the sun. If it is not the smallest black hole ever found, but in fact a neutron star, this object is still unusual because it’s not clear how a neutron star with a nearby partner could get so large.

“It’s clearly heavier than any other pair of neutron stars ever observed,” said Chatziioannou in a press conference. “The existence of a system like that challenges our current understanding of how those systems form binaries and merge to give off gravitational waves.”

We should get used to this kind of strange discovery, says Christensen. “We’re getting about one gravitational wave event a week now, and that’s a lot,” he says. “With a lot of events you inevitably see cool stuff every now and then.”

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