10.11.2017

When we watched two neutron stars smash into each other in August, producing gravitational waves and a huge explosion, we weren’t quite sure what was left over afterward: a single colossal neutron star or a black hole.

Now, Yun-Wei Yu at Central China Normal University and Zi-Gao Dai at Nanjing University in China have modelled that explosion, a so-called kilonova that could last weeks to months, and they say there is a neutron star left over at the spot where the smash-up occurred.

There are three main theories for what could be left behind when two neutron stars collide: a black hole, a single neutron star that only lasts for a few milliseconds and then collapses into a black hole, or a stable neutron star that sticks around longer. If it’s there, it’s the biggest neutron star we’ve ever seen.

The gravitational waves that the Laser Interferometer Gravitational-Wave Observatory (LIGO) saw can’t tell us what was left behind, but there may be clues in the kilonova.

Death spiral

As the original neutron stars orbit each other in their death spiral, they can accelerate up to about a third of the speed of light, says Edo Berger at Harvard University. When they crash and become one, the resulting object keeps that momentum and spins incredibly fast.

Over time, it radiates away the energy that kept it spinning and slows down. If the object is a neutron star and it slows down too much, it will collapse in on itself and become a black hole. The exact mass at which a neutron star is too big to support itself from collapsing remains an open question, but in this case the remaining neutron star would be enormous.

“If this neutron star exists, it’s spinning extremely rapidly in the beginning, something like 1000 times per second, and pumping out all of this energy,” Berger says. The energy pours into the ongoing kilonova, changing its light.

Second wind

A boost in the kilonova’s energy would point to the collision leaving behind a neutron star. “For a neutron star, that energy is sort of spewing out into the kilonova in all different directions, whereas if it’s a black hole we expect to channel it in a single jet that powers the gamma ray burst that we see,” says Berger.

Even though there does appear to be an extra burst of energy, and Yu and Dai say that the observations of the neutron star smash-up match their model with a neutron star remnant, Berger says that the burst of gamma radiation from the collision seems like a clue that it may actually be a black hole.

“The general consensus within the community is that in order to produce a gamma ray burst you need a black hole,” he says. Plus, he says that the energy from the neutron star in Yu and Dai’s model outshines the explosion that we actually saw.

But this is the first time we’ve ever had any observations of a neutron star merger, so all of the theories are still preliminary and many more will emerge in coming weeks, Berger says. “Now the models can be tuned to actual data, and that’s exactly what they’re starting to do here.”