Gas going down the wrong way could apply the brakes NASA/Goddard Space Flight Center

Gobbling gas from a neighbour should make neutron stars spin faster, but sometimes the exact opposite happens. Now there might be an explanation: the gas arrives “backwards”.

Neutron stars are dense, fast-spinning stellar corpses that can pull material from a smaller orbiting star, spooling it into a disc before gobbling it up. This material carries momentum, which is why the neutron star should end up spinning faster.

But when Demos Kazanas at the NASA Goddard Space Flight Center in Maryland and his colleagues looked at 18 years’ worth of X-ray observations of neutron stars in binary systems in the Small Magellanic Cloud, they found that half were slowing down.


“That’s harder to understand, because you’d think that they’d be tending to spin up if our current understanding of their evolution is correct,” says Tim Kallman, also at NASA Goddard, who wasn’t involved in the work.

Spooling the wrong way

Even stranger, the rate of their slowdowns seemed to be the same as the rate at which the others were accelerating.

“The two distributions are really very similar, which means that the process by which the spin up and spin down [happen] could be the same,” says Kazanas.

The team have put forward an explanation for the slowdowns: they happen when the swallowed gas is spooling around the neutron star in the opposite direction to the star’s spin.

If borne out by future observations, this idea could alter the commonly accepted view of the evolution of neutron stars in such binary systems, which is that they will keep spinning faster and faster until the end of their companion stars’ lives. Kazanas and his colleagues speculate instead that neutron stars can repeatedly speed up, slow down and switch direction.

It could also have surprising implications for the neutron stars’ less exotic partners as their gas is stripped away. “We understand those normal stars somewhat,” Kallman says. “It’s surprising that it would change [their] character so dramatically.”

It’s not yet clear whether backwards-spinning gas is the right explanation – or how that would even work. “Their explanation is charmingly simple, that it’s the same phenomenon in the opposite direction,” says Anne Archibald at the University of Amsterdam in the Netherlands. “But there are other ways [to account for the slowdown].”

Reference: arXiv.org/abs/1704.06364v1