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 鈥渂ackwards鈥�.
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 at the NASA Goddard Space Flight Center in Maryland and his colleagues looked at 18 years鈥� worth of X-ray observations of neutron stars in binary systems in the Small Magellanic Cloud, they found that half were slowing down.
Advertisement
鈥淭hat鈥檚 harder to understand, because you鈥檇 think that they鈥檇 be tending to spin up if our current understanding of their evolution is correct,鈥� says , also at NASA Goddard, who wasn鈥檛 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.
鈥淭he two distributions are really very similar, which means that the process by which the spin up and spin down [happen] could be the same,鈥� 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鈥檚 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鈥� 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鈥� less exotic partners as their gas is stripped away. 鈥淲e understand those normal stars somewhat,鈥� Kallman says. 鈥淚t鈥檚 surprising that it would change [their] character so dramatically.鈥�
It鈥檚 not yet clear whether backwards-spinning gas is the right explanation 鈥� or how that would even work. 鈥淭heir explanation is charmingly simple, that it鈥檚 the same phenomenon in the opposite direction,鈥� says at the University of Amsterdam in the Netherlands. 鈥淏ut there are other ways [to account for the slowdown].鈥�
Reference:
