Gravitational waves slow the spin of shape-shifting neutron star

Cecile G. Tamura

Put on the brakes. A spinning neutron star that shifts between two states slows at a faster rate in one of them – and gravitational waves may be responsible.

The neutron star J1023+0038 spins almost 600 times per second. But as its powerful magnetic field dissipates energy, it is slowing by about 76 rotations per second every billion years. This magnetic “spin-down” is normal, but sometimes J1023 slows at a faster rate.
The different rates are associated with two states the neutron star switches back and forth between: one where it emits mostly radio waves and one where it mainly gives off X-rays. No one knows why some neutron stars behave in this way. But when the star is emitting mostly X-rays, it slows down about 30 per cent faster.

In this X-ray phase, the star is stealing material from a smaller companion star that orbits it. Brynmor Haskell at the Polish Academy of Sciences in Warsaw and Alessandro Patruno at Leiden University, the Netherlands, argue that this stolen gas may be the key to J1023’s strange spin.

As material snatched from its companion sticks to J1023’s surface, it builds a so-called mountain. Despite being no more than a few millimetres in height, the bump crushes the atoms beneath it, pushing them deeper into the neutron star. There the higher pressure fuses them into heavier elements, giving the mountain roots in the star’s interior.

The extra surface bump and the heavier atoms below it together result in the mountain creating an asymmetry in J1023’s gravity. “Neutron stars are very compact, roughly the mass of the sun compressed in a 10-kilometre radius,” says Haskell. “This means that even very small deformations can lead to large changes in the gravitational field.”

Riding the waves

The imbalance in the neutron star’s gravitational field may cause it to radiate gravitational waves, ripples in space-time caused by the movement of massive objects. These waves would carry away some of the energy that keeps J1023 spinning.

When the star switches from its X-ray phase to its radio phase, it stops munching on its stellar partner. As a result, the mountain gradually flattens out and the star emits no more spin-stunting gravitational waves.

Last year, the LIGO collaboration announced that it had observed gravitational waves shaken off by black holes colliding. But nobody has yet seen gravitational waves from continuous, rather than catastrophic, events. Objects like J1023 are promising candidates for future gravitational wave searches, especially if they can grow larger mountains.

“If this happens, then there might be many other neutron stars that do the same,” says Patruno. “Continuous gravitational waves might really be a widespread phenomenon.”

Such a scenario could also explain the apparent cap on neutron stars’ spin. “The fastest ones we see don’t rotate as fast as we think they should be able to go,” says Nils Andersson at the University of Southampton, UK. “There’s something missing in our understanding.”

If faster-spinning stars have defects such as mountains, they would emit more gravitational waves and slow down faster, setting a cosmic speed limit for neutron stars.

https://arxiv.org/abs/1703.08374

https://www.newscientist.com/…/dn9730-neutron-star-clocked…/

http://onlinelibrary.wiley.com/jour…/10.1111/(ISSN)1365-2966

https://www.newscientist.com/…/dn9428-massive-neutron-star…/

https://www.newscientist.com/…/2077162-revolution-in-physi…/