Lumpy Neutron Stars can Generate Gravitational Waves

A new simulation of neutron stars suggest they may not be as smooth as predicted. The rapidly spinning exotic bodies may have significant topological features like mountains. These “lumps” on the star’s surface may cause fluctuations in space-time as the variation of the huge gravitational field varies on each spin. This fluctuation may generate gravitational waves, propagating into the cosmos, and could be detected here on Earth…

Neutron stars are the remnants of massive stars after they have exploded as supernovae. The dense core remains behind, spinning fast and composed of only neutrons. They have immense gravitational fields and thought to have as much mass as our Sun, but measuring only 20 kilometres across. As they conserve the angular momentum of their massive sun predecessor, as they are so small, they are expected to spin hundreds of times per second.

But how can these strange objects be detected? Well, for one, they may be seen as highly radiating pulsars (or, possibly, “magnetars“), flashing a beam of radiation past the Earth as they spin like a lighthouse, beams of high energy photons emitted from the neutron star’s poles. But what about the effect they have on space-time? Can these massive bodies create gravitational waves? (Note: A gravitational wave is a totally different creature from an atmospheric “gravity wave“.)

To picture the scene: Imagine spinning a perfectly spherical ball in a swimming pool. If the ball is perfectly stationary (not bobbing up and down and not drifting), only spinning on its axis, no ripples in the pool will be seen. Therefore, any instrument measuring ripples in the pool will not detect the presence of the spinning ball. Now spin an object not spherical (like a rugby ball, or an American football) in the pool. As this object spins, the irregularities on the surface (i.e. the pointed ends) will produce a wave on each revolution of the irregular object. The ripple instrument will detect the presence of the ball in the pool.

This is the issue facing scientists trying to detect gravitational waves from neutron stars. If they are smooth objects (perhaps spherical, or slightly flattened due to the spin), they cannot produce ripples in space-time and therefore cannot be detected. If, on the other hand, they are irregularly-shaped spinning bodies, with inhomogeneities (lumps or “mountains”) on the surface, gravitational waves may be generated. The lump will sweep out a fluctuation in space-time on each rotation. This is fine, but are neutron stars lumpy?

Well, the outlook isn’t very good. The space-time “ripple” detectors set out to observe gravitational waves have so far not detected any sign of these rapidly spinning neutron stars. This could either mean that the technology we are using is not sensitive enough to detect gravitational waves or that neutron stars are naturally smooth and cannot produce gravitational waves in the first place.

Matthias Vigelius and Andrew Melatos, researchers from University of Melbourne in Australia, think they have new hope that some types of neutron star might be detected as they are naturally lumpy. Using a new computer modelling technique, the pair believes that even a small variation in the neutron star surface will produce detectable gravitational waves. But how do these lumps form? Often, stars evolve as part of a binary system (i.e. two stars orbiting a common centre of gravity), should one die as a supernova, leaving a neutron star behind, the intense gravitational field will strip its companion star of its gases. As the gas is funnelled into the neutron star, the intense magnetic field will give structural support to the incoming gas, creating an electron-proton mix of superheated plasma sitting on top of the neutron star surface. The lumps formed at the neutron star’s magnetic poles will be a long-living feature, sweeping around the star each time it rotates. Vigelius and Melatos think that detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) may be able to detect this characteristic signature of an irregularly shaped neutron star…. in time.

As yet, these “lumpy” neutron stars have not been detected, but through continued observation (exposure time), it is hoped that Earth-based gravitational wave observatories may eventually receive the signal.

Source: RAS, New Scientist