According to new computer simulations, not only rocky moons and planets may have distinctive topographic features such as mountains, but neutron stars may have them as well. The rotational spin around their axis could produce so powerful distortions in the fabric of space-time that they could actually lead to gravitational waves. In fact, Einstein's Theory of General Relativity predicted that the motion of asymmetrical objects in space could produce gravitational waves.

The problem is that no detector has ever proved that such gravitational waves exist, not even the Laser Interferometer Gravitational-Wave Observatory or the Virgo detector. Matthias Vigelius and Andrew Melatos, researchers at the University of Melbourne, proved through computer simulations that detectable gravitational waves may be produced in neutron stars.

Neutron stars are believed to be in the core of dead stars, left behind by supernova explosions. Although being much smaller than the original body they were created from, they can be several times denser and rotate around their axis a few hundred times per second. Due to this high rotational speed and extreme gravitational attraction, neutron stars exert frame dragging on the space-time fabric, at the same time producing irregularities on their surface.

Mountains on stars

Three-dimensional simulations reveal that neutron stars may create and sustain 'mountains' on their surface, with material accreted from a star in its vicinity. This is done with the help of the powerful magnetic field produced by the neutron star. The collected material is channeled to the poles of the star and sustained by the magnetic field, so that the massive gravitational force does not flatten the whole surface of the neutron star.

Each pole should be able to maintain a mass of matter comparable to that of the planet Saturn. In the accretion phase, matter is collected in the form of a plasma mix of protons and electrons, but as matter experiences the conditions on the surface of the star, it is immediately transformed in pure neutrons.

These structures, named 'mountains' by researchers, bear no resemblance to the mountains on Earth. Although extending on surfaces more than 3 kilometers long, their height would never raise more than a few meters in relation to the surrounding surface.

Advantages opposed to mergers

"Their color would be a nice hue of X-rays, so it would in fact not be advisable to have a close-up look!" said Vigelius, evidence that they are much hotter than the surrounding material. Because the rotational and magnetic poles of neutron stars almost never coincide, the mountains would move along the surface of the star in a circular trajectory, thus producing powerful gravitational waves.

The reason why such gravitational waves would be easier to detect is that, unlike gravitational waves produced during the merging of the neutron stars of black holes, these specific gravitational waves would come in a very regular pattern that continues for indefinite amounts of time, making them easier to distinguish from the background noise collected by the detectors.

Data uncertainty

It is believed that such gravitational waves already exist in the data collected by the LIGO detector, but the computing power required to filter such data is still out of reach. Alternatively, Benjamin Owen, Pennsylvania State University Park researcher and LIGO member, says that a powerful gravitational wave could be produced by the merging of a black hole and a neutron star.

He reveals that a recent powerful gravitational wave might have been created in the Crab Nebula, during a merging. There are some uncertainties related to the size of the mountains of neutron stars and to the fact that no gravitational wave has ever been detected, which only add up to the possible theory that neutrons stars are actually smooth.