A nearly perfectly round neutron star could hide tiny “mountains” capable of continuously emitting gravitational waves.
A neutron star is the extremely dense remnant of certain stars after they explode. A mass comparable to that of the Sun is compressed into a sphere a few dozen kilometers across. Despite their name, the “mountains” studied here bear absolutely no resemblance to terrestrial landforms.
Credit: Pixabay/Public Domain
The idea mainly concerns the interior of the star. Some of the matter there can form a superfluid, meaning a liquid that flows without the usual resistance. This fluid contains microscopic vortices linked to the star’s rotation. Some may remain anchored to the solid crust.
When the superfluid and the crust do not rotate at exactly the same speed, a force acts on these vortices. It is known as the Magnus force, or Magnus effect. Researchers calculated that it could exert uneven pressure on the crust and very slightly distort the distribution of the star’s mass.
Such a distortion matters because a rotating star that is not perfectly symmetrical can produce gravitational waves. These ripples in spacetime would be emitted continuously, unlike the brief signals produced, for example, by the collision of two neutron stars.
Magnus effect on a cylinder in a smoke wind tunnel.
Image Wikimedia
The model used remains deliberately simplified. Researchers represent the star as an infinitely long cylinder containing a fluid region, a thin outer layer, and a crust. This geometry does not reproduce a real star, but it makes it possible to isolate the physical effect being studied and estimate its possible magnitude.
The calculations indicate that the force could produce deformations large enough to be detectable by current and future detectors. Their actual size would nevertheless be limited by two factors: the strength of the crust and the force with which the vortices remain anchored to it.
The next step is to move beyond cylindrical geometry in favor of more realistic stellar models. In particular, it will be necessary to calculate the shape of an entire star and better represent its crust and the anchoring of the vortices before estimating a signal that can be directly compared with observations.