Detecting the Sound of Neutron Stars

Post - September 2013 (11)

If a sound propagates and bounces off an object, its wiggly echo brings signatures of the physical properties of that object . Bats, dolphins and other cetaceans are famous for using this principle, named echolocation, to chase prey and identify objects in the environment. Something similar happens also during an earthquake, when a large number of seismic waves are produced in the Earth’s crust. Seismologists can infer the location and depth of the earthquake by looking at the physical properties of seismic waves: their speed and shape depends on the elasticity and depth of the medium through which they propagate.

Something similar can be done also for the Sun, in a branch of astronomy known as Helioseismology. By looking at small vibrations that appear on the surface of the Sun, astronomers can infer many precious information about the inner regions of our star. For example it is thanks to these studies that we know the location of the outer convective layer of the Sun (a fundamental quantity to understand the Sun’s structure and evolution).

Astronomers have long speculated that oscillations of this kind might also be present in exotic objects like neutron stars. Despite intense studies carried over the last 50 years, the internal structure of neutron stars remains today a big astrophysical puzzle. Detecting neutron star oscillations would open a window towards these mysterious inner dense regions, where atoms dissolve and exotic states of matter, never observed on Earth,  appear.

In a recent paper, scientists at the University of Maryland and NASA have discovered one such possible vibration from a neutron star (named 4U 1636-536). This system is an X-ray binary where the outer layers of a small (companion) star are being eaten by the neutron star. During the eating process, the neutron star accumulates hydrogen, helium (which are the two most common elements that compose stars) and other heavy elements stripped from the companion. Carbon, which is heavier than hydrogen and helium, sinks deep down in the bottom layer of the accumulated gas and at some point it can ignite, i.e., carbon nuclei start nuclear fusion and release a tremendous amount of energy as a huge flow of X-ray radiation. This energy flash is called a superburst,  and its name derives from the fact that other X-ray bursts, with lower energy, are seen often in these binary systems.

Since during superbursts the luminosity of the neutron star increases tremendously, it has been possible to detect the stellar oscillation in the X-ray flow recorded by the space telescope Rossi X-Ray Timing Explorer.  According to scientists, it is also possible that the superburst itself has triggered the observed stellar oscillations. An analogous case was reported a few months ago by the same team on a different source (named XTE J1751-305), which is a neutron star that is devouring its white dwarf companion. In that case the detection was less significant, and the newly reported observation of a second oscillation from a second source makes that first detection much more compelling. In that case the oscillation was not associated or observed during a superburst, but it is most likely associated with hot and dense plasma regions located at the magnetic poles of the neutron star. If these oscillations will be confirmed with further measurements and will be observed in more sources, we will have a new and truly spectacular way of determining how matter behaves inside neutron stars.

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