Anisotropic neutron star crust, solar system mountains, and gravitational waves

Neutron stars, despite their extremely dense and compact nature, are geologically active. Pulsars, which are neutron stars with strong magnetic fields, emit beams of radio energy that sweep across the sky with each rotation. These regular pulses slow down over time as the neutron star loses its rotational energy. However, sometimes a pulsar encounters a “problem” and its rotation speed increases slightly. This phenomenon, called a starquake, is caused by shifts in the star’s envelope. Similar to Earth and other geologically active bodies, neutron stars also show the formation and decay of mountains. The distribution and size of these mountains depends on the internal structure of the neutron star, which scientists still do not fully understand. A recent study aimed to shed some light on this topic. Research suggests that if a neutron star has asymmetrical distortions, such as mountains, its rotation would generate gravitational waves. Although current technology cannot detect these waves, future gravitational wave observatories may be able to do so. The configuration of these gravitational waves will be influenced by the distribution and scale of the mountains on the neutron star. To better understand this concept, the study’s authors analyzed known worlds such as Mercury and Enceladus. Mercury, with its thin shell covering a large metallic core, exhibits lobed scars due to compressive stress during cooling. On the other hand, Enceladus, with its icy shell covering the ocean layer, has a “tiger stripe” pattern on its mountains. Other icy moons, such as Europa, exhibit linear features due to interactions between the crust and interior. The study suggests that the shells and interiors of neutron stars may exhibit behavior similar to that of these worlds. The presence of large-scale asymmetry in the shell may place an upper limit on the neutron star’s rotation rate, which could be determined through gravitational wave detection. The authors also found that neutron stars can have a variety of structures, with some shell features closely resembling those of Mercury, while others resembling Europa or Enceladus. Therefore, observing gravitational waves from neutron stars will play an important role in discovering their different compositions and properties.

Source: “Anisotropic Neutron Star Shells, Solar Mountains, and Gravitational Waves.”