Pulsar as a Weber detector of gravitational waves and a probe to its internal phase transitions

Abstract

It is believed that cores of neutron stars provide a natural laboratory where exotic high baryon density QCD phases may exist.The theoretically well established {\it neutron superfluid phase} is also believed to be found only inside neutron stars. Focus on neutron stars has intensified in recent years with the direct detection of gravitational waves (GWs) from binary neutron star (BNS) merger, which has allowed the possibility of directly probing the properties of the interior of a neutron star. A remarkable phenomenon manifested by rapidly rotating neutron stars is in their {\it avatar} as {\it Pulsars}. The accuracy of pulsar timing allowed the first indirect detection of GWs from a BNS system and opened up a few exciting possibilities. Any pulsar deformation, even if incredibly tiny, can leave imprints on the pulses by introducing tiny perturbations of the moment of inertia (MI) tensor components. While the diagonal MI components of the perturbed MI tensor affect the pulse timings, the off-diagonal components lead to the pulsar's wobbling and affecting the pulse profile. This opens up an opportunity to explore various phase transitions inside a pulsar core by induced density fluctuations through the observable effects on the pulse timing and profile. Such perturbations also naturally induce a rapidly changing quadrupole moment of the star, thereby providing a new source of GW emission. Another remarkable possibility arises when we consider the effect of an external GW on a neutron star. With the possibility of detecting any minute changes in its configuration through pulse observations, the neutron star has the potential to perform as a Weber detector of GWs. This brief review focuses on these specific aspects of a pulsar, specifically on the type of physics that can be probed by utilizing the effect of changes in the MI tensor on pulse properties.