Neutron star asteroseismology: composition modes and tidal effects

Abstract

In the decade that has passed since the first observation of gravitational waves, a new opportunity to study the physics of neutron stars has presented itself. Gravitational waves emitted by neutron star oscillations can be used to obtain information about the equation of state of dense nuclear matter. The identification of such oscillations gives rise to the field of neutron star asteroseismology. In this work we focus on two types of quadrupolar oscillations: gravity g-modes and interface i-modes. We also examine the different regimes under which the mode equations are solved: Newtonian gravity; general relativity and the Cowling approximation to general relativity. Their various benefits and inaccuracies are discussed.

The effects of stratification, composition and nuclear reactions on the g-mode spectrum are studied for equations of state from the BSk family. By incorporating the reaction rates directly into the mode calculations we see that at the low temperatures expected in the neutron star during inspiral, the damping of the modes is insignificant but for the higher order g-modes. However extending the analysis to include hyperons and their strong reactions, the f -modes is affected for sufficiently massive stars, warranting further investigation.

Second, using a family of nuclear-matter equations of state generated from chiral effective field theory (EFT), we find that the detection of an interface mode would be a smoking-gun signature of a first-order phase transition, distinguishable in a single gravitational-wave event. Our estimates show that an i-mode resonance may be observable with Cosmic Explorer and the Einstein Telescope, and possibly already with LIGO A+ for sufficiently strong gravitational wave signals from binary neutron star mergers. One such phase transition that could be probed by this method is that from hadronic matter to deconfied quarks in the neutron star core, a robust prediction of quantum chromodynamics. The exact nature of this transition remains unsolved
and therefore an i-mode detection would be a breakthrough discovery for nuclear physics.

Lastly, the tidal response of a neutron star in a binary and its relation to the Love number is investigated. In this work we present a new approach to this problem, that calculates the effective Love number without using the traditional mode sum approach but instead utilizes the matching of ingoing and outgoing solutions in the weak field near zone.

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Identifiers

ORCID for Nils Andersson: orcid.org/0000-0001-8550-3843

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