A Gravitational-Wave Perspective on Neutron-Star Seismology

Nils Andersson

doi:10.3390/universe7040097

Abstract

We provide a bird’s-eye view of neutron-star seismology, which aims to probe the extreme physics associated with these objects, in the context of gravitational-wave astronomy. Focussing on the fundamental mode of oscillation, which is an efficient gravitational-wave emitter, we consider the seismology aspects of a number of astrophysically relevant scenarios, ranging from transients (like pulsar glitches and magnetar flares), to the dynamics of tides in inspiralling compact binaries and the eventual merged object and instabilities acting in isolated, rapidly rotating, neutron stars. The aim is not to provide a thorough review, but rather to introduce (some of) the key ideas and highlight issues that need further attention.

Highlights

If we want to explore aspects associated with the dense neutron star interior, it is natural to formulate a seismology strategy
An observation of the mode frequency would provide the former and the damping rate would help us infer the radius—as the scaling with the parameters is different. This is the main idea of gravitational-wave asteroseismology [7,8,9,10] and it prompts a number of follow-on questions
The last point suggests that we need to consider more realistic implementations based on relativistic neutron star models

Summary

Motivation

Neutron stars represent many extremes of physics; in density, pressure, temperature (through the early formation stages and during binary mergers), models for which require aspects that may not yet be fully (or perhaps even partially) understood. The obvious question to ask is if we can use observations to make sense of the theory mess This may seem far fetched, given that neutron stars are small and (obviously) distant. If we want to explore aspects associated with the dense neutron star interior, it is natural to (try to) formulate a seismology strategy It is well-known that the complex interior physics is reflected in a rich spectrum of oscillation modes [5] and one may hope to be able to use observations of related features to gain insight. The same technology allows us to characterise the host stars of exoplanets, as the inference of a precise stellar radius constrains the companion planetary radius, as well When it comes to neutron stars, we are not likely to be able to “resolve” surface features. This is not to say that this venture will be in any way straightforward, but the effort may pay off handsomely in the end

The First Couple of Steps

Transients

Adding Physics

Dynamical Tides

Adding a Bit of Spin

Final Comments