Spin and quadrupole contributions to the motion of astrophysical binaries

TL;DR

This paper explores how spin and quadrupole moments influence the motion of astrophysical binaries, providing a model that could help distinguish between black holes and neutron stars based on their internal structures.

Contribution

It extends the point mass action to include spin and quadrupole effects, detailing their impact on binary dynamics and potential observational signatures.

Findings

Quadrupole effects encode information about internal structure.

Distinguishes black holes from neutron stars via quadrupole signatures.

Connects oscillation modes with quadrupole evolution.

Abstract

Compact objects in general relativity approximately move along geodesics of spacetime. It is shown that the corrections to geodesic motion due to spin (dipole), quadrupole, and higher multipoles can be modeled by an extension of the point mass action. The quadrupole contributions are discussed in detail for astrophysical objects like neutron stars or black holes. Implications for binaries are analyzed for a small mass ratio situation. There quadrupole effects can encode information about the internal structure of the compact object, e.g., in principle they allow a distinction between black holes and neutron stars, and also different equations of state for the latter. Furthermore, a connection between the relativistic oscillation modes of the object and a dynamical quadrupole evolution is established.