Tidally-Induced Apsidal Precession in Double White Dwarfs: a new mass measurement tool with LISA

TL;DR

This paper explores how apsidal precession in eccentric double white dwarf binaries, detectable by LISA, can be used to measure stellar masses and properties, especially considering tidal effects across different WD ages.

Contribution

It introduces a detailed analysis of tidal, rotational, and relativistic effects on apsidal precession in DWDs, proposing a new method for mass measurement using LISA data.

Findings

Apsidal precession causes detectable GW signal shifts in eccentric DWDs.

Tidal effects dominate at higher frequencies, especially in hot WDs.

Ignoring tides biases mass estimates, risking misclassification of WDs as neutron stars or black holes.

Abstract

Galactic interacting double white dwarfs (DWD) are guaranteed gravitational wave (GW) sources for the GW detector LISA, with more than 10^4 binaries expected to be detected over the mission's lifetime. Part of this population is expected to be eccentric, and here we investigate the potential for constraining the white dwarf (WD) properties through apsidal precession in these binaries. We analyze the tidal, rotational, and general relativistic contributions to apsidal precession by using detailed He WD models, where the evolution of the star's interior is followed throughout the cooling phase. In agreement with previous studies of zero-temperature WDs, we find that apsidal precession in eccentric DWDs can lead to a detectable shift in the emitted GW signal when binaries with cool (old) components are considered. This shift increases significantly for hot (young) WDs. We find that apsidal motion in hot (cool) DWDs is dominated by tides at orbital frequencies above ~10^{-4}Hz (10^{- 3}$Hz). The analysis of apsidal precession in these sources while ignoring the tidal component would lead to an extreme bias in the mass determination, and could lead us to misidentify WDs as neutron stars or black holes. We use the detailed WD models to show that for older, cold WDs, there is a unique relationship that ties the radius and apsidal precession constant to the WD masses, therefore allowing tides to be used as a tool to constrain the source masses.