The LISA mission (Laser Interferometer Space Antenna) will observe gravitational waves in the frequency window between 0.1 mHz to 1 Hz. It will capture the gravitational signal from sources that cannot be observed from current ground based detectors such as Virgo, LIGO, KAGRA, or GEO600. Among the new sources of gravitational waves, the most numerous will be the galactic binaries, whose number of detection should be as high as several tens of thousands after a 4 years LISA mission (cf. figure 1).

Figure 1: LISA observing a galactic binary. Crédits: ESA, Virgo.

Galactic binaries are are made with neutron stars or white dwarfs in various combinations. In the low frequency regime of LISA, galactic binaries will be detected during the inspiral, namely thousand years before the merger that should be detected by ground based detectors. Observing ten or hundred thousands orbits LISA will monitor closely the orbital dynamics of galactic binairy systems. These systems are thus an interesting opportunity to learn more on the nature of gravity (see section tests of fundamental physics) and on the state of matter within neutron stars and white dwarfs (see section internal physics of galactic binaries).

Internal physics of galactic binaries

Compact objects within typical galactic binary systems have masses around the solar mass and radii from 10^4 km for white dwarfs and up to 10 km for neutron stars. These objects are among the magnetic in the universe, with magnetic fields that can reach up to 10^9 G for white dwarfs and up to 10^15 G for neutron stars. Such intense magnetic fields can significantly disrupt the orbital motion of the binary system and be characterized through their signature in the gravitational waves signal (cf. figure 2).

Figure 2: A magnetic binary system. Crédits: ESO / L. Calçada.

Since 2020, the team is involved in characterizing the magnetic signature in the gravitational waves signal. Within the magnetostatic approximation and considering that the magnetic fields are dipolar, we showed that only one secular stable equilibrium could exist for the orientation of the dipole magnetic moments of the stars [1] : the magnetic moments are expected to be orthogonal to the orbital plane and anti-aligned with respect to each other (cf. figure 3). We can thus expect that relatively old binary systems may have reached a configuration close to the secularly stable one. From this result, we were able to determine the effect of the magnetic force on the secular evolution of the system and hence to characterized the magnetic signature in the gravitational waves signal [2]. These theoretical predictions were then tested by conducting a simulation of LISA data processing assuming the gravitational signal emitted by a galactic binary in quasi-circular orbit under magnetic interaction. We demonstrated that the information related to the total magnetic energy of the system could be fully extracted the gravitational waves signal [3].

Figure 3: Equilibrium states for the orientation of the dipole magnetic moments. The only stable configuration corresponds to the case in the upper right of the plot, namely when the dipole magnetic moments are anti-aligned with respect to each other and orthogonal to the orbital plane. Crédits: image taken from [1].

Beside the magnetic effects, we also expect that the orbital dynamics of binary systems of neutron stars and white dwarfs to be controlled by dynamical tides effects and energy dissipation processes. Indeed, the internal composition changes in these stars can generate regions favoring the creation of internal vibrating modes able to efficiently dissipate the orbital energy of the binary system. If the proper modes cannot form, the internal gravity waves can take over and dissipate the orbital energy before a mode is formed. It results that the dynamical tidal effects can accelerate the inspiral phase by dissipating the orbital energy and become even more efficient than the gravitational radiation in some resonant regions. Our team in collaboration with the département d’Astrophysique at Commissariat à l’Energie Atomique is involved in the modeling of the internal dynamics of galactic binaries.

Tests of fundamental physics with galactic binaries

By following the orbital evolution of galactic binaries over many orbits, LISA will be also able to monitor closely the properties of spacetime around compact objects. In this contexte, an important point concerns the Lorentz symmetry that is a fundamental property of general relativity’s spacetime. According to unification théories, the Lorentz symmetry shall be broken at some energy level. Relying on the observation timescale and the amount of detected galactic binary systems by LISA, the mission could help in determining precisely the fundamental symmetry of spacetime by testing the Lorentz symmetry.

Bibliography

[1] C. Aykroyd, A. Bourgoin, C. Le Poncin-Lafitte, S. Mathis, and M.-C. Angonin, Secular dipole stability of mamgnetic binaries, accepted in A&A.

[2] A. Bourgoin, C. Le Poncin-Lafitte, S. Mathis, and M.-C. Angonin, Impact of dipolar magnetic fields on the gravitational wave strain by galactic binaries, Phys. Rev. D 105, 124042 (2022).

[3] E. Savalle, A. Bourgoin, C. Le Poncin-Lafitte, S. Mathis, M.-C. Angonin, and C. Aykroyd, Detection of magnetic galactic binaries in quasi-circular orbit with LISA, submitted in PRD, arXiv 2304.07294.