Continuous Gravitational Waves from Supersoft X-ray Sources: Promising Targets for deci-Hz Detectors

TL;DR

This work investigates continuous gravitational waves from supersoft X-ray sources (SSSs), where accreting, magnetized white dwarfs develop oblate deformations that emit CGWs in the deci-Hz band. The authors couple time-dependent WD evolution in MESA with 2D magnetized equilibria from the Einstein-Maxwell solver XNS to compute ellipticities and GW strains as mass, spin, and magnetic fields evolve during sustained accretion. They show that with internal toroidal fields of order 10^12–10^13 G and spin frequencies around 0.05–0.3 Hz, CGWs from SSSs should be detectable by planned deci-Hz detectors (e.g., DECIGO, BBO, ALIA, LGWA), including known systems such as CAL 83 and RX J0019+2156, and potentially reveal a large hidden SSS population. A CGW detection would directly probe WD internal magnetism and rotation, identify candidate pre-explosion Type Ia progenitors, and provide a complementary census of rapidly rotating, magnetized WDs in nearby galaxies.

Abstract

Supersoft X-ray sources (SSSs) host white dwarfs (WDs) accreting at rates that sustain steady nuclear burning, driving rapid mass growth, radial contraction, and magnetic field amplification. Angular-momentum transfer from the accretion disk naturally spins up the WD, while the amplified internal magnetic field induces a non-axisymmetric deformation in presence of a misaligned rotation. Such WDs emits continuous gravitational waves (CGWs). We model the coupled evolutions of stellar mass, spin, and magnetic structure in accreting WDs in SSSs with MESA, and compute the resulting quadrupolar deformation with the Einstein-Maxwell solver XNS. We show that WDs in SSSs, particularly near the end of thermal timescale mass transfer and close to the Chandrasekhar mass limit, produce CGWs predominantly in the deci-Hz band accessible to planned detectors such as DECIGO, BBO, Deci-Hz, ALIA, and LGWA, and are distinguishable from other Galactic CGW sources such as AM CVn systems, detached double WDs, and isolated WDs. Well studied SSSs such as CAL 83 and RX J0019+2156 can be detectable, enabling targeted CGW measurements that directly probe WD's internal magnetic fields and rotation, while blind searches can reveal hundreds of obscured SSSs otherwise missed in soft X-rays and map the hidden population of accreting, rapidly rotating, magnetized WDs in nearby galaxies. A CGW detection from WDs in SSSs could also identify potential pre-explosion Type Ia progenitors.

Figures (9)

• Figure 1: Cartoon illustration of the standard evolutionary channel leading to a SSS.
• Figure 2: Radial profiles of the internal toroidal magnetic field $B(r)$ at several stages along the accreting WD sequence (Section \ref{['sec:acc']}), showing the amplification of $B$ due to mainly stellar contraction.
• Figure 3: Cartoon illustration from side view of the accretion disk and magnetosphere. See text for notations.
• Figure 4: Evolution of the magnetospheric to corotation radius ratio $R_{\rm m}/R_{\rm c}$ (first row), individual torque contributions to $d\Omega/dt$ (second row), and spin frequency $\nu$ (third row) for SSS models. The first column shows CAL 83 with $B_p=10^{6}$ G using GL (solid) and DS (dashed) torques. The second column shows a higher-field CAL 83 sequence with $B_p=10^{7}$ G using GL. The third column shows parameter studies for RX J0019+2156 and RX J0925$-$4758 with low ($10^{-3}$ Hz; solid line) and high ($5\times10^{-2}$ Hz; dashed line) initial spins using GL. In the second row, blue, purple, pink, and orange curves denote the structural, electromagnetic, gravitational wave, and accretion torques, respectively.
• Figure 5: Evolution of the internal magnetic field $B_m$ (first panel), ellipticity $\epsilon$ (second panel), spin frequency $\nu$ (black) and obliquity angle $\chi$ (red; third panel), and characteristic GW strain $h_{\rm signal}$ (fourth panel) for CAL 83. Solid curves correspond to the fiducial model with initial magnetic field profile having maximum internal magnitude $B_m = 10^{12}$ G while dashed curves show a comparison model with initial $B_m = 10^{11}$ G.