The diverse morphology of gravitational wave signals from merging neutron-star white-dwarf binaries

TL;DR

The paper analyzes mass-transfer–driven evolution of neutron-star–white-dwarf binaries across a wide $M_{ m ns}$, $M_{ m wd}$, $e$, and accretion fraction $\alpha$ grid, revealing branched or polymorphic evolutionary pathways with distinct GW morphologies. It models white-dwarf structure, WD–NS mass transfer, disk-driven GW contributions, and orbital evolution via $\dot a$ and $f(q)$, identifying four merger regimes (G1–G4) and deriving polynomial boundaries in $M_{ m ns}$–$M_{ m wd}$ space. The study demonstrates that GW signals during Roche-lobe overflow are highly informative about binary configuration, including possible accretion-disk contributions, and provides waveform templates and SNR forecasts for space-based detectors (LISA, Taiji, TianQin, DECIGO). It further discusses multi-messenger implications, including FRB associations and EM transients, and explores EoS sensitivity by varying NS radii from 10 to 20 km. Together, these results enable informed search, classification, and interpretation of NSWD mergers in upcoming gravitational-wave observations.

Abstract

In sufficiently compact neutron star-white dwarf (NSWD) binary systems, orbital decay means the white dwarf eventually fills its shrinking Roche lobe, initiating a phase of mass transfer. The exchange of angular momentum-both internal and external-plays a critical role in determining the binary's evolutionary outcome. For neutron stars with relatively low magnetic fields and spin frequencies, whether the orbital separation continues to shrink depends on the interplay between gravitational wave (GW) radiation and mass transfer dynamics. We compute the orbital evolution of NSWD binaries across a broad parameter space, incorporating four key variables. Our results reveal distinct boundaries in the NS-WD mass-mass diagram: binaries with white dwarf masses above these thresholds undergo rapid orbital decay and direct coalescence. The dependence of these boundaries on system parameters indicates that Roche-lobe-filling NSWD binaries can follow multiple evolutionary pathways -- a phenomenon we refer to as branched or polymorphic evolution. NSWD binary systems emit strong and diverse GW signals, many of which would be detectable by space-based GW observatories. The morphology of the evolving GW waveform provides a direct diagnostic for the NSWD binary configuration, including any contribution from an accretion disk. Our models can provide critical waveform templates for identifying merging binary signals in real-time GW data.

Figures (7)

• Figure 1: Schematic illustration of the polymorphic late-stage evolution of compact NSWD binaries with varying component masses. The diagram shows four distinct evolutionary outcomes depending on the mass transfer rate ($\dot{M}$), the Eddington accretion rate ($\dot{M}_{\rm Edd}$), the orbital separation ($a$), and its rate of change ($\dot{a}$). The spherical coordinate system $(r, \theta, \phi)$ is defined with basis vectors $(n_{\rm r}, n_{\theta}, n_{\phi})$, and the orientation of the binary orbit is described by the polar and azimuthal angles $(\theta, \phi)$ in the Cartesian frame $(x, y, z)$. The rates of change of the WD and NS radii are denoted by $\dot{r}_{\rm wd}$ and $\dot{r}_{\rm ns}$, respectively.
• Figure 2: Distribution in $M_{\rm ns}$$-$$M_{\rm wd}$ (neutron star mass$-$white dwarf mass) parameter space of polymorphic evolution channels in the late-stage evolution of circular NSWD binaries ($e = 0.0$). Coloured regions represent the four distinct evolutionary outcomes: G1 (dark yellow), G2 (red), G3 (green), and G4 (blue). Computation grid points are indicated by crosses, red circles, green squares, and blue diamonds. Solid and dashed black lines delineate the boundaries between these groups. Observational data are overlaid: pulsar-white dwarf binaries in the Galactic disk and globular clusters are marked with $'+'$ and $'\star'$ symbols, respectively, while ultracompact X-ray binaries are shown as open black squares and triangles.
• Figure 3: As Fig. \ref{['fig2']}, but for different fixed values of the accretion fraction parameter $\alpha$. Panels (a)-(f) correspond to $\alpha = 0.0, 0.2, 0.4, 0.6, 0.8,$ and $1.0$, respectively. All computations assume circular orbits ($e = 0$). This figure illustrates how varying accretion efficiency influences the evolutionary outcomes of NSWD binaries.
• Figure 4: The evolution of gravitational wave (GW) signals generated by four NSWD binaries, each evolved from known pulsar--white dwarf systems, modelled under our fiducial assumptions. We assume that the pulsar component in each binary is a rapidly rotating neutron star. Top and bottom panels (labeled $\ast.1$ and $\ast.2$) show the evolution of the GW plus polarisation $h_+$ and its corresponding dynamic power spectral density (DPSD), respectively. The initial parameters for each binary are indicated in the lower panels. In each top panel, the red dotted line represents the GW signal contribution from the accretion disk surrounding the neutron star.
• Figure 5: As Fig. \ref{['fig4']}, except that all the mass loss from the WD component is assumed to be accreted by the NS component.