Thermal Deformations in Super-Eddington Magnetized Neutron Stars: Implications for Continuous Gravitational-Wave Detectability

TL;DR

The paper investigates thermal deformations in magnetized neutron stars undergoing super-Eddington column accretion and their potential to emit continuous gravitational waves. By modeling the accretion column, computing magnetically induced crustal temperature perturbations, and deriving the resulting mass quadrupole $Q_{22}$ and ellipticity $\\epsilon \approx 2 \times 10^{-7} B_{12}$, the authors connect accretion physics to GW observables. They show that Galactic NS ULXs with spin periods $P \\lesssim 20$ ms could be detectable by next-generation detectors like the Einstein Telescope and Cosmic Explorer, while LIGO O5 could probe $P \\lesssim 6$ ms, providing a new channel to study NS crusts. The work highlights a promising class of CGW sources that links crustal microphysics with accretion dynamics and GW astronomy, motivating targeted searches and refined modeling with future detectors.

Abstract

Rapidly rotating neutron stars (NSs) are promising targets for continuous gravitational-wave (CGW) searches with current and next-generation ground-based GW detectors. In this Letter, we present the first study of thermal deformations in super-Eddington magnetized NSs with column accretion, where magnetic fields induce anisotropic heat conduction that leads to crustal temperature asymmetries. We compute the resulting mass quadrupole moments and estimate the associated CGW strain amplitudes. Our results show that Galactic magnetized NSs undergoing super-Eddington column accretion can emit detectable CGWs in upcoming observatories. Assuming a 2-yr coherent integration, the Einstein Telescope and Cosmic Explorer could detect such CGW signals from rapidly spinning NSs with spin periods $P \lesssim 20\,\rm ms$, while the LIGO O5 run may detect systems with $P \lesssim 6 \,{\rm ms}$. These findings suggest that super-Eddington magnetized NSs could represent a new class of CGW sources, providing a unique opportunity to probe the NS crust and bridge accretion physics with GW astronomy.

Figures (3)

• Figure 1: Profiles of physical quantities within the accretion column above super-Eddington magnetized NSs as functions of height. Different colors indicate different mass accretion rates. Note that we adopt a typical NS model with $M = 1.4 \, M_{\odot}$ and $R = 10^{6}\, \rm cm$. The left panels present the radiation energy density and mass density, while the right panels show the corresponding pressure and temperature profiles. The characteristic temperature, $T_{\rm C}$, is calculated via $T_{\rm C} = (\rho \varepsilon/a)^{1/4}$, where $a$ is the radiation constant. Assuming a surface magnetic field strength of $B = 10^{12}\, \rm G$, we define the normalization parameters as $\varepsilon_{0} = GM/R$, $\rho_{0}=3 B^2 R/ (8 \pi G M)$, and $p_0 =B^2/ (8\pi)$.
• Figure 2: Temperature perturbations in the crust of ULXPs for different magnetic field strengths, assuming a fixed accretion rate of $\dot{M} = 5 \times 10^{18}\, \rm g\, s^{-1}$.
• Figure 3: GW strain amplitude as a function of the frequency. Open blue squares represent predicted Galactic NS ULXs with spin periods of ${P = 20 \,{\rm ms}}$, ${P = 12.5 \,{\rm ms}}$, ${P =6\,{\rm ms}}$, ${P =5\,{\rm ms}}$, and ${P =3\,{\rm ms}}$. The filled red star marks a predicted Galactic NS ULX system with PSR J1928$+$1815-like spin period of ${P = 10.55 \,{\rm ms}}$. All these predicted NS ULXs are assumed to be located at a distance of $d = 8\, {\rm kpc}$. Sensitivity curves for LIGO O5, ET, and CE assume a coherent integration time of two years Watts:2008qw.