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Colloquium: Multimessenger astronomy with continuous gravitational waves and future detectors

Benjamin J. Owen

TL;DR

This Colloquium surveys continuous gravitational waves from rapidly rotating neutron stars as a multimessenger frontier, detailing emission mechanisms (mass quadrupoles and r-modes), detector developments, and detectability across accreting NS, millisecond pulsars, and other populations. It outlines how the GW frequency, spin-down parameters, and distance measurements can constrain neutron star interior physics, including elasticity, internal magnetic fields, and the equation of state, especially when combined with electromagnetic observations. The work highlights the role of next-generation detectors like Cosmic Explorer and the Einstein Telescope in enabling multiple detections and in reshaping our understanding of neutron-star life cycles, even in cases of non-detection. Overall, continuous gravitational waves offer a unique window into extreme matter and will richly augment multimessenger astrophysics alongside forthcoming radio and high-energy observatories.

Abstract

Continuous gravitational waves from rapidly rotating neutron stars are on the new frontiers of gravitational wave astrophysics and have strong connections to electromagnetic astronomy, nuclear astrophysics, and condensed matter physics. In this Colloquium I survey prospects for detection of continuous gravitational waves from various neutron star populations, especially aided by electromagnetic observations. Although there are caveats, current theories and observations suggest that the first detections are likely within a few years, and that many are likely in the era of next generation detectors such as Cosmic Explorer and the Einstein Telescope. I also survey what can be learned from these signals, each one of which will contain more cycles than all the compact binary mergers ever detected. Since continuous gravitational wave emission mechanisms depend on aspects of neutron star physics, such as crustal elasticity, which are not well constrained by current astronomical observations and physical experiments, their detection can tell us a great deal that is new about extreme matter. Even more can be learned by combining gravitational wave observations with data from the Square Kilometre Array, the Next Generation Very Large Array, FAST, and other electromagnetic detectors operating in the next generation era.

Colloquium: Multimessenger astronomy with continuous gravitational waves and future detectors

TL;DR

This Colloquium surveys continuous gravitational waves from rapidly rotating neutron stars as a multimessenger frontier, detailing emission mechanisms (mass quadrupoles and r-modes), detector developments, and detectability across accreting NS, millisecond pulsars, and other populations. It outlines how the GW frequency, spin-down parameters, and distance measurements can constrain neutron star interior physics, including elasticity, internal magnetic fields, and the equation of state, especially when combined with electromagnetic observations. The work highlights the role of next-generation detectors like Cosmic Explorer and the Einstein Telescope in enabling multiple detections and in reshaping our understanding of neutron-star life cycles, even in cases of non-detection. Overall, continuous gravitational waves offer a unique window into extreme matter and will richly augment multimessenger astrophysics alongside forthcoming radio and high-energy observatories.

Abstract

Continuous gravitational waves from rapidly rotating neutron stars are on the new frontiers of gravitational wave astrophysics and have strong connections to electromagnetic astronomy, nuclear astrophysics, and condensed matter physics. In this Colloquium I survey prospects for detection of continuous gravitational waves from various neutron star populations, especially aided by electromagnetic observations. Although there are caveats, current theories and observations suggest that the first detections are likely within a few years, and that many are likely in the era of next generation detectors such as Cosmic Explorer and the Einstein Telescope. I also survey what can be learned from these signals, each one of which will contain more cycles than all the compact binary mergers ever detected. Since continuous gravitational wave emission mechanisms depend on aspects of neutron star physics, such as crustal elasticity, which are not well constrained by current astronomical observations and physical experiments, their detection can tell us a great deal that is new about extreme matter. Even more can be learned by combining gravitational wave observations with data from the Square Kilometre Array, the Next Generation Very Large Array, FAST, and other electromagnetic detectors operating in the next generation era.
Paper Structure (13 sections, 5 equations, 4 figures)

This paper contains 13 sections, 5 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Improving detectors
  3. Neutron stars
  4. Continuous gravitational waves
  5. Detectability
  6. Accreting neutron stars
  7. Millisecond pulsars
  8. Other populations
  9. What can be learned from a detection
  10. Frequency ratio
  11. Distance
  12. Spin-down parameters
  13. Summary and outlook

Figures (4)

  • Figure 1: Projected noise curves (one-sided strain noise amplitude spectral densities) of some near future and next generation detectors. For reference I have plotted the best current (O4) noise curve, from the LIGO Livingston Observatory Capote2025.
  • Figure 2: Projected detectability of the most rapidly accreting neutron stars. The slanted lines (Sco X-1 etc.) show the intrinsic strain $h_0$ as a function of the unknown spin frequency for the brightest few sources, assuming that accretion torque is balanced by gravitational wave emission. The curves show the sensitivities of various detector networks, assuming a conservative sensitivity depth of 39 Hz$^{-1/2}.$ Adapted from Owen2025.
  • Figure 3: Spin periods and period derivatives of millisecond pulsars, showing evidence for a quadrupole cutoff corresponding to a minimum ellipticity of about $10^{-9}.$ Reproduced from Woan2018 under https://creativecommons.org/licenses/by/3.0/.
  • Figure 4: Projected detectability of currently known millisecond pulsars. The sensitivity depth is assumed to be 500 Hz$^{-1/2}.$ Diamonds show intrinsic strain amplitudes of known pulsars assuming mass quadrupole emission at twice the spin frequency with an elliptcity of $10^{-9}$ or the spin-down limit, whichever is lower. Adapted from Owen2025.