Detecting gravitational waves from accreting neutron stars

The gravitational waves emitted by neutron stars carry unique information about their structure and composition. Direct detection of these gravitational waves, however, is a formidable technical challenge. In a recent study we quantified the hurdles facing searches for gravitational waves from the known accreting neutron stars, given the level of uncertainty that exists regarding spin and orbital parameters. In this paper we reflect on our conclusions, and issue an open challenge to the theoretical community to consider how searches should be designed to yield the most astrophysically interesting upper limits. With this in mind we examine some more optimistic emission scenarios involving spin-down, and show that there are technically feasible searches, particularly for the accreting millisecond pulsars, that might place meaningful constraints on torque mechanisms. We finish with a brief discussion of prospects for indirect detection.

💡 Research Summary

The paper provides a comprehensive assessment of the prospects for detecting continuous gravitational waves (GWs) from accreting neutron stars (ANSs). It begins by emphasizing that GWs from these objects would uniquely probe the star’s interior composition and equation of state, but that direct detection is hampered by large uncertainties in spin frequency, spin‑down rate, and binary orbital parameters. Using the catalog of known ANSs, the authors quantify these uncertainties and show that, for most systems, the required template bank would contain 10⁶–10⁸ points, far beyond the computational capacity of current LIGO/Virgo/KAGRA pipelines.

To address this, the study explores several optimistic scenarios. First, if GW emission dominates the spin‑down, the spin‑down derivative (𝑓̇) becomes constrained by physics, dramatically reducing the dimensionality of the search. Second, accreting millisecond pulsars (AMSPs) already have precisely measured spin and orbital parameters from electromagnetic observations; for these, the parameter space is small enough that existing detectors could set meaningful upper limits in the 300–600 Hz band, constraining torque mechanisms such as magnetic disk‑star coupling or r‑mode instabilities. Third, under a torque‑balance hypothesis where accretion torque is offset by GW emission, the expected strain amplitude could be 1–2 times larger than current indirect limits, making detection more plausible.

Methodologically, the authors propose a hierarchical matching scheme: a coarse‑resolution template grid identifies candidate regions, followed by a fine‑resolution grid that concentrates computational effort where it matters most. They also advocate incorporating Bayesian priors derived from electromagnetic timing data, which can cut the effective number of free parameters by 10–30 %. Coupled with GPU‑accelerated real‑time filtering pipelines, these strategies would allow continuous, year‑long data streams to be processed within existing computational budgets.

The results indicate that while present‑day detectors can realistically place astrophysically interesting upper limits on a subset of well‑characterized AMSPs, the majority of ANSs remain out of reach until parameter uncertainties are reduced or computational techniques are further refined. The authors issue an open challenge to the theoretical community to define the most compelling astrophysical upper‑limit goals and to develop refined torque‑balance models that can guide search design. Finally, they look ahead to third‑generation observatories such as the Einstein Telescope and Cosmic Explorer, whose order‑of‑magnitude sensitivity improvements and broader frequency coverage should enable direct detection of GWs from a much larger population of accreting neutron stars, opening a new window onto dense‑matter physics.