Gravitational signatures of axion dark matter via parity-violating interactions

We investigate axionlike particles coupled to gravity through a parity-violating Chern-Simons (CS) interaction. In this framework, axion dark matter (DM) can decay into pairs of circularly polarized gravitons, producing a persistent, nearly monochromatic gravitational wave signal. We compute the expected signal at Earth assuming a Navarro-Frenk-White Galactic halo model with the corresponding velocity distribution, and compare it with the narrow-band sensitivities of the LIGO O4 run and the projected reach of the Einstein Telescope. The resulting bounds on the axion-graviton coupling alpha improve upon the cosmological stability requirement for axion masses m phi less than or similar to 10-11 eV, excluding values up to four orders of magnitude below the stability limit. This constitutes a robust direct terrestrial constraint on the axion-gravity CS coupling. We also discuss distinctive observational signatures, such as circular polarization asymmetries, annual modulation, and potential enhancements from DM substructures, which could serve as smoking-gun evidence for parity-violating gravitational interactions.