Observational constraint on axion dark matter with gravitational waves

Most matter in the Universe is invisible and unknown, and is called dark matter. A candidate of dark matter is the axion, which is an ultralight particle motivated as a solution for the C P problem. Axions form clouds in a galactic halo, and amplify and delay a part of gravitational waves propagating in the clouds. The Milky Way is surrounded by the dark matter halo composed of a number of axion patches. Thus, the characteristic secondary gravitational waves are always expected right after the reported gravitational-wave signals from compact binary mergers. In this paper, we derive a realistic amplitude of the secondary gravitational waves. Then we search the gravitational waves having the characteristic time delay and duration with a method optimized for them. We find no significant signal. Assuming the axions are a dominant component of dark matter, we obtain the constraints on the axion coupling to the parity-violating sector of gravity for the mass range, [1.7 ×10^-13,8.5 ×10^-12] eV , which is at most ∼10 times stronger than that from Gravity Probe B.