Constraining asteroid-mass primordial black hole abundance using continuous gravitational waves

We present new constraints on the merging rates of planetary-mass and asteroid-mass primordial black hole binaries using limits on continuous gravitational waves (quasi-monochromatic, quasi-infinite duration signals) derived from an all-sky search for isolated compact objects in the third observing run of LIGO/Virgo. We calculate the merging rates of these binaries in a model-independent way, and convert them to constraints on the primordial black hole abundance with minimal modelling assumptions. Our results show that we are sensitive to sources at most O(10 pc) away for systems with chirp masses of O(10⁻⁵M_?) at gravitational-wave frequencies around 30-40 Hz. These results also show that continuous-wave searches could in the future directly probe the existence of planetary-mass and asteroid-mass primordial black holes, especially those in binaries with asymmetric mass ratios. Furthermore, they demonstrate that new methods accounting for the full nonlinear gravitational-wave frequency evolution are needed to improve constraints on primordial black holes.