Observing gravitational waves from sub-solar-mass, inspiraling compact binaries would provide almost smoking-gun evidence for primordial black holes. Here, we develop a method to search for ultracompact binaries with chirp masses ranging from $\mathcal{M}\in [{10}^{-2},{10}^{-1}]M_{\odot }$. This mass range represents a previously unexplored gap in gravitational-wave searches for compact binaries: it was thought that the signals were too long for matched-filtering analyses but too short for time-frequency pattern-recognition techniques. Despite this, we show that a pattern-recognition technique, the generalized frequency-Hough transform, can be employed with particular modifications that allow us to handle rapidly spinning-up binaries and to increase the statistical robustness of our method; we call this improved method BinaryGFH-v2. We then design a hypothetical search for binaries in this mass regime, compare the empirical and theoretical sensitivities of this method, and project constraints on formation rate densities and the fraction of dark matter that primordial black holes could compose in both current- and future-generation gravitational-wave detectors. Our results show that our method can be used to search for sub-solar-mass, ultracompact objects in a mass regime that remains to-date unconstrained with gravitational waves.

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• Classical black holes
• Gravitational waves
• Particle dark matter
• Primordial black holes