Bridging the chirp-mass gap: searching for gravitational waves from inspiraling ultra-compact binaries using LIGO O4ab data

Gravitational-wave searches for subsolar-mass compact binaries have left a gap between the planetary-mass regime accessible to long-duration searches and the higher masses probed by conventional compact-binary analyses. We close this gap by searching for the inspiral of ultra-compact binaries with chirp masses ${10}^{-2}$–${10}^{-1}{M}_{\odot }$ in LIGO O4ab data. Using the statistically robust BinaryGFH-v2 method, a time-frequency technique to search for rapidly evolving inspirals lasting minutes to hours, we cover this previously challenging parameter space with a computational cost of only $\sim 350$ CPU-hours, and find no significant candidates. We thus set 90% confidence upper limits on the binary merger-rate density, providing the first gravitational-wave constraints across this chirp-mass range. Assuming that primordial-black-hole binaries formed in the early Universe with no binary suppression ($f_{\mathrm{s}\mathrm{u}\mathrm{p}}=1$), these results provide the first gravitational-wave constraints on the primordial-black-hole \dmh fraction in this mass range, yielding upper limits with $f_{\mathrm{P}\mathrm{B}\mathrm{H}}<1$ for equal-mass binaries with component masses ${10}^{-2}$–${10}^{-1}{M}_{\odot }$ and asymmetric binaries with secondary masses $m_2=3\times {10}^{-4}$–${10}^{-2}{M}_{\odot }$ for a primary mass $m_1=2.5M_{\odot }$.