A Search for Low-mass Neutron Stars in the Third Observing Run of Advanced LIGO and Virgo

Abstract Most observed neutron stars have masses around 1.4 M ⊙ , consistent with current formation mechanisms. To date, no subsolar-mass neutron star has been observed. Observing one would provide crucial constraints on the nuclear equation of state, unveil a new neutron star population, and advance understanding of their formation mechanisms. We present the first targeted search for tidally deformed subsolar-mass binary neutron stars (BNSs), with primary masses ranging from 0.1 to 2 M ⊙ and secondary masses from 0.1 to 1 M ⊙ , using data from the third observing run of the Advanced LIGO and Virgo gravitational-wave detectors. We account for the tidal deformabilities of up to O (10 4 ) of these systems, as low-mass neutron stars are more easily distorted by their companions. Previous searches that neglect tidal deformability lose sensitivity to low-mass sources, potentially missing more than ∼30% of detectable signals from a system with a chirp mass of 0.6 M ⊙ binaries. No statistically significant detections were made. In the absence of a detection, we place a 90% confidence upper limit on the local merger rate for subsolar BNSs, constraining it to be <6.4 × 10 4 Gpc −3 yr −1 for a chirp mass of 0.2 M ⊙ and <2.2 × 10 3 Gpc −3 yr −1 for 0.7 M ⊙ . With future upgrades to detector sensitivity, development of next-generation detectors, and ongoing improvements in search pipelines, constraints on the minimum mass of neutron stars will improve, providing the potential to constrain the nuclear equation of state, reveal new insights into neutron star formation channels, and potentially identify new classes of stars.