Using Equation of State Constraints to Classify Low-Mass Compact Binary Mergers

Compact objects observed via gravitational waves are classified as blackholes or neutron stars primarily based on their inferred mass with respect tostellar evolution expectations. However, astrophysical expectations for thelowest mass range, ≲ 1.2 M_⊙, are uncertain. If such low-masscompact objects exist, ground-based gravitational wave detectors may observethem in binary mergers. Lacking astrophysical expectations for classifying suchobservations, we go beyond the mass and explore the role of tidal effects. Weevaluate how combined mass and tidal inference can inform whether each binarycomponent is a black hole or a neutron star based on consistency with thesupranuclear-density equation of state. Low-mass neutron stars experience alarge tidal deformation; its observational identification (or lack thereof) cantherefore aid in determining the nature of the binary components. Usingsimulated data, we find that the presence of a sub-solar mass neutron star(black hole) can be established with odds ∼ 100:1 when two neutron stars(black holes) merge and emit gravitational waves at signal-to-noise ratio ∼20. For the same systems, the absence of a black hole (neutron star) can beestablished with odds ∼ 10:1. For mixed neutron star-black hole binaries,we can establish that the system contains a neutron star with odds ≳5:1. Establishing the presence of a black hole in mixed neutron star-blackhole binaries is more challenging, except for the case of a ≲1 M_⊙ black hole with a ≳ 1 M_⊙ neutron star companion.On the other hand, classifying each individual binary component suffers from aninherent labeling ambiguity.