Inferring tidal deformability in the Black Hole compactness limit

One of the distinguishing features between Black Holes and other categories of astrophysical compact objects is that the gravitational multipole moments of nonrotating Black Holes are unaffected by the strong tidal forces from a binary companion near their gravitational-wave induced merger.

The induced tidal deformation, being dependent on the compactness of the astrophysical object, is generally distinct between Black Holes and other astrophysical objects (such as Neutron Stars), but has for certain classes of uniform-density ultracompact stars (Schwarzschild stars) been shown to follow an Equation of State with continuous support up to the Black Hole compactness limit.

Assuming that the population of Binary Black Holes, as observed in Gravitational Waves by LIGO/Virgo/KAGRA, instead can be described as binaries of these Schwarzschild stars we present strong constraints on their compactness and tidal deformability, both as individual objects and for the population at large.

Through these constraints, we comment on the viability of other categories of Black Hole mimickers as constituents of the ultracompact-object binaries observed so far.