A recent study explores how gravitational wave echoes, potentially observed after neutron star mergers like GW170817, can reveal properties of exotic compact stars. These echoes, with a significance of 4.2σ and a dominant frequency near 72 Hz, could originate from ultracompact remnants possessing photon spheres that partially trap gravitational perturbations. While standard General Relativity imposes strict limits on stellar compactness for photon sphere formation, this work investigates the possibility within the framework of quadratic curvature gravity coupled to matter.

The researchers employed a modified gravity model, considering strange stars described by the MIT bag model equation of state. By solving the modified Tolman-Oppenheimer-Volkoff equations, they obtained mass-radius relations and identified configurations capable of supporting photon spheres and, consequently, producing gravitational wave echoes. The proposed framework allows for more compact stellar solutions than the Buchdahl limit of General Relativity, although photon sphere constraints limit the viable parameter space.

Results indicate that increasing the bag constant decreases the maximum mass and echo time, shifting the echo frequency towards the kHz regime. The constraints imposed by gravitational wave echoes are found to be more stringent for the maximum mass and radius of strange stars than those derived from hydrostatic equilibrium. This suggests a revision of the maximum mass bounds for these stars. The study highlights the potential of post-merger strange stars as sources of gravitational wave echoes and demonstrates the role of these echoes as probes of modified gravity and high-frequency gravitational waves.