Gravitational wave echoes from strange quark stars in the equation of state with density-dependent quark masses

According to the recent studies, the gravitational wave (GW) echoes are expected to be generated by quark stars composed of ultrastiff quark matter. The ultrastiff equations of state (EOS) for quark matter were usually obtained either by a simple bag model with artificially assigned sound velocity or by employing interacting strange quark matter (SQM) depicted by simple reparameterization and rescaling. In this study, we investigate GW echoes with EOSs for SQM in the framework of the equivparticle model with density-dependent quark masses and pairing effects. We conclude that strange quark stars (SQSs) can be sufficiently compact to possess a photon sphere capable of generating GW echoes with frequencies in the range of approximately 20 kHz. However, SQSs cannot account for the observed 72 Hz signal in GW170817 event. Furthermore, we determined that quark-pairing effects play a crucial role in enabling SQSs to satisfy the necessary conditions for producing these types of echoes.