Gravitational wave echoes from quark stars

It has recently been reported that gravitational wave echoes at a frequency of about 72 Hz have been detected with the significance of 4.2σ from two neutron star merger event GW170817. Alternatively, the echoes of a gravitational wave can be obtained from ultra-compact stars, i.e., stars with compactness between 1/3 and 4/9. The mass-radius relation of ultra-compact stars is crossing the photon sphere line, where the line obeys the relation compactness equal to 1/3. Furthermore, it has recently been reported that strange stars within a simplest MIT-Bag model with an ultra-stiff equation of state (EoS) where the speed of sound equal to speed of light in vacuum can produce gravitational wave echoes with frequencies on the order of tens of kilohertz. For more realistic models of strange star equation of state, the speed of sound in the matter is physically less than one, and it should be density-dependent. In this report, we will check whether the more realistic EoS of quark stars can emit gravitational wave echoes or not. Here we employ two models of EoSs such as MIT-Bag models with speed of sound =1/√3 and CIDDM models with the density-dependent speed of sound.