Gravitational wave echoes from strange stars

It has recently been claimed, with a 4.2s significance level, that gravitational wave echoes at a frequency of about 72 Hz have been produced in the GW170817 event. The merging of compact stars can lead to the emission of gravitational wave echoes if the postmerger object features a photon sphere capable of partially trapping the gravitational waves. If the postmerger source is a black hole, a second internal reflection surface, associated with quantum effects near the black hole horizon, must be present to avoid the gravitational wave capture. Alternatively, gravitational wave echoes can be produced by ultracompact stars crossing the photon-sphere line in the mass-radius diagram during the neutron star merging. In this case, the second reflection surface is not needed. A recently proposed preliminary analysis using an incompressible (and so unphysical) equation of state suggests that gravitational wave echoes at a frequency of tens of hertz can be produced by an ultracompact star. Since strange stars are extremely compact, we examine the possibility that strange stars emit gravitational wave echoes at such a frequency. Using parametrized models of the equation of state of ultrastiff quark matter we find that a strange star can emit gravitational wave echoes, but the corresponding frequencies are on the order of tens of hilohertz, and thus not compatible with the 72 Hz signal.