Gravitational wave echoes from strange stars

TL;DR

This paper investigates whether strange stars can produce gravitational wave echoes at frequencies around 72 Hz, concluding that their natural frequencies are much higher, around tens of kHz, and thus unlikely to explain the observed signal.

Contribution

The study models strange stars with ultra-stiff quark matter equations of state to assess their potential to generate low-frequency gravitational wave echoes.

Findings

Strange stars emit gravitational wave echoes at tens of kHz frequencies.

The predicted echo frequencies are incompatible with the 72 Hz signal observed.

Strange stars are unlikely sources of the gravitational wave echoes at 72 Hz.

Abstract

It has recently been claimed, with a $4.2 \sigma$ 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 waves echoes if the post-merger object features a photon-sphere capable of partially trapping the gravitational waves. If the post-merger source is a black hole, a second internal reflection surface, associated to 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 Hz 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 parameterized models of the equation of state of ultra-stiff quark matter we find that a strange star can emit gravitational wave echoes, but the corresponding frequencies are of the order of tens of kHz, thus not compatible with the $72$ Hz signal.