Gravitational wave echoes from strange stars

Abstract

It has recently been claimed, with a 4.2σ 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 photonsphere 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.

On gravitational echoes from ultracompact exotic stars

“…This result is in agreement with the findings of Ref. [35], that we complemented here with a comprehensive analysis of rotation.…”

“…11 shows that, for these light objects, the detectability of GW echoes must rely on an unphysical EoS as the physical branch ω 1 cannot sustain visible signals. As already noticed in [32] (see also [35]), a compactness extremely close to the Buchdahl limit (at least 10 −2 ) is indeed needed. We remark that this conclusion depends on the value of M chosen to compute the echo frequency In such a case, it would be possible to move the shaded region in Fig.…”

“…[32] considered constant-density stars (CDS) as a toy model for exotic stars capable of producing gravitational echoes -in this case stable configurations arbitrarily close to the Buchdahl limit exist. However -as already noticed in [32], and reiterated in [35] -a CDS represents an unphysical system because an incompressible fluid violates causality. The aim of this article is to investigate under which physical conditions, if any, gravitational echoes 1 Throughout this paper, we shall use a geometrized unit system with c = G N = 1.…”

Effect of scalar boson on fermionic dark stars

“…43 has shown that strange stars can emit gravitational wave echoes on the order of tens of kilohertz (see details in Ref. 43 and the references therein). It is obvious from the upper panel of Fig.…”

Constraining values of bag constant for strange star candidates

“…In the present paper we take quark EOS in general linear form and then we utilized MIT bag model as a special case. We have not used any specific value of bag constants rather we determine all the possible values of bag constants by studying compact 4 stars. The purpose of the paper is not only to provide possible range of B but also to present a general model for strange star with general linear quark EOS.…”