Gravitational signals emitted by a point mass orbiting a neutron star: Effects of stellar structure

The right bottom panel of Fig. 4 in the main text refers to the Lorentz factor E = 1.01 and not to E = 1.5 as reported in the letter. For completeness, we show here the waveforms for both E = 1.01 and E = 1.5. All our conclusions remain unchanged. Is the Gravitational-Wave Ringdown a Probe of the Event Horizon? It is commonly believed that the ringdown signal from a binary coalescence provides a conclusive proof for the formation of an event horizon after the merger. This expectation is based on the assumption that the ringdown waveform at intermediate times is dominated by the quasinormal modes of the final object. We point out that this assumption should be taken with great care, and that very compact objects with a light ring will display a similar ringdown stage, even when their quasinormal-mode spectrum is completely different from that of a black hole. In other words, universal ringdown waveforms indicate the presence of light rings, rather than of horizons. Only precision observations of the late-time ringdown signal, where the differences in the quasinormalmode spectrum eventually show up, can be used to rule out exotic alternatives to black holes and to test quantum effects at the horizon scale.

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“…Fig. 2) which correspond to narrow resonances in the flux [37,38]. Right panels: the corresponding GW waveforms compared to the BH case.…”

mentioning

confidence: 99%

“…The spectra coincide only at low frequencies, but are generically very different. Furthermore, in the BH limit, the long-lived QNMs of the wormhole can be excited and correspond to narrow, Breit-Wigner resonances in the spectrum [37,38].…”

mentioning

confidence: 99%

Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?

2016

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“…A weakfield study should provide a reasonably accurate estimate of the orbital frequency at which the resonance occurs, but it can only provide a leading-order estimate of the radiated energy. A more precise characterization requires the numerical integration of the perturbation equations [67][68][69][70]. In a follow-up paper we will compare gravitational radiation from extreme mass-ratio inspirals in thin-shell gravastar space-times to the BH results of Refs.…”

Section: Discussionmentioning

confidence: 99%

Gravitational wave signatures of the absence of an event horizon: Nonradial oscillations of a thin-shell gravastar

et al. 2009

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Gravitational waves from compact objects provide information about their structure, probing deep into strong-gravity regions. Here we illustrate how the presence or absence of an event horizon can produce qualitative differences in the gravitational waves emitted by ultracompact objects. In order to set up a straw-man ultracompact object with no event horizon, but which is otherwise almost identical to a black hole, we consider a nonrotating thin-shell model inspired by Mazur and Mottola's gravastar, which has a Schwarzschild exterior, a de Sitter interior and an infinitely thin shell with finite tension separating the two regions. As viewed from the external space-time, the shell can be located arbitrarily close to the Schwarzschild radius, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We study the linearized dynamics of the system, and, in particular, the junction conditions connecting internal and external gravitational perturbations. As a first application of the formalism we compute polar and axial oscillation modes of a thin-shell gravastar. We show that the quasinormal mode spectrum is completely different from that of a black hole, even in the limit when the surface redshift becomes infinite. Polar quasinormal modes depend on the equation of state of matter on the shell and can be used to distinguish between different gravastar models. Our calculations suggest that low-compactness gravastars could be unstable when the sound speed on the shell v s =c * 0:92.

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“…For the sake of reference, the first frequencies of the polar part of the spectrum of the two stellar models considered are listed in Table I. They have been obtained using a frequencydomain code based on the classical Lindblom-Detweiler formulation of the perturbed Einstein equations [33] and described in [34].…”

Section: A Unperturbed Stellar Modelsmentioning

confidence: 99%

Accretion-driven gravitational radiation from nonrotating compact objects: Infalling quadrupolar shells

et al. 2004

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This paper reports results from numerical simulations of the gravitational radiation emitted from non-rotating compact objects (both neutron stars and Schwarzschild black holes) as a result of the accretion of matter. We adopt a hybrid procedure in which we evolve numerically, and assuming axisymmetry, the linearized equations describing metric and fluid perturbations coupled to a fully nonlinear hydrodynamics code that calculates the motion of the accreting matter. The initial matter distribution, which is initially at rest, is shaped in the form of extended quadrupolar shells of either dust or obeying a perfect fluid equation of state. Self-gravity of the accreting layers of fluid is neglected, as well as radiation reaction effects. We use this idealized setup in order to understand the qualitative features appearing in the energy spectrum of the gravitational wave emission from compact stars or black holes, subject to accretion processes involving extended objects. A comparison for the case of point-like particles falling radially onto black holes is also provided. Our results show that, when the central object is a black hole, the spectrum is far from having only one clear, monochromatic peak at the frequency of the fundamental quasi-normal mode. On the contrary, it shows a complex pattern, with distinctive interference fringes produced by the interaction between the infalling matter and the underlying perturbed spacetime, in close agreement with results for point-like particles. Remarkably, most of the energy is emitted at frequencies lower than that of the fundamental mode of the black hole. Similar results are obtained for extended shells accreting onto neutron stars, but in this case the contribution of the stellar fundamental mode stands clearly in the energy spectrum. Our analysis illustrates that the gravitational wave signal driven by accretion onto compact objects is influenced more by the details and dynamics of the process, and the external distribution of matter, than by the quasi-normal mode structure of the central object. The gravitational waveforms from such accretion events appear to be much more complex than former simplified assumptions predicted.

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Gravitational signals emitted by a point mass orbiting a neutron star: Effects of stellar structure

Cited by 55 publications

References 18 publications

Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?

Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?

Gravitational wave signatures of the absence of an event horizon: Nonradial oscillations of a thin-shell gravastar

Accretion-driven gravitational radiation from nonrotating compact objects: Infalling quadrupolar shells

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