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Register a new userProbing resonant excitations in exotic compact objects via gravitational waves
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Gravitational waves (GWs) from presumed binary black hole mergers are now being detected on a regular basis with the Advanced LIGO and Advanced Virgo interferometers. Exotic compact objects (ECOs) have been proposed that differ from Kerr black holes, and which could leave an imprint upon the GW signal in a variety of ways. Here we consider excitations of ECOs during inspiral, which may occur when the monotonically increasing GW frequency matches a resonant frequency of an exotic object. This causes orbital energy to be taken away, leading to a speed-up of the orbital phase evolution. We show that resonances with induced phase shifts $lesssim 10$ radians can be detectable with second-generation interferometers, using Bayesian model selection. We apply our methodology to detections in the GWTC-1 catalog from the first and second observing runs of Advanced LIGO and Advanced Virgo, finding consistency with the binary black hole nature of the sources.
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This paper reviews a phenomenological approach to the gravitational lensing by exotic objects such as the Ellis wormhole lens, where exotic lens objects may follow a non-standard form of the equation of state or may obey a modified gravity theory. A gravitational lens model is proposed in the inverse powers of the distance, such that the Schwarzschild lens and exotic lenses can be described in a unified manner as a one parameter family. As observational implications, the magnification, shear, photo-centroid motion and time delay in this lens model are discussed.
Dark matter could be composed of compact dark objects (CDOs). A close binary of CDOs orbiting in the interior of solar system bodies can be a loud source of gravitational waves (GWs) for the LIGO and VIRGO detectors. We perform the first search ever for this type of signal and rule out close binaries, with separations of order 300 m, orbiting near the center of the Sun with GW frequencies (twice the orbital frequency) between 50 and 550 Hz and CDO masses above $approx 10^{-9} M_odot$. This mass limit is eight orders of magnitude lower than the mass probed in a LIGO search at extra galactic distances.
Exotic compact objects (ECOs) have recently become an exciting research subject, since they are speculated to have a special response to the incident gravitational waves (GWs) that leads to GW echoes. We show that energy carried by GWs can easily cause the event horizon to form out of a static ECO --- leaving no echo signals towards spatial infinity. To show this, we use the ingoing Vaidya spacetime and take into account the back reaction due to incoming GWs. Demanding that an ECO does not collapse into a black hole puts an upper bound on the compactness of the ECO, at the cost of less distinct echo signals for smaller compactness. The trade-off between echoes detectability and distinguishability leads to a fine tuning of ECO parameters for LIGO to find distinct echoes. We also show that an extremely compact ECO that can survive the gravitational collapse and give rise to GW echoes might have to expand its surface in a non-causal way.
We present numerical waveforms of gravitational-wave echoes from spinning exotic compact objects (ECOs) that result from binary black hole coalescence. We obtain these echoes by solving the Teukolsky equation for the $psi_4$ associated with gravitational waves that propagate toward the horizon of a Kerr spacetime, and process the subsequent reflections of the horizon-going wave by the surface of the ECO, which lies right above the Kerr horizon. The trajectories of the infalling objects are modified from Kerr geodesics, such that the gravitational waves propagating toward future null infinity match those from merging black holes with comparable masses. In this way, the corresponding echoes approximate to those from comparable-mass mergers. For boundary conditions at the ECO surface, we adopt recent work using the membrane paradigm, which relates $psi_0$ associated with the horizon-going wave and $psi_4$ of the wave that leaves the ECO surface. We obtain $psi_0$ of the horizon-going wave from $psi_4$ using the Teukolsky-Starobinsky relation. The echoes we obtain turn out to be significantly weaker than those from previous studies that generate echo waveforms by modeling the ringdown part of binary black hole coalescence waveforms as originating from the past horizon.
Spinning horizonless compact objects may be unstable against an ergoregion instability. We investigate this mechanism for electromagnetic perturbations of ultracompact Kerr-like objects with a reflecting surface, extending previous (numerical and analytical) work limited to the scalar case. We derive an analytical result for the frequency and the instability time scale of unstable modes which is valid at small frequencies. We argue that our analysis can be directly extended to gravitational perturbations of exotic compact objects in the black-hole limit. The instability for electromagnetic and gravitational perturbations is generically stronger than in the scalar case and it requires larger absorption to be quenched. We argue that exotic compact objects with spin $chilesssim 0.7$ ($chilesssim 0.9$) should have an absorption coefficient of at least $0.3%$ ($6%$) to remain linearly stable, and that an absorption coefficient of at least $approx60%$ would quench the instability for any spin. We also show that - in the static limit - the scalar, electromagnetic, and gravitatonal perturbations of the Kerr metric are related to one another through Darboux transformations.
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