547 research outputs found
Intrinsic and extrinsic geometries of a tidally deformed black hole
A description of the event horizon of a perturbed Schwarzschild black hole is
provided in terms of the intrinsic and extrinsic geometries of the null
hypersurface. This description relies on a Gauss-Codazzi theory of null
hypersurfaces embedded in spacetime, which extends the standard theory of
spacelike and timelike hypersurfaces involving the first and second fundamental
forms. We show that the intrinsic geometry of the event horizon is invariant
under a reparameterization of the null generators, and that the extrinsic
geometry depends on the parameterization. Stated differently, we show that
while the extrinsic geometry depends on the choice of gauge, the intrinsic
geometry is gauge invariant. We apply the formalism to solutions to the vacuum
field equations that describe a tidally deformed black hole. In a first
instance we consider a slowly-varying, quadrupolar tidal field imposed on the
black hole, and in a second instance we examine the tide raised during a close
parabolic encounter between the black hole and a small orbiting body.Comment: 27 pages, 4 figure
Tidal response from scattering and the role of analytic continuation
The tidal response of a compact object is a key gravitational-wave observable encoding information about its interior. This link is subtle due to the nonlinearities of general relativity. We show that considering a scattering process bypasses challenges with potential ambiguities, as the tidal response is determined by the asymptotic in- and outgoing waves at null infinity. As an application of the general method, we analyze scalar waves scattering off a nonspinning black hole and demonstrate that the frequency-dependent tidal response calculated for arbitrary dimensions and multipoles reproduces known results for the Love number and absorption in limiting cases. In addition, we discuss the definition of the response based on gauge-invariant observables obtained from an effective action description, and clarify the role of analytic continuation for robustly (i) extracting the response and the physical information it contains, and (ii) distinguishing high-order post-Newtonian corrections from finite-size effects in a binary system. Our work is important for interpreting upcoming gravitational-wave data for subatomic physics of ultradense matter in neutron stars, probing black holes and gravity, and looking for beyond standard model fields
Relativistic effective action of dynamical gravitomagnetic tides for slowly rotating neutron stars
Gravitomagnetic quasi-normal modes of neutron stars are resonantly excited by tidal effects during a binary inspiral, leading to a potentially measurable effect in the gravitational wave signal. We take an important step towards incorporating these effects in waveform models by developing a relativistic effective action for the gravitomagnetic dynamics that clarifies a number of subtleties. Working in the slow-rotation limit, we first consider the post-Newtonian approximation and explicitly derive the effective action from the equations of motion. We demonstrate that this formulation opens a novel way to compute mode frequencies, yields insights into the relevant matter variables, and elucidates the role of a shift symmetry of the fluid properties under a displacement of the gravitomagnetic mode amplitudes. We then construct a fully relativistic action based on the symmetries and a power counting scheme. This action involves four coupling coefficients that depend on the internal structure of the neutron star and characterize the key matter parameters imprinted in the gravitational waves. We show that, after fixing one of the coefficients by normalization, the other three directly involve the two kinds of gravitomagnetic Love numbers (static and irrotational), and the mode frequencies. We discuss several interesting features and dynamical consequences of this action. Our results provide the foundation for deriving precision predictions of gravitomagnetic effects, and the nuclear physics they encode, for gravitational-wave astronomy
Spin effects on neutron star fundamental-mode dynamical tides: phenomenology and comparison to numerical simulations
Gravitational waves from neutron star binary inspirals contain information on strongly-interacting matter in unexplored, extreme regimes. Extracting this requires robust theoretical models of the signatures of matter in the gravitational-wave signals due to spin and tidal effects. In fact, spins can have a significant impact on the tidal excitation of the quasi-normal modes of a neutron star, which is not included in current state-of-the-art waveform models. We develop a simple approximate description that accounts for the Coriolis effect of spin on the tidal excitation of the neutron star's quadrupolar and octupolar fundamental quasi-normal modes and incorporate it in the SEOBNRv4T waveform model. We show that the Coriolis effect introduces only one new interaction term in an effective action in the co-rotating frame of the star, and fix the coefficient by considering the spin-induced shift in the resonance frequencies that has been computed numerically for the mode frequencies of rotating neutron stars in the literature. We investigate the impact of relativistic corrections due to the gravitational redshift and frame-dragging effects, and identify important directions where more detailed theoretical developments are needed in the future. Comparisons of our new model to numerical relativity simulations of double neutron star and neutron star-black hole binaries show improved consistency in the agreement compared to current models used in data analysis
Empiric Models of the Earth's Free Core Nutation
Free core nutation (FCN) is the main factor that limits the accuracy of the
modeling of the motion of Earth's rotational axis in the celestial coordinate
system. Several FCN models have been proposed. A comparative analysis is made
of the known models including the model proposed by the author. The use of the
FCN model is shown to substantially increase the accuracy of the modeling of
Earth's rotation. Furthermore, the FCN component extracted from the observed
motion of Earth's rotational axis is an important source for the study of the
shape and rotation of the Earth's core. A comparison of different FCN models
has shown that the proposed model is better than other models if used to
extract the geophysical signal (the amplitude and phase of FCN) from
observational data.Comment: 8 pages, 3 figures; minor update of the journal published versio
Detection of Phase Jumps of Free Core Nutation of the Earth and their Concurrence with Geomagnetic Jerks
We detected phase jumps of the Free Core Nutation (FCN) of the Earth directly
from the analysis of the Very Long Baseline Interferometer (VLBI) observation
of the Earth rotation for the period 1984-2003 by applying the Weighted Wavelet
Z-Transform (WWZ) method and the Short-time Periodogram with the Gabor function
(SPG) method. During the period, the FCN had two significant phase jumps in
1992 and 1998. These epochs coincide with the reported occurrence of
geomagnetic jerks.Comment: 8 pages, 4 figure
Unconventional Charge Ordering in Na0.70CoO2 below 300 K
We present the results of measurements of the dc-magnetic susceptibility
chi(T) and the 23Na-NMR response of Na_{0.70}CoO_{2} at temperatures between 50
and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an
effective configuration of Co^{3.4+}. The 23Na-NMR response reveals pronounced
anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is
found in chi(T). Our data suggest the onset of a dramatic change in the Co
3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our
results maybe interpreted as evidence for either a tendency to electron
localization or an unconventional charge-density wave phenomenon within the
cobalt oxide layer, CoO_2, 3d electron system near room temperature.Comment: 4 pages, 4 figures, re-submitted to Physical Review Letters. The
manuscript has been revised following the recommendations of the referees.
The discussion section contains substantial change
Aligned-spin neutron-star-black-hole waveform model based on the effective-one-body approach and numerical-relativity simulations
After the discovery of gravitational waves from binary black holes (BBHs) and binary neutron stars (BNSs) with the LIGO and Virgo detectors, neutron-star--black-holes (NSBHs) are the natural next class of binary systems to be observed. In this work, we develop a waveform model for aligned-spin neutron-star--black-holes (NSBHs) combining a binary black-hole baseline waveform (available in the effective-one-body approach) with a phenomenological description of tidal effects (extracted from numerical-relativity simulations), and correcting the amplitude during the late inspiral, merger and ringdown to account for the NS's tidal disruption. In particular, we calibrate the amplitude corrections using NSBH waveforms obtained with the numerical-relativity spectral Einstein code (SpEC) and the SACRA code. Based on the simulations used, and on checking that sensible waveforms are produced, we recommend our model to be employed with NS's mass in the range , tidal deformability 0\mbox{-}5000, and (dimensionless) BH's spin magnitude up to . We also validate our model against two new, highly accurate NSBH waveforms with BH's spin 0.9 and mass ratios 3 and 4, characterized by tidal disruption, produced with SpEC, and find very good agreement. Furthermore, we compute the unfaithfulness between waveforms from NSBH, BBH, and BNS systems, finding that it will be challenging for the advanced LIGO-Virgo--detector network at design sensitivity to distinguish different source classes. We perform a Bayesian parameter-estimation analysis on a synthetic numerical-relativity signal in zero noise to study parameter biases. Finally, we reanalyze GW170817, with the hypothesis that it is a NSBH. We do not find evidence to distinguish the BNS and NSBH hypotheses, however the posterior for the mass ratio is shifted to less equal masses under the NSBH hypothesis
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