arXiv subjects

Zhong-Wu Xia

Publications and source records attributed to Zhong-Wu Xia.

8 recordsLinked to original sources

arXiv2026

Stationary scalar clouds around a rotating Kalb-Ramond BTZ black hole

We investigate the scalar clouds around a rotating Kalb-Ramond (KR) BTZ black hole under Robin boundary conditions. The clouds are obtained as stationary bound states at the superradiant threshold $ω=mΩ_H$, where the KR parameter, the rotation and the Robin boundary jointly determine their existence. It is shown that the KR parameter qualitatively changes the existence lines of clouds. For a nonpositive KR parameter, the lines remain monotonic, whereas for a positive KR parameter they can become nonmonotonic, so that a fixed boundary condition may admit clouds in disconnected regions of parameter space. Quasinormal modes (QNMs) and horizon fluxes are further used as consistency checks, confirming that the cloud solutions correspond to non-damping modes at the superradiant threshold where the energy flux changes sign. The KR parameter also shifts the critical Robin parameter at which the clouds exist. These results establish stationary scalar clouds as sensitive probes of the interplay between the Robin boundary conditions and KR gravity.

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arXiv2026

Quasinormal modes and AdS/CFT correspondence of a rotating BTZ-like black hole in the Einstein-bumblebee gravity

We obtain exact expressions for the quasinormal modes (QNMs) of the massive scalar, fermionic and vector perturbations around a rotating BTZ-like black hole in the Einstein-bumblebee gravity. We find that the Lorentz symmetry breaking (LSB) parameter $\ell$ leaves its imprint only on the imaginary parts of the quasinormal frequencies and the corresponding perturbation field decays more slowly for a larger $\ell$, except for the left-moving quasinormal frequencies $ω_L$ with positive mass and the right-moving ones $ω_R$ with negative mass for the fundamental modes under the vector perturbation where the imaginary parts are independent of $\ell$. We also note that, regardless of the kind of perturbations, the real parts depend only on the angular quantum number, which are the same as those in the standard BTZ black hole. Furthermore, we investigate the AdS/CFT correspondence from the QNMs and observe that the expected universal relation for the left and right conformal weights ($h_L,h_R$) of the boundary operators dual to various fields still holds even for the BTZ-like black hole in the Einstein-bumblebee gravity. These results strongly support the AdS/CFT correspondence and could help us better understand the Einstein-bumblebee gravity with the Lorentz symmetry violation.

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arXiv2026

Bayesian inference for tidal heating with extreme mass ratio inspirals

Extreme mass ratio inspirals (EMRIs) provide unique probes of near-horizon dissipation through the tidal heating. We present a full Bayesian analysis of tidal heating in equatorial eccentric EMRIs by performing injection-recovery studies and inferring posterior constraints on the reflectivity parameter $|\mathcal{R}|^2$ while sampling in the full EMRI parameter space. We find that in the strong-field regime the posterior uncertainties are smaller, indicating a stronger constraining capability on the tidal heating. Using two-year signals with an optimal signal-to-noise ratio (SNR) of $ρ=50$, EMRIs can put bounds on $|\mathcal{R}|^2$ at the level of $10^{-3}$--$ 10^{-4}$ for a rapidly spinning central object. Moreover, we show that neglecting the tidal heating can induce clear systematic biases in the intrinsic parameters of the EMRI system. These results establish EMRIs as promising precision probes for detecting and constraining black hole event horizons.

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arXiv2025

Scalar perturbation around a rotating Kalb-Ramond BTZ black hole

We investigate the scalar perturbation of a newly proposed Kalb-Ramond (KR) BTZ-like black hole. After the separation of variables for the Klein-Gordon equation, we find that the radial part reduces to the general Heun equation. Using the Heun function, we compute quasinormal modes (QNMs) subject to generic Robin boundary conditions, which shows that the KR parameter substantially modifies the QNM spectrum and only the fundamental mode on the left branch has an instability. To ascertain whether the instability is superradiant, we further analyze how the KR field changes the energy and angular momentum fluxes. Our results show that the KR parameter shifts the threshold and the range of the Robin coupling parameter where the superradiance occurs, underscoring the importance of the KR field in modeling black hole perturbations.

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arXiv2025

Probing Kalb-Ramond field with extreme mass ratio inspirals

The extreme-mass-ratio inspirals (EMRIs) are emerging as precision laboratories for testing the gravity beyond general relativity. In this work, we investigate the Lorentz symmetry breaking (LSB) effect induced by the Kalb-Ramond (KR) field on the gravitational waveforms from the EMRI system. We observe that the LSB parameter $l$ appears in the leading order for the corrections of energy and angular momentum fluxes, and as $|l|$ increases, the differences in EMRI waveforms between the KR black hole and Schwarzschild black hole become more pronounced. We note that the LSB effect becomes detectable by LISA for values of $|l|\sim 10^{-6}$ with a one-year observation period. Furthermore, we use the Fisher information matrix (FIM) approach for the parameter estimation and find the detection error for $l$ can be constrained to $Δl \sim 10^{-5}$ at $\mathrm{SNR} = 20$, demonstrating the potential of space-based gravitational wave detectors to rigorously test the KR field.

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arXiv2025

Echoes and quasi-normal modes of perturbations around Schwarzchild traversable wormholes

We investigate the waveforms and quasi-normal modes around Schwarzschild traversable wormholes under different field perturbations, including the scalar field, the electromagnetic (vector) field, and the axial gravitational (tensor) field perturbations. Our results indicate that under the influence of the matter at the throat of the wormhole, a Dirac $δ$-function distribution of matter appears in the effective potentials of the scalar and axial gravitational perturbations and it affects the propagation of these two types of perturbations in spacetime. However, the matter at the throat has no influence on the propagation of electromagnetic perturbations. Furthermore, we quantify the impact of throat matter on both the perturbation waveforms and quasi-normal modes for all three field types. Through comparative studies between Schwarzschild traversable wormholes and Schwarzschild black holes, we identify two distinct features. Firstly, the perturbation waveforms exhibit echoes and damping oscillations around wormholes, whereas they solely display damping oscillations around black holes. Secondly, the difference between the adjacent peaks varies with the mass parameter and the throat radial coordinate in the waveform around Schwarzschild traversable wormholes, while a constant peak spacing occurs, which is determined solely by mass, in the waveform around Schwarzschild black holes. Based on these findings, we propose a framework to estimate the mass parameter and throat radial coordinate of Schwarzschild traversable wormholes through waveforms and quasi-normal modes. Our analyses provide a more profound comprehension of the inherent characteristics of Schwarzschild traversable wormholes.

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arXiv2024

Gravitational waves from extreme mass ratio inspirals in Kerr-MOG spacetimes

This work elaborates on a detailed analysis of the novel characteristics of gravitational waves (GWs) generated by extreme mass ratio inspirals (EMRIs) within the framework of modified gravity (MOG). Our study begins by exploring the geometrical and dynamical properties of the Kerr-MOG spacetime. We employ the numerical kludge (NK) method for waveform simulations and reveal that the parameter $α$, representing deviations from general relativity (GR), significantly impacts the frequencies of geodesic orbits and, consequently, the EMRI waveforms. However, the waveform confusion problem remains mainly unresolved, posing a challenge in distinguishing between the underlying gravitational theories based on the observed EMRI waveforms. Notably, by incorporating the effects of radiation reaction, we observe a substantial reduction in the waveform overlap over time. This reduction could enhance our ability to discern between different waveforms over an extended period.

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arXiv2024

Scalar fields around a rotating loop quantum gravity black hole: Waveform, quasi-normal modes and superradiance

The rotating loop quantum gravity black hole is a newly proposed non-singular black hole, which eliminates spacetime singularities when a regularization parameter is introduced through loop quantum corrections. This parameter is expected to give rise to observable effects. In this paper, the dynamical behavior of a scalar field near a rotating loop quantum gravity black hole is investigated. Given a small initial perturbation, we obtain the waveform of massless scalar fields evolving over time. By analyzing the waveform, we find that the regularization parameter only affects the damping oscillation of waveform, but not the initial outburst and late-time tail stages. This behavior is characterized by quasi-normal modes. Under scalar field perturbations, the loop quantum black holes remain stable. Moreover, we calculate the quasi-normal modes of massive scalar fields by three numerical methods, which are the Prony, WKB, and shooting methods, respectively. Our results indicate that the real part of quasi-normal modes depends only on the regularization parameter, while the imaginary part does not only on the regularization parameter but also on the angular momentum. Finally, we study the amplification effect of rotating black holes, i.e., the superradiance. Our analyses indicate the existence of stronger superradiance around loop quantum gravity black holes compared to Kerr ones.

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