arXiv subjects

Mou Xu

Publications and source records attributed to Mou Xu.

7 recordsLinked to original sources

arXiv2026

Photon Motion and Shadows of Rotating Black Holes with Nonlinear Electromagnetic and Anisotropic Matter Fields

This paper investigates the effects of the nonlinear electromagnetic field and the anisotropic matter field on photon motion, shadow structures, and the energy emission rate of a rotating black hole (BH). Using the Hamilton-Jacobi formalism, we derive the photon motion equations and analyze the distribution and stability of photon regions. The results show that the anisotropic matter field parameters affect the size and shape of the photon region outside the event horizon more significantly than the nonlinear electromagnetic field parameter. As the anisotropic matter field parameter $K$ decreases, the unstable photon region outside the BH gradually expands and becomes increasingly flattened. Furthermore, we construct the BH shadow in terms of the celestial coordinates and obtain the corresponding shadow images by backward ray tracing. Several shadow observables, including the shadow radius, distortion parameter, shadow area, and oblateness, are also analyzed. The results indicate that the anisotropic matter field affects the shadow size more strongly than the shadow shape. Specifically, the shadow radius and area both decrease significantly as the parameter $K$ decreases, but increase markedly as the anisotropic matter state parameter $\omega$ increases. In addition, we analyze the energy emission rate of the BH and find that decreasing $K$ or increasing magnetic charge $Q$ suppresses its peak value, while the influence of $\omega$ remains comparatively mild. These results provide a useful reference for understanding the effects of nonlinear electromagnetic and anisotropic matter fields on rotating BH shadows and related observational signatures.

gr-qc↗

arXiv2025

Constraint on magnetized black bounce spacetime from HFQPOs data and the selection of resonance models via information criterion

This paper primarily explores the dynamics of charged particle in the magnetized SV spacetime, and constrains the parameters of the SV spacetime along with its surrounding magnetic fields. The constraints are given by using $\chi^2$ analysis combined with high-frequency quasi-periodic oscillation (HFQPO) data observed from three microquasars: GRS 1915+105, XTE 1550-564, and GRO J1655-40. The results indicate that the magnetic field significantly influences the position of the innermost stable circular orbit of charged particle and frequency distribution of epicyclic motion, which excites more resonance model variants, enhancing observational effects. Additionally, we employ the Akaike Information Criterion (AIC) to evaluate resonance model and its various variants. The support for different models from observational data shows significant variation: $E R_8$ as the best model is supported strongly, $ER_3$ model has moderate evidence of support, $ER_6$ and $ER_7$ models are considerably less support, while other resonance models have essentially no support. For models more supported by the observational data, the allowed ranges of the regularization parameter: $0\leq a<0.736$ ($68\%$ confidence level) suggests that HFQPOs data support the magnetized black bounce spacetime as a regular black hole, and the smaller value of the regularization parameter indicates a possibility of the presence of quantum effects. According to the constraint results, we get the best-fit values of magnetic field strength around $10^{-5}\sim 10^{-4}$ GS for electrons and around $10^{-2}\sim 10^{-1}$ GS for protons. Finally, as a comparison, we test the SV spacetime without a magnetic field using microquasar observational data, and the calculated results of AIC show that this case is incompatible with the HFQPOs data, further supporting the existence of a magnetic field in SV spacetime.

astro-ph.HE↗

arXiv2025

Particle dynamics and optical appearance of charged spherically symmetric black holes in bumblebee gravity

In this paper, we study the particle dynamics, shadow, and optical appearance of charged black holes (BHs) in bumblebee gravity. Firstly, we find that the Lorentz-violation parameter l and charge parameter Q have opposite effects on the peak of the effective potential by analyzing timelike geodesics, and the radius of the innermost stable circular orbit (ISCO) decreases as the BH parameters l and Q increase. We also explore the behaviors of particle energy, angular momentum, and Keplerian frequency. Secondly, for null geodesics, both the photon sphere radius and the shadow radius decrease with increasing l and Q, and are consistently smaller than those of the Reissner-Nordstrom black hole (RNBH) and Schwarzschild-like BH. And based on observational data reported by the Event Horizon Telescope (EHT) Collaboration, we constrain the parameters l and Q by using the shadow radius data of Sgr A*. Thirdly, we explore the observation characteristics of charged BHs under three thin disk accretion models. The results show that, compared to RNBH, the increase of l leads to a greater thickness of the photon rings and lensed rings. However, due to their extremely narrow ranges, the contributions are small, and the observed intensities are mainly contributed by the direct emissions. Moreover, when the parameter l is fixed, the peaks of the observed intensities of rings decrease with increasing Q for the same emission model, and it is always lower than the corresponding value for Schwarzschild-like BH. These findings contribute to distinguishing bumblebee charged black holes (BCBHs) from other types of BHs based on their optical appearance.

gr-qc↗

arXiv2025

Optical Appearance and Shadow of Kalb-Ramond Black Hole: Effects of Plasma and Accretion Models

In this paper, we study the effect of the presence of plasma and different accretion models on the shadow and optical appearance of static spherically symmetric black holes containing the Kalb-Ramond field. We derive the motion equations for photons around the Kalb-Ramond black hole and constrain the Lorentz symmetry breaking parameters $\lambda$ and $\gamma$ using observational data released by the Event Horizon Telescope collaboration. The results indicate that, under the static spherical accretion model, as $\lambda$ or $\gamma$ increase, the peak value of the observed intensity for Kalb-Ramond black holes is enhanced and consistently exceeds that of the corresponding Schwarzschild black holes. In the presence of plasma, we find that as the plasma frequency increases, the photon sphere radius increases, whereas the black hole shadow radius decreases. In addition, compared to inhomogeneous plasma, the effect of homogeneous plasma on these features is more significant. Specifically, when the plasma is homogeneous, an increase in plasma frequency further enhances the observed intensity peaks. This suggests that the shadow of the Kalb-Ramond black hole is brighter due to the presence of plasma. Additionally, for the same Kalb-Ramond black hole model parameters and plasma frequency, the shadow of the Kalb-Ramond black hole in inhomogeneous plasma is larger than in the case of homogeneous plasma. Under the thin disk accretion model, an increase in the Lorentz symmetry breaking parameters decreases the observed intensity peak and increases the thickness of the photon ring and the lensed ring. The presence of plasma significantly alters the optical appearance of Kalb-Ramond black hole, providing a possible way to distinguish Kalb-Ramond black hole from Schwarzschild black hole.

gr-qc↗

arXiv2025

Observational constraints on the Kerr and its several single-parameter modified spacetimes using quasi-periodic oscillation data

This paper investigates the dynamical effects of particles moving in the Kerr spacetime and its nine single-parameter modified spacetimes, including Bardeen, Ayon-Beato and Garcia (ABG), Hayward, Kerr-Newman (KN), Kerr-Taub-NUT (KTN), Braneworld Kerr (BK), Kerr-MOG, Kerr-Sen, and Perfect Fluid Dark Matter (PFDM) black holes. Using quasi-periodic oscillation (QPO) observational data, we constrain the free parameters of the ten spacetimes through $\chi^2$ analysis under the relativistic precession model of QPO. We constrain the modification parameters for the nine single-parameter modified spacetimes and provide the spin and mass ranges of three microquasars within the ten spacetime models (including Kerr) at the $68\%$ confidence level (CL). The results demonstrate that, at the $68 \%$ CL, the QPO data impose stringent constraints on the free parameters, as evidenced by the narrow confidence intervals. Among them, only the KN spacetime yields a modification parameter constraint spanning both negative and positive values (encompassing the Kerr case at zero). In contrast, all other tested geometries mandate positive-definite parameters at $68 \%$ CL, demonstrating statistical deviation of the Kerr solution. This highlights the significance of exploring modifications to the Kerr spacetime. Finally, we evaluate the spacetime models using the Bayes factor and the Akaike Information Criterion (AIC). Based on the current QPO observational data, the Bayesian factor analysis indicates that the ABG, Hayward, KN, BK, and Kerr-MOG spacetime have a slight advantage over the Kerr solution, while the Bardeen, KTN, Kerr-Sen, and PFDM spacetime are somewhat inferior to the Kerr model. In contrast, the AIC analysis shows that the Kerr spacetime remains the optimal model under the current QPO data.

gr-qc↗

arXiv2024

Research on high-frequency quasi-periodic oscillations in black bounce-type spacetime

This paper investigates the high frequency quasi-periodic oscillations (HFQPOs) phenomenon around the black bounce-type (BBT) spacetime using the resonance models. We calculated the location of the innermost stable circular orbit (ISCO) for different types of celestial bodies, and derived the expression for the epicyclic frequencies of test particles. The results show that the BBT spacetime possesses unique observational characteristics, where the ordering of epicyclic frequencies varies with the regularization parameter $a$, enabling the excitation of low-order resonances and producing stronger observational signals. Using parametric and forced resonance models, we compared theoretical results with the observed 3:2 twin-peak HFQPOs in microquasars (GRO 1655-40, XTE 1550-564, GRS $1915+105$ ), analyzed the formation mechanisms of HFQPOs, constrained the parameters of the BBT model, and explored the possible types of celestial objects corresponding to microquasars. The study indicates that, certain parametric resonance conditions (e.g., $n=1, 2$) lead to traversable wormhole models in BBT that closely align with observations. And forced resonance corresponding to BH or wormhole models can be verified through observations. These results deviate from the data fits of the original black-bounce model. It is found that the oscillatory behavior of three types of microquasars can also be explained by particle oscillations generated in BBT theory, providing evidence for exploring the existence of wormholes, under the assumptions of parametric resonance and forced resonance.

gr-qc↗

arXiv2024

Investigating the Physical Properties of Traversable Wormholes in the Modified $f(R,T)$ Gravity

In the paper, we analyze some physical properties of static traversable WH within the framework of $f(R,T)$ modified gravitational theory. Firstly, we explore the validity of the null, weak, dominant and strong energy conditions for wormhole matter for the considered $f(R,T)=R+\alpha R^2+\lambda T$ model. Research shows that it is possible to obtain traversable WH geometry without bring in exotic matter that violates the null energy condition in the $f(R,T)$ theory. The violation of the dominant energy condition in this model may be related to quantum fluctuations or indicates the existence of special matter that violates this EC within the wormhole. Moreover, it is found that in the $f(R,T)=R+\alpha R^2+\lambda T$ model, relative to the GR, the introduction of the geometric term $\alpha R^2$ has no remarkable impact on the wormhole matter components and their properties, while the appearance of the matter-geometry coupling term $\lambda T$ can resolve the question that WH matter violates the null, weak and strong energy condition in GR. Additionally, we investigate dependency of the valid NEC on model parameters and quantify the matter components within the wormhole using the ``volume integral quantifier". Lastly, based on the modified Tolman-Oppenheimer-Volkov equation, we find that the traversable WH in this theory is stable. On the other hand, we use the classical reconstruction technique to derive wormhole solution in $f(R,T)$ theory and discuss the corresponding ECs of matter. It is found that all four ECs (NEC, WEC, SEC and DEC) of matter in the traversable wormholes are valid in this reconstructed $f(R,T)$ model, i.e we provide a wormhole solution without introducing the exotic matter and special matter in $f(R,T)$ theory.

gr-qc↗