arXiv subjects

Songbai Chen

Publications and source records attributed to Songbai Chen.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of Lorentz violation in the bright ring for Sgr A* images from radiatively inefficient accretion flows

We have investigated the effects of Lorentz violation (LV) on the bright ring in Sgr A* images illuminated by the 230 GHz thermal synchrotron emission from radiatively inefficient accretion flows around a rotating LV black hole within the low-energy Hořava gravity framework. Our results reveal that the LV parameter reduces the bright ring diameter yet increases its width, luminosity, azimuthal asymmetry and orientation angle. A higher spin parameter strengthens the LV-induced effects on the bright ring properties. Increasing disk thickness reduces the ring diameter and enhances the LV parameter's effects on this diameter. The ring width shows no systematic dependence on the disk thickness. These quantities of the bright ring display similar trends against the black hole spin and the LV parameter for the rotating LV black hole. Using EHT observational data of Sgr A*, we find that, at fixed disk thickness, the parameter-space map illustrates that the bounded range of the LV parameter first broadens and then contracts with the growing black hole spin, and shifts toward smaller LV parameter values. In addition, the LV parameter narrows the bounded range of black hole spin: negative LV parameter values shift this range to a higher spin, while positive values shift it to a lower spin. Finally, we probe effects of the LV parameter on the peak position value and the width of the primary image and the $n=1$ photon ring for the rotating LV black hole. The peak positions and their widths nearly always decrease with the LV parameter. The peak position differences for various LV parameters are more pronounced for a purely Keplerian accretion flow. In addition, the primary image and the $n=1$ photon ring produced by purely radially free-falling flows are broader than their counterparts generated by purely Keplerian flows.

astro-ph.HE↗

arXiv2026

Polarization Signatures of Sgr A* with Tilted Hybrid Magnetic Fields

The magnetic field geometry in the near-horizon region is very important in shaping polarimetric signatures of black hole images. With a hybrid magnetic field consisting of an axisymmetric split-monopole field and a tilted uniform external field, we investigate the effects of external magnetic field inclination on the polarimetric signatures of Sgr A* by employing a stationary semi-analytical radiatively inefficient accretion flow (RIAF) model and general relativistic polarized radiative transfer simulations. Our results show that the external magnetic field tilt significantly modifies the electric vector position angle (EVPA) pattern and brightness distribution. The induced non-axisymmetry from the tilted magnetic filed component renders polarization morphology dependent on the observer's azimuthal angle, which is a novel behavior absent in purely axisymmetric magnetic field configurations. In combination with Event Horizon Telescope (EHT) data, we analyse the possibility of such hybrid magnetic fields around Sgr A*.

astro-ph.HE↗

arXiv2026

Polarized image of an equatorial emitting ring around a Konoplya-Zhidenko rotating non-Kerr black hole

We investigate the polarized images of an equatorial emitting ring around a Konoplya-Zhidenko rotating non-Kerr black hole, which introduces an additional deformation parameter. The deformation parameter $η$ allows the spin parameter to extend beyond the bounds imposed by the standard Kerr black hole. The results indicate that the polarized images depend not only on the magnetic field configuration, fluid velocity, and observer inclination angle, but also on the deformation parameter and the spin parameter. As the deformation parameter increases, the polarization intensity decreases monotonically. However, the magnitude of the Electric Vector Position Angle (EVPA) increases with $η$. Furthermore, we note that the parameter $η$ may induce subtle yet discernible azimuthal separation features, which could potentially distinguish it from the spin parameter and the magnetic field orientation angle. Nevertheless, these features are difficult to resolve under current observational conditions and await verification by future high-resolution facilities such as the next-generation Event Horizon Telescope (ngEHT).

gr-qc↗

arXiv2026

Circular polarization images of Sgr A* for different magnetic field geometries

Sgr A* exhibits a persistent negative circular polarization (CP) at 230\,GHz, offering a powerful probe of the magnetic field geometry in its accretion flow. Using a stationary semi-analytic radiatively inefficient accretion flow (RIAF) model in Kerr spacetime with polarized radiative transfer, we systematically analyze CP images for six poloidal magnetic field configurations across varying black hole spins, inclinations, and field polarities. We find that CP production is dominated by Faraday conversion in radial, parabolic, quadrupole, and combined geometries, but by intrinsic emission in dipole and vertical fields. The radial and parabolic configurations produce the polarity-invariant net CP, while dipole and vertical fields yield the polarity-sensitive one. As the accretion disk is prograde with respect to the black hole spin, the CP production across all six field geometries is found to be lower at high spin case, while the situation is more complicated in the retrograde case. Moreover, the net CP observed from edge-on views $V_{\rm net} \approx 0$ except for the quadrupole geometry. Comparing with ALMA data, the reversed-field model is excluded at high inclinations and then the magnetic field geometry of Sgr A* is constrained.

astro-ph.HE↗

arXiv2026

Vacuum breakdown in a misaligned magnetized Kerr spacetime

Electron-positron ($e^{+}e^{-}$) pair creation by vacuum breakdown around compact objects is believed to power high-energy astrophysical transients like gamma-ray bursts (GRBs). In this work, we focus on vacuum breakdown around a Kerr black hole (BH) immersed in an asymptotically uniform magnetic field that is inclined with respect to the BH spin axis. The dyadoregion, the region where the induced electric field exceeds the critical value $E_{\text{c}}=m_{e}^{2}c^{3}/(e\hbar)$, is identified via the electromagnetic invariants. It is found that the dyadoregion consists of several lobes whose number, size, and orientation vary with the inclination. We also estimate the electromagnetic energy available for pair creation and derive a beaming factor that allows a conversion between the intrinsic dyadoregion energy and the observed isotropic energy. The thermodynamic properties of the resulting electron-positron-photon ($e^{+}e^{-}γ$) plasma are included, revealing an initial magnetic dominance. The evaluation of the minimum magnetic field required shows that misaligned magnetic fields generally favor pair creation more than aligned ones.

gr-qc↗

arXiv2026

Bright ring features and polarization structures in Kerr-Sen black hole images illuminated by radiatively inefficient accretion flows

Using general relativistic radiative transfer (GRRT) simulations, we investigate the bright ring features and polarization structures in images of the Kerr-Sen black hole associated with Sgr A*, as illuminated by 230 GHz thermal synchrotron emission from radiatively inefficient accretion flows (RIAF). Our findings reveal that an increase in the dilaton parameter leads to a shrinking of the bright ring, accompanied by enhancements in both its width and brightness. As the disk thickness grows, the bright ring's diameter and width both decrease. The brightness enhancement induced by the disk thickness is less prominent than that driven by the dilaton parameter. Comparing with the Event Horizon Telescope (EHT) observational data of SgrA*, we present the allowed ranges of black hole parameters, and find that effects of the disk thickness on the allowed parameter space are stronger than those of the observer's inclination. Furthermore, we analyze the coefficient $β_2$, associated with the two-fold rotational symmetry of the electric vector position angles (EVPA), to probe the polarization structure of the black hole images, and reveal that effects of the disk thickness on $β_2$ are much weaker than those from the dilaton parameter.

gr-qc↗

arXiv2026

Shadow of a circular disformal Kerr black hole beyond GR

We have studied the shadows of a circular disformal Kerr black hole with a deformation parameter, which represents a rotating solution within a generalized scalar-tensor framework of Horndeski gravity that characterized by second-order field equations. Our result show that for the non-rotating case, the shadow remains perfectly circular, with its radius independent of the deformation parameter. For the rotating case, the size of the shadow decreases as the deformation parameter decreases, and the shape of the shadow gradually becomes more flattened. It is worth noting that while the shadow of a rotating black hole remains north-south symmetric for equatorial observers, this symmetry is broken once the observer moves away from the equatorial plane. For an observer in the northern hemisphere, the geometric center of the shadow shifts northward when $D_0<0$ and southward when $D_0>0$, while the opposite behavior occurs for observers in the southern hemisphere. These features in the black hole shadow originating from the scalar field could help us to understand the circular disformal Kerr black hole and generalized scalar-tensor framework of Horndeski gravity.

gr-qc↗

arXiv2025

Effects of tidal charge on Blandford-Znajek process around braneworld black holes

The Blandford-Znajek (BZ) process is a pivotal mechanism to efficiently extract the energy from a rotating black hole (BH) via its plasma-filled magnetosphere in relativistic astrophysics. Within the framework of extended BZ monopole expansion, we have studied BZ process in the Randall-Sundrum braneworld BH spacetime and analyzed effects of the tidal charge on the energy and angular momentum extraction rates. It is found that the positive tidal charge reduces the BZ power of a braneworld BH, while the negative tidal charge enhances the power. Compared with a Kerr BH of the same mass and angular velocity, the BZ power exhibits a maximum reduction of approximately $15.2\%$ in positive cases, whereas in negative cases, it achieves a maximum enhancement of $66.5\%$ in power output. A similar qualitative trend is also observed for the relative angular momentum extraction rate, albeit with different magnitudes.

gr-qc↗

arXiv2025

The images of Brans-Dicke-Kerr type naked singularities

We have studied the images of the Brans-Dicke-Kerr spacetime with a dimensionless Brans-Dicke parameter $ω$, which belongs to axisymmetric rotating solutions in the Brans-Dicke theory. Our results show that the Brans-Dicke-Kerr spacetime with the parameter $ω>-3/2$ represents naked singularities with distinct structures. For the case with $a \leq M$, the shadow in the Brans-Dicke-Kerr spacetime persists, gradually becomes flatter and smaller as $ω$ decreases. Especially when $ω<1/2$, the shadow in the image exhibit a very special ``jellyfish" shape and possesses a self-similar fractal structure. For the case with $a > M$, a distinct gray region consisting of two separate patches appears in the image observed by equatorial observers. This indicating that the Brans-Dicke-Kerr spacetime can be distinguished from the Kerr and Kerr-de Sitter cases based on its image. These effects of the Brans-Dicke parameter could help us to reveal the intrinsic structure of the Brans-Dicke-Kerr spacetimes and provide a foundation for testing Brans-Dicke theory through future high-precision observations.

gr-qc↗

arXiv2025

Quantum-classical correspondence in resonant and nonresonant Rabi-Stark model

Testing the correspondence principle in nonlinear quantum systems is a fundamental pursuit in quantum physics. In this paper, we employed mean field approximation theory to study the semiclassical dynamics in the Rabi-Stark model (RSM) and showed that the nonlinear Stark coupling significantly modulates the semiclassical phase space structure. By analyzing the linear entanglement entropy of coherent states prepared in the classical chaotic and regular regions of the semiclassical phase space, we demonstrate that quantum-classical correspondence can be achieved in the RSM with large atom-light frequency ratios. While this correspondence fails in the resonant Rabi model because its truncated photon number is insufficient to approach the large quantum number limit, we discovered that in the resonant RSM when the nonlinear Stark coupling $U \to \pm 1$, the time-averaged linear entanglement entropy correlates strongly with the semiclassical phase space. In particular, when $U \to -1$, the truncated photon number in the resonant RSM is very close to that in the resonant Rabi model, but the time-averaged linear entanglement entropy still corresponds well with the semiclassical phase space. This result demonstrates that quantum-classical correspondence can be realized in the few-body resonant RSM.

quant-ph↗

arXiv2025

Dynamic shadow of a black hole with a self-interacting massive complex scalar hair

We investigate the dynamic shadows of a black hole with a self-interacting massive complex scalar hair. The complex scalar field ψevolves with time t, and its magnitude on the apparent horizon |ψ_{h}| starts from zero, undergoes a sharp rise followed by rapid oscillations, and eventually converges to a constant value. The variation in the photon sphere radius r_{ps} is similar to that of the magnitude |ψ_{h}|. Owing to the emergence of the complex scalar hair ψ, the apparent horizon radius r_{h} starts increasing sharply and then smoothly approaches a stable value eventually. The shadow radius R_{sh} of the black hole with an accretion disk increases with time t_{o} at the observer's position. In the absence of an accretion disk, the shadow radius R_{sh} is larger and also increases as t_{o} increases. Furthermore, we slice the dynamical spacetime into spacelike hypersurfaces for all time points t. For the case with an accretion disk, the variation in R_{sh} is similar to that in the apparent horizon r_{h}, because the inner edge of the accretion disk extends to the apparent horizon. In the absence of an accretion disk, the variation in R_{sh} is similar to that in the photon sphere radius r_{ps}, because the black hole shadow boundary is determined by the photon sphere. As the variation in r_{ps} is induced by ψ, it can be stated that the variation in the size of the shadow is similarly caused by the change in ψ. Regardless of the presence or absence of the accretion disk, the emergence of the complex scalar hair ψcauses the radius R_{sh} of the shadow to start changing. Moreover, we investigate the time delay Δt of lights propagating from light sources to the observer. These findings not only enrich the theoretical models of dynamic black hole shadows but also provide a foundation for testing black hole spacetime dynamics.

gr-qc↗

arXiv2025

Chaotic motion of particles around a dyonic Kerr-Newman black hole immersed in the Melvin-swirling universe

We employ the Poincaré section, fast Lyapunov indicator, recurrence analysis, bifurcation diagram and basins of attraction to investigate the dynamical behaviors of the motion of particles around a new dyonic Kerr-Newman black hole immersed in the Melvin-swirling universe presented in [A. Di Pinto, S. Klemm, and A. Viganò, J. High Energy Phys. {\bf 06}, 150 (2025)]. We note that the swirling parameter $j$ and magnetic field strength $B$ make the equations of motion for particles nonseparable, and confirm the presence of chaotic behavior in the motion in this dyonic Kerr-Newman-Melvin-swirling spacetime and its sub-cases by removing the conical singularities and removing both the conical singularities and the Dirac strings. We observe that both the number of chaotic orbits and the chaotic region increase with the increase of the parameters $j$ and $B$, but decrease as the electric charge $Q$, magnetic charge $H$ or spin parameter $a$ increases. Moreover, we find that the presence of $j$ changes the ranges of $B$, $Q$, $H$ and $a$ where the chaotic motion appears for particles. The swirling parameter together with the magnetic field strength, electric charge, magnetic charge and spin parameter yields richer physics in the motion of particles for the spacetime of a dyonic Kerr-Newman black hole immersed in the Melvin-swirling universe.

gr-qc↗

arXiv2025

Gravitational waves from equatorially eccentric extreme mass ratio inspirals around swirling Kerr black holes

The swirling-Kerr black hole is a novel solution of vacuum general relativity and has an extra swirling parameter characterizing the rotation of spacetime background. We have studied the gravitational waves generated by extreme mass ratio inspirals (EMRIs) along eccentric orbits on equatorial plane in this novel swirling spacetime. Our findings indicate that this swirling parameter leads to a delayed phase shift in the gravitational waveforms. Furthermore, we have investigated effects of the swirling parameter on the potential issue of waveform confusion caused by the orbital eccentricity and semi-latus rectum parameters. As the swirling parameter increases, the relative variations in the eccentricity increase, while the variations in the semi-latus rectum decrease rapidly. These trends of the changes related to the orbital eccentricity and the semi-latus rectum with the swirling parameter resemble those observed with the MOG parameter in the Scalar-Tensor-Vector-Gravity (STVG) theory, but with different rates of change. Furthermore, our results also reveal that effects of the background swirling parameter on the relative variations in the eccentricity and the semi-latus rectum are distinctly different from those of the black hole spin parameter. These results provide deeper insights into the properties of EMRI gravitational waves and the background's swirling.

gr-qc↗

arXiv2025

Extracting energy from plunging region of a Kerr-Taub-NUT black hole by magnetic reconnection

We have studied the energy extraction from a Kerr-Taub-NUT black hole via magnetic reconnection occurring in the plunging region. Our results show that the gravitomagnetic charge suppresses the energy extraction process through magnetic reconnection and reduces the corresponding extraction efficiency, which is opposite to the effects of the black hole spin and the magnetization parameter. Finally, we treat the energy extraction process through magnetic reconnection as a mechanism to revisit the problem of the observed jet power and radiative efficiency of GRS 1915+105. Our results show that the allowed black hole parameter region originating from the jet power has an intersection with the region from the radiative efficiency. This means that with this mechanism related to magnetic reconnection the Kerr-Taub-NUT metric can simultaneously explain the observed jet power and radiative efficiency for GRS 1915+105, which is not explained by other mechanisms in previous studies.

gr-qc↗

arXiv2025

Semi-analytical Study on the Polarized Images of Black Hole due to Frame Dragging

An initially retrograde accretion flow is transformed into a prograde configuration before plunging into the black hole, as a result of the frame-dragging effect induced by the black hole's rotation. The polarized image of a black hole shaped by such an accretion flow manifests three distinctive critical locations: the turning point of the flow's primary image, the polarization-flip location on the image plane, and the position of the primary image corresponding to the flow's actual turning point in spacetime. Due to the influences of gravitational lensing and gravitational Faraday rotation, these three positions generally do not coincide. In this work, we examine a thin equatorial accretion disk composed of initially retrograde, geodesically moving flows, and conduct a systematic investigation into the interrelations and discrepancies among these critical locations. We elucidate the spatial hierarchy among the three, and for an on-axis observer, we derive approximate analytic expressions characterizing their positions.

gr-qc↗

arXiv2025

Thin accretion disk around a Kerr black hole immersed in swirling universes

We have studied the properties of thin accretion disks around swirling-Kerr black holes, which own an extra swirling parameter describing the rotation of the immersed universe. Our results show that the swirling parameter leaves distinct imprints on the energy flux, temperature distribution and emission spectra of the disk and gives rise to some new effects that differ from those induced by the black hole's spin. With the increasing of the swirling parameter, both the energy flux and radiated temperature in the disk increase in the inner region where circular orbital radii are smaller and decrease in the outer region where circular orbital radii are larger. In contrast, these quantities consistently increase with the black hole's spin. Although the swirling parameter and the black hole's spin parameter lead to higher cut-off frequencies, the background swirling reduces the observed luminosity of the disk at lower frequencies and enhances it only at higher frequencies, which is quite distinct from that of the black hole's spin. Furthermore, the conversion efficiency increases with the black hole's spin parameter, but decreases with the swirling parameters. Additionally, the effects of the swirling parameter are found to be suppressed by the black hole's spin parameter. These results could help us further understand the properties of thin accretion disks and the swirling of the universe background.

gr-qc↗

arXiv2025

Quantum Signatures of Chaos in Anisotropic Quantum Rabi Model

Quantum chaos is an intriguing topic and has attracted a great deal of interests in quantum mechanics and black hole physics. Recently, the exponential growth of out-of-time-ordered correlator (OTOC) has been proposed to diagnose quantum chaos and verify the correspondence principle. Here, good correspondence is found between the linear entanglement entropy and the semiclassical phase space structures in the anisotropic quantum Rabi model. The Loschmidt echo in the chaotic sea decays more faster than that in the stable island. However, the OTOCs grow exponentially at early times for the initial states centered both in the chaotic and stable regions. The exponential growth of the OTOC is attributed to quantum collapse that provides a novel mechanism of yielding exponential growth of the OTOC in quantum systems. Moreover, the quantum collapse effect is more obvious for the initial states centered in the chaotic one. The results show that in the anisotropic quantum Rabi model, the linear entanglement entropy, and Loschmidt echo are more effective than OTOC for diagnosing quantum chaotic signals.

quant-ph↗

arXiv2025

Long-time signatures of chaos in large atom-light frequency ratios Rabi model

As one of the famous effects in quantum Rabi model (QRM), Rabi oscillation may lead to the occurrence of quantum dynamics behaviors without classical dynamic counterparts, such as quantum collapse and revival effects. In this paper, we focus on studying the long-time quantum signatures of chaos in the large atom-light frequency ratios Rabi model. It is shown that the saturated values of the entanglement entropy for initial states located in chaotic sea are higher than that in the regular regions, and the Husimi Q function are more dispersed in phase space. Moreover, we observed that the long-time average entanglement entropy and spin variance correspond well with the semiclassical phase space. Our results imply that the correspondence principle does not invalidated by quantum collapse and revival effects in the large atom-light frequency ratios Rabi model.

quant-ph↗