arXiv subjects

Yiqian Chen

Publications and source records attributed to Yiqian Chen.

15 recordsLinked to original sources

arXiv2026

Optical Appearance of Scalarized Kerr-Newman Black Holes with Multiple Light Rings

This study investigates the optical appearance of rotating scalarized Kerr-Newman black holes in the Einstein-Maxwell-scalar theory with exponential coupling. By analyzing equatorial null geodesics, these black holes are classified into six types according to the number and properties of their light rings. Combining slow-rotation analysis with full numerical ray tracing, we investigate images of black holes illuminated by a geometrically and optically thin accretion disk. Unlike the Kerr case, where the image is typically governed by a single outer photon shell and a single critical curve, scalarized Kerr-Newman black holes can develop an additional inner photon shell outside the event horizon. This extra shell gives rise to an inner critical curve inside the usual outer one, which may be absent, partial, or closed depending on the black hole parameters and the observer's inclination. Moreover, it generates new higher-order images in the region between the two critical curves, some of which exhibit crescent-like morphologies distinct from the nearly circular higher-order images familiar from Kerr black holes. These features enrich the optical appearance of scalarized black holes and could serve as distinctive observational signatures in future high-resolution observations.

gr-qc↗

arXiv2025

Schwarzschild Black Holes Immersed in Born-Infeld Magnetic Fields and Their Observational Signatures

We investigate the influence of Born-Infeld (BI) nonlinear electrodynamics on magnetic field configurations, photon orbits, and black hole shadows for Schwarzschild black holes immersed in magnetic fields. Assuming that the BI magnetic fields are asymptotically uniform and aligned with the polar axis, we solve the nonlinear magnetic field equations numerically using pseudospectral methods. Our analysis shows that nonlinear electromagnetic effects become prominent near the event horizon, particularly in the polar regions, where the magnetic field strength is significantly enhanced. This enhancement leads to closed photon orbits on the meridional plane becoming prolate, with noticeable stretching along the polar axis. Simulations of black hole images reveal that, at high observer inclinations, the shadow, which is circular in the Maxwell limit, becomes increasingly elongated along the polar direction as the nonlinear effects increase.

gr-qc↗

arXiv2024

Observational Signatures of Traversable Wormholes

In this paper, we study the observational signatures of traversable Simpson-Visser wormholes illuminated by luminous celestial spheres and orbiting hot spots. We demonstrate that when light sources and observers are on the same side of the wormholes, the images of the wormholes mimic those of black holes. However, when the light sources are positioned on the opposite side from observers, photons traversing the wormhole throat generate distinct observational signatures. Specifically, unlike black hole images, the wormhole images are confined within the critical curve, resulting in smaller centroid variations. Furthermore, the light curve of hot spots can exhibit additional peaks.

gr-qc↗

arXiv2024

Polarized Image of a Synchrotron-emitting Ring in Einstein-Maxwell-scalar Theory

This study investigates polarized images of an equatorial synchrotron-emitting ring surrounding hairy black holes within the Einstein-Maxwell-scalar theory. Our analysis demonstrates qualitative similarities between the polarization patterns of hairy black holes and Schwarzschild black holes. However, due to the non-minimal coupling between the scalar and electromagnetic fields, an increase in black hole charge and coupling constant can substantially amplify polarization intensity and induce deviations in the electric vector position angle. These effects may offer observational signatures to distinguish hairy black holes from Schwarzschild black holes.

gr-qc↗

arXiv2024

Distinguishing the Observational Signatures of Hot Spots Orbiting Reissner-Nordström Spacetime

This paper delves into observable signatures of hot spots orbiting Reissner-Nordström (RN) black holes and naked singularities. In a RN black hole case, we find two discernible lensing image tracks in time integrated images capturing a complete orbit of hot spots, and a image shadow within the critical curve where photons with a small impact parameter fall into the event horizon. Conversely, in RN singularities, additional image tracks can be found inner the critical curve, originating from photons reflected by the infinitely high effective potential well. Moreover, we found incomplete and converge tracks from the time integrated images of hot spot orbiting RN singularities lacking of a photon sphere. The presence of these additional image tracks exerts a significant influence on temporal magnitudes at their local maxima, allowing us to differentiate between RN black holes and RN naked singularities.

gr-qc↗

arXiv2024

Interferometric Signatures of Black Holes with Multiple Photon Spheres

It has been reported that the photon ring structure in black hole images produces strong and universal interferometric signatures on long interferometric baselines, holding promise for measuring black hole parameters and testing general relativity. This paper investigates the interferometric signatures of black holes with one or two photon spheres, specifically within the framework of Einstein-Maxwell-Scalar models. Notably, for black holes possessing two photon spheres, interference between light rays orbiting the inner and outer photon spheres manifests as beat signals in the visibility amplitude, deviating from the universal signatures observed in the single-photon sphere case.

gr-qc↗

arXiv2024

Observations of Orbiting Hot Spots around Scalarized Reissner-Nordström Black Holes

This paper investigates the observational signatures of hot spots orbiting scalarized Reissner-Nordström black holes, which have been reported to possess multiple photon spheres. In contrast to the single-photon sphere case, hot spots orbiting black holes with two photon spheres produce additional image tracks in time integrated images capturing a complete orbit of hot spots. Notably, these newly observed patterns manifest as a distinct second-highest peak in temporal magnitudes when observed at low inclination angles. These findings offer promising observational probes for distinguishing black holes with multiple photon spheres from their single-photon sphere counterparts.

gr-qc↗

arXiv2023

Observations of Orbiting Hot Spots around Naked Singularities

Recently, it has been reported that photons can traverse naked singularities in the Janis-Newman-Winicour and Born-Infeld spacetimes when these singularities are appropriately regularized. In this paper, we investigate observational signatures of hot spots orbiting these naked singularities, with a focus on discerning them from black holes. In contrast to Schwarzschild black holes, we unveil the presence of multiple additional image tracks within critical curves in time integrated images capturing a complete orbit of hot spots. Moreover, these new images manifest as a more pronounced second-highest peak in temporal magnitudes when observed at low inclinations.

gr-qc↗

arXiv2023

Gravitational Lensing by Transparent Janis-Newman-Winicour Naked Singularities

The Janis-Newman-Winicour (JNW) spacetime can describe a naked singularity with a photon sphere that smoothly transforms into a Schwarzschild black hole. Our analysis reveals that photons, upon entering the photon sphere, converge to the singularity in a finite coordinate time. Furthermore, if the singularity is subjected to some regularization, these photons can traverse the regularized singularity. Subsequently, we investigate the gravitational lensing of distant sources and show that new images emerge within the critical curve formed by light rays escaping from the photon sphere. These newfound images offer a powerful tool for the detection and study of JNW naked singularities.

gr-qc↗

arXiv2023

Gravitational Lensing by Born-Infeld Naked Singularities

We examine the gravitational lensing phenomenon caused by photon spheres in the Born-Infeld naked singularity spacetime, where gravity is coupled with Born-Infeld electrodynamics. Specifically, our focus lies on relativistic images originating from a point-like light source generated by strong gravitational lensing near photon spheres, as well as images of a luminous celestial sphere. It shows that Born-Infeld naked singularities consistently exhibit one or two photon spheres, which project onto one or two critical curves on the image plane. Interestingly, we discover that the nonlinearity nature of the Born-Infeld electrodynamics enables photons to traverse the singularity, leading to the emergence of new relativistic images within the innermost critical curve. Furthermore, the presence of two photon spheres doubles the number of relativistic images compared to the scenario with only a single photon sphere. Additionally, the transparency inherent to Born-Infeld naked singularities results in the absence of a central shadow in the images of celestial spheres.

gr-qc↗

arXiv2022

Observational Appearance of a Freely-falling Star in an Asymmetric Thin-shell Wormhole

It has been recently reported that, at late times, the total luminosity of a star freely falling in black holes decays exponentially with time, and one or two series of flashes with decreasing intensity are seen by a specific observer, depending on the number of photon spheres. In this paper, we examine observational appearances of an infalling star in a reflection-asymmetric wormhole, which has two photon spheres, one on each side of the wormhole. We find that the late-time total luminosity measured by distant observers gradually decays with time or remains roughly constant due to the absence of the event horizon. Moreover, a specific observer would detect a couple of light flashes in a bright background at late times. These observations would offer a new tool to distinguish wormholes from black holes, even those with multiple photon spheres.

gr-qc↗

arXiv2022

Appearance of an Infalling Star in Black Holes with Multiple Photon Spheres

Photon spheres play a pivotal role in the imaging of luminous objects near black holes. In this paper, we examine observational appearances of a star freely falling in hairy black holes, which can possess one or two photon spheres outside the event horizon. When there exists a single photon sphere, the total luminosity measured by distant observers decreases exponentially with time at late times. Due to successive arrivals of photons orbiting around the photon sphere different times, a specific observer would see a series of light flashes with decreasing intensity, which share a similar frequency content. Whereas in the case with two photon spheres, photons temporarily trapped between the photon spheres can cause a peak of the total luminosity, which is followed by a slow exponential decay, at late times. In addition, these photons lead to one more series of light flashes seen by the specific observer.

gr-qc↗

arXiv2022

Equilibrium and stiffness study of clustered tensegrity structures with the consideration of pulley sizes

This paper presents the equilibrium and stiffness study of clustered tensegrity structures (CTS) considering pulley sizes. We first derive the geometric relationship between clustered strings and pulleys, where the nodal vector is chosen as the generalized coordinate. Then, the equilibrium equations of the clustered tensegrity structure with pulleys based on the Lagrangian method are given. Since the stiffness of a structure is usually weakened when using clustering strings, we formulate the tangent stiffness matrix equations for analysis. It is also shown that as pulley sizes go to zero, the governing equations of the clustered tensegrity system with pulleys yield to the classical clustered tensegrity structure without pulleys, which is consistent with the existing literature. Three examples are demonstrated to validate the given theory. The proposed method allows one to conduct equilibrium, stiffness, and robustness studies of cluster tensegrity structures with pulleys. Nevertheless, the approach developed in this paper is not limited to the tensegrity structures. It can also be applied to a wide range of applications with pulley-rope systems, such as drilling rigs, ocean platform anchors, and cargo cranes.

eess.SY↗

arXiv2021

Holographic Superconductors in a Non-minimally Coupled Einstein-Maxwell-scalar Model

In this paper, we investigate holographic superconductors dual to asymptotically anti-de Sitter black holes in an Einstein-Maxwell-scalar model with a non-minimal coupling between the scalar and Maxwell fields. In the probe limit, it shows that the scalar condensate occurs below the critical temperature $T_{c}$, and decreases with the increase of the coupling constant $α$. On the other hand, the the critical temperature $T_{c}$ increases as the coupling constant $α$ grows. We also calculate the optical conductivity of the holographic superconductor, and observe that a gap forms below $T_{c}$. Interestingly, the non-minimal coupling can lead to a spike occurring in the gap at a low temperature.

hep-th↗

arXiv2019

Strong Cosmic Censorship for a Scalar Field in a Logarithmic-de Sitter Black Hole

It has been shown that the quasinormal modes of perturebated fields can be used to investigate the validity of strong cosmic censorship (SCC). Relevant issues for Reissner-Nordstrom-de Sitter (RNdS) black holes and Born-Infeld-de Sitter (BI-dS) black holes have been discussed. In this paper, we investigate SCC in an asymptotic RN-dS black hole with logarithmic nonlinear electromagnetic field perturbed by massless scalar fields. It has been argued that SCC can be violated in a near-extremal RN-dS black hole. However, we find that the NLED effect can rescue SCC for a near-extremal logarithmic-de Sitter black hole. Compared with Born-Infeld model, we find that the NLED effect have similar behavior.

gr-qc↗