arXiv subjects

Che-Yu Chen

Publications and source records attributed to Che-Yu Chen.

At least 19 recordsLinked to original sources

arXiv2026

Imprints of core/cusp dark matter distributions on black hole signatures in galaxies

In galactic environments, a host dark matter (DM) halo can imprint weak but coherent corrections on black hole (BH) strong-field observables. We construct an exact family of static and spherically symmetric BH spacetimes sourced by a generic core/cusp DM halo, described by an anisotropic stress-energy tensor with nonvanishing radial pressure. The resulting geometry is determined self-consistently from the Einstein equations for a broad $\{\alpha,\beta,\gamma\}$ density profile, including the NFW, Moore, Hernquist, Jaffe, and core/cusp Dehnen models as special cases. We discuss the asymptotic structure, horizon location, curvature scale, and energy conditions of the corresponding geometries, emphasizing that the inner logarithmic slope $\gamma$ controls the amount of DM probed by the relativistic region. We then obtain perturbative analytic estimates for the characteristic circular geodesics, demonstrating that the leading strong-field corrections are controlled by the dimensionless compactness $q_\gamma$, rather than by the total halo mass alone. To leading order in $q_\gamma$, we derive analytic expressions for the light ring radius, angular frequency, critical impact parameter, Lyapunov exponent, innermost stable circular orbit (ISCO) radius, and ISCO frequency. For cuspy profiles, the light ring and ISCO are displaced outward, while the corresponding orbital frequencies are redshifted; for cored profiles, the light ring radius is unchanged at this order, although its frequency and capture impact parameter still carry finite environmental corrections. We also investigate the weak- and strong-deflection angles and their dependence on the halo's inner structure. We further discuss how these environmental corrections may affect ringdown physics, in particular through the perturbative imprint of the halo on quasinormal-mode redshifts and late-time wave propagation.

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arXiv2026

Twisted doughnuts: Thick disk torus around equatorial asymmetric black hole

The Kerr black hole spacetime is symmetric with respect to a well-defined equatorial plane. When such a symmetry is broken, for instance, by some putative effects beyond general relativity, the Keplerian circular orbits around the black hole are distorted vertically away from the equatorial plane by an amount depending on the orbital radius. As a result, the Keplerian thin disk acquires a curved surface. In this work, we extend such results to thick tori configurations by considering non-self-gravitating Polish doughnut models. We show that due to the equatorial asymmetry of the spacetime, the centers and the cusps of tori are distorted away from the original equatorial plane toward the same direction as that experienced by the stable Keplerian orbits, and the entire tori configurations are twisted toward that direction as well. The shape of the distorted tori is demonstrated explicitly using a constant specific angular momentum profile $\ell(r,y)=\ell_0$ of the disk fluid. However, the result also applies to non-constant profiles of $\ell(r,y)$ generically in the sense that any asymmetric profile of $\ell(r,y)$ that attempts to produce a symmetric tori configuration either turns out to be ill-defined near the equatorial plane or suffers from fine-tuning issues.

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arXiv2026

Shadow signatures and energy accumulation in Lorentzian-Euclidean black holes

The Lorentzian-Euclidean black hole has been recently introduced as a geodesically complete spacetime featuring a signature shift at the event horizon where causal geodesics are precluded from reaching the central $r=0$ singularity. In this paper, we investigate the shadows produced by this geometry to identify deviations from the standard Schwarzschild solution. Our analysis reveals an excess intensity in the inner shadow region that points to a potential observational signature of the novel behavior of light rays propagating near the event horizon. This excess could be a probe for horizon-scale modifications of black hole geometries. Furthermore, although the horizon surface of the Lorentzian-Euclidean black hole continuously accumulates photons and energy, we show that its backreaction response differs from that of stable light rings found in various exotic compact objects.

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arXiv2025

Shadow of rotating black holes with consistent thermodynamics

Quantum effects in general induce scale dependence in the coupling constants. We explore this possibility in gravity, with a scale-dependent Newton coupling. When applied to Kerr black holes with such a running coupling, the consistency of black hole thermodynamics requires that the Newton coupling have a specific dependence on the black hole parameters. In this work, we consider such a class of Newton couplings and look for the possible observational implications on the highly lensed images of the black holes. In addition to placing constraints on the parameter space of the model through the latest Sgr A* images, we find that the variations in the shape of shadows in a large portion of the parameter space can be qualitatively captured by a quantity solely defined by the event horizon. Most importantly, the consistency of thermodynamics suggests a lower bound on the shadow size, beyond which either horizon disappears, or the shadow cannot keep the standard D-shaped structure. The possibility that the black holes in this model could spin faster than the Kerr bound, and the physical implications of the resulting cuspy shadows, are also discussed.

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arXiv2025

Vector Horndeski black holes in nonlinear electrodynamics

On a spherically symmetric and static background, we study the existence of linearly stable black hole (BH) solutions in nonlinear electrodynamics (NED) with a Horndeski vector-tensor (HVT) coupling, with and without curvature singularities at the center ($r=0$). Incorporating the electric charge $q_E$ and the magnetic charge $q_M$, we first show that nonsingular BHs can exist only if $q_M = 0$. We then study the stability of purely electric BHs by analyzing the behavior of perturbations in the metric and the vector field. Nonsingular electric BHs are unstable due to a Laplacian instability in the vector perturbation near the regular center. In the absence of the HVT coupling ($\beta=0$), singular BHs in power-law NED theories can be consistent with all linear stability conditions, while Born-Infeld BHs encounter strong coupling due to a vanishing propagation speed as $r \to 0$. In power-law NED and Born-Infeld theories with $\beta \neq 0$, the electric fields for singular BHs are regular near $r=0$, while the metric functions behave as $\propto r^{-1}$. Nevertheless, we show that Laplacian instabilities occur for regions inside the outer horizon $r_h$, unless the HVT coupling constant $\beta$ is significantly smaller than $r_h^2$. For $\beta \neq 0$, we also reconstruct the NED Lagrangian so that one of the metric functions takes the Reissner-Nordstr\"om form. In this case, there exists a branch where all squared propagation speeds are positive, but the ghost and strong coupling problems are present around the BH center. Thus, the dominance of the HVT coupling generally leads to BH instability in the high-curvature regime.

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arXiv2025

Insights in $f(Q)$ cosmology: the relevance of the connection

We explore the role of the affine connection in $f(Q)$ gravity, a modified theory where gravity is governed by non-metricity within the symmetric teleparallel framework. Although the connection is constrained to be flat and torsionless, it is not uniquely determined by the metric, allowing for multiple physically distinct formulations. We analyze three such connections compatible with a homogeneous and isotropic universe to show that they yield markedly different cosmological dynamics, even under the same functional form of $f(Q)$. Using both analytical and numerical methods, including a Born-Infeld type model of $f(Q)$, we demonstrate that specific connections can resolve cosmological singularities like the Big Bang and Big Rip, replacing them with smooth de Sitter phases. Others retain singularities but with notable modifications in their behavior. These findings highlight the physical relevance of connection choice in $f(Q)$ gravity and its potential to address fundamental cosmological questions.

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arXiv2024

Modeling the Far-Infrared Polarization Spectrum of a High-Mass Star Forming Cloud

The polarization spectrum, or wavelength dependence of the polarization fraction, of interstellar dust emission provides important insights into the grain alignment mechanism of interstellar dust grains. We investigate the far-infrared polarization spectrum of a realistic simulated high-mass star forming cloud under various models of grain alignment and emission. We find that neither a homogeneous grain alignment model nor a grain alignment model that includes collisional dealignment is able to produce the falling spectrum seen in observations. On the other hand, we find that a grain alignment model with grain alignment efficiency dependent on local temperature is capable of producing a falling spectrum that is in qualitative agreement with observations of OMC-1. For the model most in agreement with OMC-1, we find no correlation between temperature and the slope of the polarization spectrum. However, we do find a positive correlation between column density and the slope of the polarization spectrum. We suggest this latter correlation to be the result of wavelength-dependent polarization by absorption.

astro-ph.GA↗

arXiv2024

Linear stability of vector Horndeski black holes

Horndeski's vector-tensor (HVT) gravity is described by a Lagrangian in which the field strength $F_{\mu \nu}=\partial_{\mu} A_{\nu}-\partial_{\nu} A_{\mu}$ of a vector field $A_{\mu}$ interacts with a double dual Riemann tensor $L^{\mu \nu \alpha \beta}$ in the form $\beta L^{\mu \nu \alpha \beta} F_{\mu \nu} F_{\alpha \beta}$, where $\beta$ is a constant. In Einstein-Maxwell-HVT theory, there are static and spherically symmetric black hole (BH) solutions with electric or magnetic charges, whose metric components are modified from those in the Reissner-Nordstr\"om geometry. The electric-magnetic duality of solutions is broken even at the background level by the nonvanishing coupling constant $\beta$. We compute a second-order action of BH perturbations containing both the odd- and even-parity modes and show that there are four dynamical perturbations arising from the gravitational and vector-field sectors. We derive all the linear stability conditions associated with the absence of ghosts and radial/angular Laplacian instabilities for both the electric and magnetic BHs. These conditions exhibit the difference between the electrically and magnetically charged cases by reflecting the breaking of electric-magnetic duality at the level of perturbations. In particular, the four angular propagation speeds in the large-multipole limit are different from each other for both the electric and magnetic BHs. This suggests the breaking of eikonal correspondence between the peak position of at least one of the potentials of dynamical perturbations and the radius of photon sphere. For the electrically and magnetically charged cases, we elucidate parameter spaces of the HVT coupling and the BH charge in which the BHs without naked singularities are linearly stable.

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arXiv2024

Observational features of reflection asymmetric black holes

The Kerr spacetime is symmetric with respect to a well-defined equatorial plane. When testing the equatorial reflection symmetry of an isolated black hole, one is at the same time testing the Kerr hypothesis in General Relativity. In this work, we investigate the possible observational features when a Keplerian disk is surrounding a rotating black hole without reflection symmetry. When such symmetry is broken, generically, the photon trajectories around the black hole and the Keplerian orbits on the accretion disk are distorted vertically away from the equatorial plane by an amount that depends on their distance to the black hole. In the reflection asymmetric spacetime we are considering, these two kinds of orbits are distorted in opposite directions. Interestingly, while the size and shape of black hole shadows closely resemble those of Kerr black holes, distinct observational characteristics can emerge in the disk image and emission line profiles. When observing the disk edge-on, a pronounced concave shape may appear along its innermost edge on the incoming side. Furthermore, distinctive horn-like features might be observed on the spectral line profile at the blue-shifted side. These special features can serve as compelling indicators of the reflection asymmetry present in rotating black holes.

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arXiv2024

Imaging a semi-classical horizonless compact object with strong redshift

The recent advancements in black hole imaging have opened a new era of probing horizon-scale physics with electromagnetic radiation. However, a feature of the observed images, a bright ring encircling a relatively dark region, has not sufficiently proved the existence of event horizons. It thus requires extreme care when studying the possibility of using such image features to examine quantum effects that may change the classical picture of black holes slightly or drastically. In this work, we investigate the image of a horizonless compact object, whose interior metric satisfies the 4D semi-classical Einstein equation non-perturbatively for the Planck constant, and whose entropy agrees with the Bekenstein-Hawking formula. Although the absence of an event horizon allows light rays to pass through the dense interior, the extremely strong redshift significantly darkens the image, making it almost identical to the classical black-hole image. In particular, if there is light emission a bit inside the surface of the object, the intensity around the inner shadow is slightly enhanced, which could be a future observable prediction to characterize the object. We also find through a phenomenological parameter that the image is further darkened due to interactions inside. Thus, the image is consistent with current observations, and the object could be a candidate for black holes in quantum theory.

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arXiv2023

Dynamics in Star-forming Cores (DiSCo): Project Overview and the First Look toward the B1 and NGC 1333 Regions in Perseus

The internal velocity structure within dense gaseous cores plays a crucial role in providing the initial conditions for star formation in molecular clouds. However, the kinematic properties of dense gas at core scales (~0.01 - 0.1 pc) has not been extensively characterized because of instrument limitations until the unique capabilities of GBT-Argus became available. The ongoing GBT-Argus Large Program, Dynamics in Star-forming Cores (DiSCo) thus aims to investigate the origin and distribution of angular momenta of star-forming cores. DiSCo will survey all starless cores and Class 0 protostellar cores in the Perseus molecular complex down to ~0.01 pc scales with < 0.05 km/s velocity resolution using the dense gas tracer N$_2$H$^+$. Here, we present the first datasets from DiSCo toward the B1 and NGC 1333 regions in Perseus. Our results suggest that a dense core's internal velocity structure has little correlation with other core-scale properties, indicating these gas motions may be originated externally from cloud-scale turbulence. These first datasets also reaffirm the ability of GBT-Argus for studying dense core velocity structure and provided an empirical basis for future studies that address the angular momentum problem with a statistically broad sample.

astro-ph.GA↗

arXiv2023

Modeling CN Zeeman Effect Observations of the Envelopes of a Low-Mass Protostellar Disk and a Massive Protostar

We use the POLARIS radiative transfer code to produce simulated circular polarization Zeeman emission maps of the CN $J = 1 - 0$ molecular line transition for two types of protostellar envelope magnetohydrodynamic simulations. Our first model is a low mass disk envelope system (box length $L = 200\text{ au}$), and our second model is the envelope of a massive protostar ($L = 10^4\text{ au}$) with a protostellar wind and a CN enhanced outflow shell. We compute the velocity-integrated Stokes $I$ and $V$, as well as the implied $V/I$ polarization percentage, for each detector pixel location in our simulated emission maps. Our results show that both types of protostellar environment are in principle accessible with current circular polarization instruments, with each containing swaths of envelope area that yield percentage polarizations that exceed the 1.8\% nominal sensitivity limit for circular polarization experiments with the Atacama Large Millimeter/submillimeter Array (ALMA). In both systems, high polarization ($\gtrsim$1.8\%) pixels tend to lie at an intermediate distance away from the central star and where the line-center opacity of the CN emission is moderately optically thin ($\tau_{LC} \sim 0.1-1$). Furthermore, our computed $V/I$ values scale roughly with the density weighted mean line-of-sight magnetic field strength, indicating that Zeeman observations can effectively diagnose the strength of envelope-scale magnetic fields. We also find that pixels with large $V/I$ are preferentially co-located where the absolute value of the velocity-integrated $V$ is also large, suggesting that locations with favorable percentage polarization are also favorable in terms of raw signal.

astro-ph.SR↗

arXiv2023

Quasinormal modes of black holes encircled by a gravitating thin disk

The ringdown phase of gravitational waves emitted by a perturbed black hole is described by a superposition of exponentially decaying sinusoidal modes, called quasinormal modes (QNMs), whose frequencies depend only on the property of the black hole geometry. The extraction of QNM frequencies of an isolated black hole would allow for testing how well the black hole is described by general relativity. However, astrophysical black holes are not isolated. It remains unclear whether the extra matter surrounding the black holes such as accretion disks would affect the validity of the black hole spectroscopy when the gravitational effects of the disks are taken into account. In this paper, we study the QNMs of a Schwarzschild black hole superposed with a gravitating thin disk. Considering up to the first order of the mass ratio between the disk and the black hole, we find that the existence of the disk would decrease the oscillating frequency and the decay rate. In addition, within the parameter space where the disk model can be regarded as physical, there seems to be a universal relation that the QNM frequencies tend to obey. The relation, if it holds generically, would assist in disentangling the QNM shifts caused by the disk contributions from those induced by other putative effects beyond general relativity. The QNMs in the eikonal limit, as well as their correspondence with bound photon orbits in this model, are briefly discussed.

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arXiv2023

Resonant orbits of rotating black holes beyond circularity: Discontinuity along parameter shift

According to general relativity, an isolated black hole in vacuum shall be described by the Kerr metric, whose geodesic equations are integrable. The violation of integrability leads to chaos for particles moving around the black hole. This chaotic dynamics could leave imprints on the associated gravitational waveform and could be tested with upcoming observations. In this paper, we investigate the chaotic orbital dynamics induced by the violation of a certain spacetime symmetry, the circularity. Specifically, we focus on the resonant orbits of a particular noncircular spacetime as an example and find that they form chains of Birkhoff islands on Poincar\'e surfaces of section. We compare the island structures with those generated in typical nonintegrable but circular spacetimes. The islands of stability induced by noncircularity appear asymmetric on the most common Poincar\'e surface of section at the equatorial plane. The asymmetric patterns of islands vary discontinuously when the spacetime parameters transit through integrable regions. The origin of such features is explained in the context of perturbation analysis by considering the orbits associated with stable fixed points on the section. Possible observational implications about testing circularity through gravitational wave detection are discussed.

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arXiv2023

Relative Alignment Between Magnetic Fields and Molecular Gas Structure in Molecular Clouds

We compare the structure of synthetic dust polarization with synthetic molecular line emission from radiative transfer calculations using a 3-dimensional, turbulent collapsing-cloud magnetohydrodynamics simulation. The histogram of relative orientations (HRO) technique and the projected Rayleigh statistic (PRS) are considered. In our trans-Alfv\'enic (more strongly magnetized) simulation, there is a transition to perpendicular alignment at densities above $\sim$$4 \times 10^{3}$ cm$^{-3}$. This transition is recovered in most of our synthetic observations of optically thin molecular tracers, however for $^{12}$CO it does not occur and the PRS remains in parallel alignment across the whole observer-space. We calculate the physical depth of the optical depth $\tau = 1$ surface and find that for $^{12}$CO it is largely located in front of the cloud midplane, suggesting that $^{12}$CO is too optically thick and instead mainly probes low volume density gas. In our super-Alfv\'enic simulation, the magnetic field becomes significantly more tangled, and all observed tracers tend toward no preference for perpendicular or parallel alignment. An observable difference in alignment between optically thin and optically thick tracers may indicate the presence of a dynamically important magnetic field, though there is some degeneracy with viewing angle. We convolve our data with a Gaussian beam and compare it with HRO results of the Vela C molecular cloud. We find good agreement between these results and our sub-Alfv\'enic simulations when viewed with the magnetic field in the plane-of-the-sky (especially when sensitivity limitations are considered), though the observations are also consistent with an intermediately inclined magnetic field.

astro-ph.GA↗

arXiv2022

A novel test of gravity via black hole eikonal correspondence

When adopted in black hole spacetimes, geometric-optics approximations imply a mapping between the quasinormal mode (QNM) spectrum of black holes in the eikonal limit and black hole images. In particular, the real part and the imaginary part of eikonal QNM frequencies are associated with the apparent size and the detailed structure of the ring images, respectively. This correspondence could be violated when going beyond general relativity. We propose a novel method to test the eikonal correspondence via the comparison of two sets of observables from a nonrotating black hole, one extracted from QNM spectra and the other from the lensed photon rings on the image plane. Specifically, the photon ring observables robustly capture the information of the black hole spacetime itself regardless of the surrounding emission models. Therefore, the proposed test of eikonal correspondence can be validated in quite broad scenarios.

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arXiv2022

Revealing the drag instability in one-fluid nonideal MHD simulations of a 1D isothermal C-shock

C-type shocks are believed to be ubiquitous in turbulent molecular clouds thanks to ambipolar diffusion. We investigate whether the drag instability in 1D isothermal C-shocks, inferred from the local linear theory of Gu & Chen, can appear in non-ideal magnetohydrodynamic simulations. Two C-shock models (with narrow and broad steady-state shock widths) are considered to represent the typical environment of star-forming clouds. The ionization-recombination equilibrium is adopted for the one-fluid approach. In the 1D simulation, the inflow gas is continuously perturbed by a sinusoidal density fluctuation with a constant frequency. The perturbations clearly grow after entering the C-shock region until they start being damped at the transition to the postshock region. We show that the profiles of a predominant Fourier mode extracted locally from the simulated growing perturbation match those of the growing mode derived from the linear analysis. Moreover, the local growth rate and wave frequency derived from the predominant mode generally agree with those from the linear theory. Therefore, we confirm the presence of the drag instability in simulated 1D isothermal C-shocks. We also explore the nonlinear behavior of the instability by imposing larger-amplitude perturbations to the simulation. We find that the drag instability is subject to wave steepening, leading to saturated perturbation growth. Issues concerning local analysis, nonlinear effects, one-fluid approach, and astrophysical applications are discussed.

astro-ph.GA↗

arXiv2022

On the Possible Spacetime Structures of Rotating Loop Quantum Black Holes

To date, a mathematically consistent construction of effective rotating black hole models in the context of Loop Quantum Gravity (LQG) is still lacking. In this work, we start with the assumption that rotating LQG black hole metrics can be effectively obtained using Newman-Janis Algorithm. Then, based on a few extra fair assumptions on the seed metric functions, we make a conjecture on what a rotating LQG black hole would generically look like. Our general arguments and conclusions can be supported by some known specific examples in the literature.

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