arXiv subjects

Junji Jia

Publications and source records attributed to Junji Jia.

At least 19 recordsLinked to original sources

arXiv2026

The brachistochrone problem for a constant velocity traveler in static and spherically symmetric spacetimes

This work investigates the brachistochrone problem for a traveler with constant local velocity within static and spherically symmetric (SSS) spacetimes. The brachistochrone trajectory (BT) equations for ultra-relativistic travelers are derived for general SSS metrics, and the solution is formally obtained in an integral form. We then apply the result to two representative spacetimes corresponding to the singular isothermal sphere (SIS) with a finite boundary and the relativistic Plummer mass profile, respectively. For the SIS spacetime, the BT inside the boundary is solved analytically and found always to bend. As the equation of state index $w$ increases, the turning radius $r_0$ of the BT, and consequently the total time, also increase. For the BT outside the boundary, it is found that the heavier the central object, the larger the $r_0$ and the total travel time. For the relativistic Plummer model, the BT will be a straight line passing through the origin when the initial and final points' radii are comparable or smaller than the size of the core region of the mass distribution. When the end points lie well outside the core region, the BT bends, exhibiting a larger turning radius for a more concentrated core. We then extend the consideration to travelers with subluminal constant velocity $v$ and show through the generalized Fermat's principle that the BT will be the same geodesic in the optical metric as ultra-relativistic travelers, with the total travel time scaled by a factor of $1/v$.

gr-qc↗

arXiv2026

Light deflection in general static and spherically symmetric spacetime with a homogeneous plasma

We developed in this work a perturbative technique to compute the deflection angle of light rays in general static and spherically symmetric spacetimes with a homogeneous non-magnetic plasma. The deflection angle is expressed as a power series of $M/b$ with $M$ and $b$ being the spacetime mass and the impact parameter of the ray. The series coefficients are polynomials of the asymptotic expansion coefficients of the metric functions and the reciprocal of the asymptotic refractive index. When the plasma is dilute, the deflection angle can also be expressed as a dual series of $M/b$ and the frequency ratio between the electron plasma frequency and the asymptotic photon frequency. The series result reveals that for general SSS spacetime, the plasma at the leading order always enhances the deflection angle and therefore increases the apparent angles of the gravitational lensing images. These series results of the deflection angle are then shown to have excellent agreement with those obtained using numerical integration. The general formula of the deflection angle is then applied to the Reissner-Nordstr\"{o}m, charged Horndeski and charged Galileon spacetimes. The effect of the plasma and characteristic parameter of the spacetimes on the deflection angles in these spacetimes was briefly discussed. In the appendix, the deflection angles in previously attempted spacetimes are re-computed and compared with the literature.

gr-qc↗

arXiv2026

Matter around Schwarzschild black holes in scalar-tensor theories: Absorption and Scattering

We investigate the absorption and scattering by a Schwarzschild black hole in scalar--tensor theories of gravity, where the coupling between matter and the scalar field induces different models for the effective mass of the scalar field. In model~I, a Bondi-type mass model described by the asymptotic mass $\mu_c$, horizon mass $\mu_H$, and profile slope $\lambda$, it is found that the absorption cross section increases with steeper $\lambda$, larger $\mu_c$ (especially at higher frequencies), or smaller $\mu_H$. The differential scattering cross section in this model shows the strongest dependence on the horizon mass $\mu_H$. When $\mu_H$ exceeds a critical value for a fixed incoming wave frequency $\omega$, no partial wave transmits into the black hole, flattening the differential scattering cross section as a function of angle before it increases again with further increase of $\mu_H$. Model~II, which considers a truncated accretion region outside some radius $r_0$, contains a potential well in its effective scattering potential. Its absorption cross section decreases in the low-frequency region as the accretion radius $r_0$ decreases, and more importantly, it shows resonance peaks at the quasibound wave frequencies due to resonances induced by the potential well. The differential scattering cross sections show dips around intermediate scattering angles when the parameters (mainly $\mu_H$ and $\omega$) are such that the resonantly scattered and non-resonant waves interfere destructively around these angles. In both models, absorption exhibits a zero-absorption band as $\omega$ approaches $\mu_c$ from above, and in both absorption and scattering, the effects of the parameters are found to diminish in the high-frequency limit.

gr-qc↗

arXiv2025

Scattering of charged massive scalar waves by Kerr-Newman black holes

The scattering of charged massive scalar waves by Kerr-Newman black holes, with incidence along the equatorial plane, is investigated in this work. The differential scattering cross section is computed using the partial wave method, with the forward divergence handled via the series reduction technique. For the first time, we systematically examine the influence of the black hole charge, electromagnetic interactions, and field mass on the equatorial cross section. Our results reveal that regardless of whether the electromagnetic interaction is present or not, the frame-dragging effect shifts the glory away from the exact backward direction and can place interference minima there, contrasting with the on-axis scattering case. The average scattered flux intensity at the medium to large scattering angles exhibits a large enhancement as the Lorentz attraction or field mass increases, particularly in the slowly rotating regime, with the enhancement being frequency-dependent. When superradiance occurs, we observe that the cross section in the prograde scattering angles ($\sim 135^{\circ} < \phi < 270^{\circ}$) increases as the black hole spin increases, due to enhanced prograde partial wave contributions. Meanwhile, the superradiant scattering cross section increases in all (except the forward) directions when the Lorentz force becomes more repulsive. These findings highlight unique equatorial-plane signatures of charged, rotating spacetimes, distinguishing them from prior on-axis analyses.

gr-qc↗

arXiv2025

Spin precession in general stationary and axisymmetric spacetimes

This paper investigates the spin precession of test particles moving in the equatorial plane of general stationary and axisymmetric spacetimes using the Mathisson-Papapetrou-Dixon equations. The spin precession angles for two cases, the small-spin case and the spin-orbital plane parallel case, are derived using different approximations. For the small-spin case, the precession angle of the spin components in the equatorial plane along circular orbits is found, and perpendicular component is shown to be a constant of motion. For the spin-orbital plane parallel case, it is shown that in general the orbital and spin motions generally do not affect each other, and the spin precession angle is calculated using the post-Newtonian method to an arbitrarily high order of the orbital semi-latus rectum p. The precession angles in both cases are analyzed both qualitatively and quantitatively in the Kerr-Newman spacetime to elucidate their features. For large orbital radii, it is shown that the leading order of the precession angle series is generally proportional to the spacetime mass while the Lense-Thirring effect always appears from the subleading order. These precession results are then applied to various astronomical systems to determine their spin precession rates. For systems with observational data, our results show excellent agreement. For systems without observational data, we predict their spin precession rates at both the leading and Lense-Thirring effect orders. These predictions indicate that Jupiter's satellites exhibit exceptionally large geodetic spin precession and their Lense-Thirring effect may be detectable with current technology.

gr-qc↗

arXiv2025

Periapsis shift in magnetized stationary and axisymmetric spacetimes

In this work, we conduct a detailed study of the precession of charged particles in stationary and asymmetric spacetimes with external magnetic fields. Specifically, we develop the post-Newtonian method and the quasi-circular approximation to derive the periapsis shift respectively for two common types of magnetic fields, the dipolar one and the asymptotically uniform one. It is found using the PN method for magnetic fields decaying as fast or faster than a magnetic dipole that the magnetic effect in the periapsis shift appears from the same order of the traditional frame-dragging term due to the spacetime spin. The magnetic field is found to enhance (or decrease) the periapsis shift when the Lorentz force is attractive (or repulsive). When the repulsive Lorentz force is strong enough, the periapsis shift can become negative. For magnetic fields with a slower decay rate, the periapsis shift of quasi-circular orbits exhibits a more complex dependence on the Lorentz force. The periapsis shift increases as the attractive Lorentz force increases from zero but will decrease eventually. However, when the Lorentz force is repulsive, the orbit develops local helical loops and the periapsis shift approaches $-2\pi$. The results for the dipolar magnetic field are applied to the periapsis shift of Mercury around the Sun and the S2 around the Sgr A* to constrain the dipole moment of the center and the charge of the orbiting object.

gr-qc↗

arXiv2025

On-axis absorption and scattering of charged massive scalar waves by Kerr-Newman black-bounce spacetime

We investigate the absorption and scattering of charged massive scalar waves by the Kerr-Newman black-bounce spacetime when the waves are incident along the rotation axis. Our findings indicate that a faster (slower) rotating spacetime or a more repulsive (attractive) electric force tends to reduce (increase) the absorption cross section and results in larger (smaller) angular widths of the scattered wave oscillations. We find that the rotation parameter exerts a suppressive influence on superradiance, which contrasts with the enhancing effect of the repulsive electric force. It is worth mentioning that the regularization parameter $k$ is found to modify the absorption or scattering cross sections only weakly, but can cause a noticeable reduction of superradiance. To further clarify the role of the parameters in superradiance, we study the energy extraction efficiency in the electric Penrose process. For particles moving along the rotation axis, we find that the influence of the parameters ($a, q, k$) on this efficiency is consistent with their effects on superradiance. We also discuss potential astrophysical applications, showing that particles in this process could be accelerated to ultrahigh energies in realistic environments, and could therefore be used to constrain black hole parameters. For the effect of field mass, it is found that a heavier scalar field leads to a larger absorption cross section and a wider interference fringe of the differential scattering cross section. When superradiance happens, i.e., the absorption cross section becomes negative, it is also found that the differential scattering cross section only changes smoothly, with no apparent qualitative feature showing up.

gr-qc↗

arXiv2025

From Matter Density to Deflection Angle and Gravitational Lensing Using a Perturbative Method

In this work, we develop a perturbative method to compute the deflection angle of null or timelike signals in spacetimes filled with a static and spherically symmetric (SSS) perfect fluid with fairly arbitrary density distributions. After solving the Tolman-Oppenheimer-Volkoff equations, the metric functions of the spacetime are obtained either as asymptotic series or as expansions around a finite boundary. The deflection angles of null or timelike signals in the weak-field limit in such spacetimes can then be expressed as series expansions in terms of the impact parameter, with coefficients determined by the metric expansions and, in turn, the density distribution function. Gravitational lensing equations are also solved perturbatively to derive the apparent angles of the lensed images. Comparing our analytical formulas with numerical results demonstrates the validity and efficiency of our method and results. This procedure establishes a direct connection between the mass density, the deflection angle, and the apparent angles of gravitationally lensed images. We apply these methods and results to the generalized Navarro-Frenk-White model and some other density profiles to analyze the influence of the density parameters.

gr-qc↗

arXiv2025

Weak deflection angle of charged signal in magnetic fields

We use the perturbative method to study the influence of the magnetic field on the weak deflection angle of charged signals in magnetized stationary and axisymmetric spacetimes within general electromagnetic potentials. The deflection angle is expressed as a series expansion of the inverse of the impact parameter $b$, with coefficients determined by the asymptotic expansions of the metric functions and the electromagnetic four-potential. It is found that in general, the deflection angle can always be separated into two parts, the usual gravitational part as for neutral particles, and the electromagnetic part due to the interaction between the (electro)magnetic field and the signal. The leading order of the gravitational, electrostatic (from nonzero spacetime charge) and magnetic (from nonzero magnetic dipole moment) contributions are $b^{-1},\,b^{-1}$ and $b^{-2}$ respectively. The entire electromagnetic part is enhanced by the large specific charge of elementary particles but suppressed by the reciprocal Lorentz factor. The deflection angle result is then applied to three spacetimes with intrinsic or externally enforced magnetic fields. Effects of the magnetic field on the deflection angle from various parameters, including the spacetime spin, magnetic dipole moment and magnetic parameters, are analyzed. In all these cases, it is found that in the weak deflection limit, these effects agree with the expectation for a Lorentz force; that is, an attractive (or repulsive) one will enlarge (or decrease) the deflection angle.

gr-qc↗

arXiv2024

Absorption and scattering of charged scalar waves by charged Horndeski black hole

We investigate the absorption and scattering of a charged massive scalar field by a charged Horndeski black hole using both the approximation or classical geometric method and the partial wave method and compare the numerical and analytical results, which are found to agree with each other very well. We observe that an increase in either the BH charge $Q$ or the field charge $q$ when $qQ>0$ leads to a smaller absorption cross section and a widening of the interference fringes in the scattering cross section, while the increase in the field mass enlarges the absorption cross section and the width of the interference fringes. Compared to the Reissner-Nordstr$\ddot{\rm{o}}$m BH with the same charge and other parameter settings, the absorption and scattering cross sections of the charged Horndeski BH are higher, and its interference fringes are narrower. We also investigate the effect of the field charge $q$ on the absorption and scattering cross sections when superradiance is triggered. It is shown that the total absorption cross section can be negative, and the scattering intensity can be significantly enhanced by superradiance.

gr-qc↗

arXiv2024

Vortex Lines in Ultralight Bosonic Dark Matter around Rotating Supermassive Black Holes

Theoretical analysis of the interaction between superfluid dark matter and rotating supermassive black holes offers a promising framework for probing quantum effects in ultralight dark matter and its role in galactic structure. We study how black hole rotation influences the state of ultralight bosonic dark matter, focusing on the stability and dynamics of vortex lines. The gravitational effects of both dark matter and the black hole on the physical properties of these vortex lines, including their precession around the black hole, are analyzed.

hep-ph↗

arXiv2024

Dynamical friction in rotating ultralight dark matter galactic cores

Dynamical friction and stellar orbital motion in spiral galaxies with dark matter composed of ultralight bosons in the state of rotating Bose-Einstein condensate (BEC) are studied. It is found that the dynamical friction force is significantly affected by the topological charge of the vortex structure of the BEC core with the strongest effect at distances near the galactic center. It is also shown that the ultralight dark matter self-interaction plays an important role in studying the dynamical friction.

astro-ph.GA↗

arXiv2024

Off-equatorial deflections and gravitational lensing. II. In general stationary and axisymmetric spacetimes

In this work, we develop a general perturbative procedure to find the off-equatorial plane deflections in the weak deflection limit in general stationary and axisymmetric spacetimes, allowing the existence of the generalized Carter constant. Deflections of both null and timelike rays, with the finite distance effect of the source and detector taken into account, are obtained as dual series of $M/r_0$ and $r_0/r_{s,d}$. These deflections allow a set of exact gravitational lensing equations from which the images' apparent angular positions are solved. The method and general results are then applied to the Kerr-Newmann, Kerr-Sen, and rotating Simpson-Visser spacetimes to study the effect of the spin and characteristic (effective) charge of the spacetimes and the source altitude on the deflection angles and image apparent angles. It is found that, in general, both the spacetime spin and charge only affect the deflections from the second non-trivial order, while the source altitude influences the deflection from the leading order. Because of this, it is found that, in gravitational lensing in realistic situations, it is hard to measure the effects of the spacetime spin and charge from the images' apparent locations. We also presented the off-equatorial deflections in the rotating Bardeen, Hayward, Ghosh, and Tinchev black hole spacetimes.

gr-qc↗

arXiv2024

Dragging of the particle spin and spin-spin coupling effect on its periapsis shift

The periapsis shift (PS) of spinning test particles in the equatorial plane of arbitrary stationary and axisymmetric spacetime is studied using the post-Newtonian method. The result is expressed as a half-integer power series of $M/p$ where $M$ is the spacetime mass and $p$ is the semilatus rectum. The coefficients of the series are polynomials of the particle spin, the asymptotic expansion coefficients of the metric functions and the eccentricity of the orbit. The particle spin is shown to have a similar effect as the Lense-Thirring (LT) effect on the PS, and both of them appear from the $(M/p)^{-3/2}$ order in the PS. The coupling between the spacetime and particle spins will increase (or decrease) the PS if they are parallel (or antiparallel). For Jupiter and Saturn rotating around the Sun and exceptionally designed satellites around Mercury and Moon, the particle spin effect can be comparable to the LT one in size. The PS in other spacetime studied are not distinguishable from that in the Kerr spacetime to the $(M/p)^{-3/2}$ order.

gr-qc↗

arXiv2024

Generating New Spacetimes through Zermelo Navigation

Zermelo navigation is not only a fundamental tool in Finsler geometry but also a fundamental approach to the geometrization of dynamics in physics. In this paper, we consider the Zermelo navigation problem on optical Riemannian space and, via Zermelo/Randers/spacetime triangle, explore the generation of new spacetimes from pre-existing ones. Whether the Randers metric has reversible geodesics corresponds to the presence of time-reversal symmetry in the generated spacetime. In cases where the Randers metric has reversible geodesics, we utilize a radial vector field to generate new static spacetimes from existing ones. For example, we can generate Schwarzschild, Rindler, de Sitter, and Schwarzschild-de Sitter spacetimes from flat spacetime. In fact, the Zermelo navigation method allows for the derivation of a variety of static spacetimes from flat spacetime. For multi-parameter spacetimes, they can be generated through various navigation paths. However, for some spacetimes, not all navigation paths may exist. In the second scenario, when the Randers metric does not have reversible geodesics, we employ a rotational vector field to transform non-flat static metrics into slowly rotating spacetimes. Alternatively, using a mixed vector field, we generate slowly rotating spacetimes starting from flat spacetime. We provide examples of generating Kerr spacetimes and Kerr-de Sitter spacetimes.

gr-qc↗

arXiv2023

Periapsis shift in spherically symmetric spacetimes and effect of electric interaction

The periapsis shift of charged test particles in arbitrary static and spherically symmetric charged spacetimes are studied. Two perturbative methods, the near-circular approximation and post-Newtonian methods, are developed, and shown to be very accurate when the results are found to high orders. The former method is more precise when the eccentricity $e$ of the orbit is small while the latter works better when the orbit semilatus rectum $p$ is large. Results from these two methods are shown to agree with each other when both $e$ is small and $p$ is large. These results are then applied to the Reissner-Nordstr\"om spacetime, the Einstein-Maxwell-dilation gravity and a charged wormhole spacetime. The effects of various parameters on the periapsis shift, especially that of the electrostatic interaction, are carefully studied. The periapsis shift data of the solar-Mercury is then used to constrain the charges of the Sun and Mercury, and the data of the Sgr A$^*$-S2 periapsis shift is used to find, for the first time using this method, constraints about the charges of Sgr A$^*$ and S2.

gr-qc↗

arXiv2023

Deflection of charged signals in a dipole magnetic field in Kerr background

This paper investigates charged particle deflection in a Kerr spacetime background with a dipole magnetic field, focusing on the equatorial plane and employing the weak field approximation. We employ the Jacobi-Randers metric to unify the treatment of the gravitational and electromagnetic effects on charged particles. Furthermore, we utilize the Gauss-Bonnet theorem to calculate the deflection angle through curvature integrals. The difference between the prograde and retrograde deflection angles is linked to the non-reversibility of metrics and geodesics in Finsler geometry, revealing that this difference can be considered a Finslerian effect. We analyze the impact of both gravitomagnetic field and dipole magnetic field on particle motion and deflection using the Jacobi-Randers magnetic field. The model considered in this paper exhibits interesting features in the second-order approximation of ($M/b$). When $q\mu=2MaE$, the Jacobi-Randers metric possesses reversible geodesics, leading to equal prograde and retrograde deflection angles. In this case, the gravitomagnetic field and dipole magnetic field cancel each other out, distinguishing it from scenarios involving only the gravitomagnetic field or the dipole magnetic field. We also explore the magnetic field's impact on gravitational lensing of charged particles.

gr-qc↗

arXiv2023

Periapsis precession in general stationary and axisymmetric spacetimes

This work studies the periapsis shift in the equatorial plane of arbitrary stationary and axisymmetric spacetimes. Two perturbative methods are systematically developed. The first work for small eccentricity but very general orbit size and the second, which is post-Newtonian and includes two variants, is more accurate for orbits of large size but allows general eccentricity. Results from these methods are shown to be equivalent under small eccentricity and large size limits. The periapsis shift of Kerr-Newman, Kerr-Sen and Kerr-Taub-NUT spacetimes are computed to high orders. The electric charge and NUT charge are shown to contribute to the leading order but with opposite signs. The frame-dragging term and high-order effect of spacetime spin are given. The electric and NUT changes of the Earth, Sun and Sgr A* are constrained using the Mercury, satellite and S2 precession data. Periapsis shifts of other spacetimes are obtained too.

gr-qc↗