Takahisa Igata

We investigate the deflection of photons in the strong deflection limit within static, axisymmetric spacetimes possessing reflection symmetry. As the impact parameter approaches its critical value, the deflection angle exhibits a logarithmic divergence. This divergence is characterized by a logarithmic rate and a constant offset, which we express in terms of coordinate-invariant curvature evaluated at the unstable photon circular orbit. The curvature contribution is encoded in the electric part of the Weyl tensor, reflecting tidal effects, and the matter contribution is encoded in the Einstein tensor, capturing the influence of local energy and pressure. We also express these coefficients using Newman--Penrose scalars. By exploiting the relationship between the strong deflection limit and quasinormal modes, we derive a new expression for the quasinormal mode frequency in the eikonal limit in terms of the curvature scalars. Our results provide a unified and coordinate-invariant framework that connects observable lensing features and quasinormal modes to the local geometry and matter distribution near compact objects.

We investigate general relativistic effects on the photon spectrum emitted from decaying (or annihilating) particle dark matter in the halo surrounding a primordial black hole. The spectrum undergoes significant modification due to gravitational redshifts, which induces broadening as a result of the intense gravitational field near the black hole. This characteristic alteration in the photon spectrum presents a unique observational signature. Future observations of such spectral features may provide critical evidence for a mixed dark matter scenario, involving both primordial black holes and particle dark matter.

We investigate the effects of extended mass and spheroidal deformation on the periapsis shift of quasi-circular orbits inside a gravitating mass distribution in the Newtonian framework. Focusing on the internal gravitational potential of a spheroidal body with both homogeneous and inhomogeneous density profiles, we elucidate how the ratio of local density to average density governs the extended mass effect on the periapsis shift. By analyzing the orbital angular frequency, along with the radial and vertical epicyclic frequencies, we demonstrate that in the uniform density case (i.e., the Maclaurin spheroid), where the potential takes the form of a harmonic oscillator, the periapsis exhibits a constant retrograde shift of $-\pi$. In contrast, in regions where density inhomogeneity and spheroidal deformation (in both prolate and oblate forms) are significant, the periapsis shift varies with the guiding orbital radius due to local density contrast and deformation effects. The results indicate that oblate deformation suppresses the extended mass effect associated with the ratio of local density to average density, whereas prolate deformation amplifies it. Furthermore, by varying the density distribution parameters, we establish the conditions for orbital stability and identify the emergence of marginally stable orbits.

In static, spherically symmetric spacetimes, the deflection angle of photons in the strong deflection limit exhibits a logarithmic divergence. We introduce an analytical framework that clarifies the physical origin of this divergence by employing local, coordinate-invariant geometric quantities alongside the properties of the matter distribution. In contrast to conventional formulations -- where the divergence rate $\bar{a}$ is expressed via coordinate-dependent metric functions -- our approach relates $\bar{a}$ to the components of the Einstein tensor in an orthonormal basis adapted to the spacetime symmetry. By applying the Einstein equations, we derive the expression \begin{align*} \bar{a}=\frac{1}{\sqrt{1-8\pi R_{\mathrm{m } }^2\left(\rho_{\mathrm{m } }+\Pi_{\mathrm{m } }\right) } }, \end{align*} where $\rho_{\mathrm{m } }$ and $\Pi_{\mathrm{m } }$ denote the local energy density and tangential pressure evaluated at the photon sphere of areal radius $R_{\mathrm{m } }$. This result reveals that $\bar{a}$ is intrinsically governed by the local matter distribution, with the universal value $\bar{a}=1$ emerging when $\rho_{\mathrm{m } }+\Pi_{\mathrm{m } }=0$. Notably, this finding resolves the long-standing puzzle of obtaining $\bar{a}=1$ in a class of spacetimes supported by a massless scalar field. Furthermore, these local properties are reflected in the frequencies of quasinormal modes, suggesting a profound connection between strong gravitational lensing and the dynamical response of gravitational wave signals. Our framework, independent of any specific gravitational theory, offers a universal tool for testing gravitational theories and interpreting astrophysical observations.

In this paper, we investigate the gravitational lensing and accretion disk imaging characteristics of a dense core modeled by the Buchdahl spacetime. By imposing the appropriate energy conditions and ensuring the absence of curvature singularities, we delineate the parameter space in which the dense core mimics key gravitational features of black holes while exhibiting unique deviations. We derive the photon orbital equation and calculate deflection angles, clearly distinguishing between weak- and strong-deflection regimes. Furthermore, we construct a mapping from the illuminated, geometrically thin accretion disk onto the observer's screen -- focusing on the isoradial curves corresponding to a representative source ring. For compactness values below a critical threshold, only a finite number of disk images are formed. In this range, their secondary and higher-order images typically display double-loop structures, with each loop individually capturing the entire source ring. Notably, the highest-order image sometimes appears as a single, crescent-shaped loop that does not enclose the screen's center, implying the existence of a cutoff angle that restricts the imaged portion of the source ring. In contrast, for compactness values above the critical threshold, an infinite sequence of double-loop structures appears -- a behavior closely linked to the presence of a photon sphere. These findings suggest that the lensing signatures of dense cores can distinguish them from black holes, offering new insights for high-resolution observations.