arXiv subjects

Yuan-Xing Gao

Publications and source records attributed to Yuan-Xing Gao.

5 recordsLinked to original sources

arXiv2025

Analytical description for light propagation with the source inside the black hole photon sphere

The photon sphere defines the unstable circular orbit of photons in a black hole spacetime. Photons emitted by a source located inside the photon sphere can be gravitationally lensed by the black hole and have time delays when reaching the observer. These delays may lead to light echoes produced in the light curve if an accretion event in the vicinity of the horizon can be observed. In this work, we present fully analytical formulas with high accuracy to describe the change of the azimuthal angle and the travel time of those photons. By employing the analytical approaches, we find that the time delay between photons emitted from the interior of the photon sphere has a typical time scale of $2(π- ϕ_\mathrm{S}) u_\mathrm{m}$ with $ϕ_\mathrm{S}$ and $u_\mathrm{m}$ being respectively the azimuthal angle of the source and the impact parameter evaluated at the photon sphere, which can provide some clues on the future search for gravitational lensing signatures in the accretion inflow event.

astro-ph.HE↗

arXiv2025

Higher-order inner photon rings of a horizonless ultracompact object with an antiphoton sphere and their interferometric pattern

A horizonless ultracompact object can have a stable antiphoton sphere, which causes the strong deflection of photons inside the unstable photon sphere, leading to the formation of distinctive inner photon rings. In this work, we present analytical descriptions for the shape, thickness and interference pattern of higher-order inner photon rings. By taking the static spherically symmetric Schwarzschild star with a photon sphere as an example, we find that its inner photon rings can be more non-circular and thicker than the outer ones, and show that the inclusion of the inner photon rings can give rise to new features in the interferometric pattern. Our formulae can also be applied to other ultracompact objects, providing a convenient way to study the observational properties of their higher-order photon rings.

gr-qc↗

arXiv2020

Strong gravitational lensing of a 4-dimensional Einstein-Gauss-Bonnet black hole in homogeneous plasma

We investigate the strong gravitational lensing of spherically symmetric black holes in the novel Einstein-Gauss-Bonnet(EGB) gravity surrounded by unmagnetised plasma medium. The deflection angle in the strong deflection limit in EGB spacetime with homogeneous plasma is derived. We find that both the coupling constant $α$ in the novel EGB gravity and the presence of plasma can affect the radius of photon sphere, strong field limit coefficient and other lensing observables significantly. While plasma has little effect on the angular image separation and the relative magnifications as $α/M^2\to -8$ and $α/M^2\to 1$, respectively.

gr-qc↗

arXiv2019

Analytically approximated scalarized black holes and their thermodynamic stability

It is recently shown that, besides the Schwarzshcild black hole solution, there exist also scalarized black hole solutions in some Einstein-scalar-Gauss-Bonnet theories. In this paper, we construct analytical expressions for the metric functions and scalar field configurations for these scalarized black hole solutions approximately by employing the continued fraction parametrization method and investigate their thermodynamic stability. It is found that the horizon entropy of a scalarized black hole is always smaller than that of a Schwarzschild black hole, which indicates that these scalarized black holes may decay to Schwarzschild black holes by emission of scalar waves. This fact also implies the possibility to extract the energy of scalar charges.

gr-qc↗

arXiv2019

Scalar perturbations on the background of Kerr black holes in the quadratic dynamical Chern-Simons gravity

We study the scalar perturbation on the background of a Kerr black hole in the dynamical Chern-Simons modified gravity with a quadratic coupling between the scalar field and Chern-Simons term. In particular, the late-time tails of scalar perturbations are investigated numerically in time domain by using the hyperboloidal foliation method. It is found that the Kerr black hole becomes unstable under linear perturbations in a certain region of the parameter space, which depends on the harmonic azimuthal index $m$ of the perturbation's mode. This may indicate that some Kerr black holes in this theory will get spontaneously scalarized into a non-Kerr black hole.

gr-qc↗