arXiv subjects

Keisuke Nakashi

Publications and source records attributed to Keisuke Nakashi.

11 recordsLinked to original sources

arXiv2026

Ellis-Bronnikov Wormhole Shadows with Spherically Symmetric Accretion Flow

We investigate the observational differences between the Ellis--Bronnikov (EB) wormhole and the Schwarzschild black hole (BH) by performing general relativistic radiative transfer (GRRT) simulations. We consider a spherically symmetric steady-state accretion flow and perform GRRT simulations incorporating synchrotron emission. For both the EB wormhole and the Schwarzschild BH, the simulated images consist of a central shadow region and a bright photon ring. We find that both the shadow region and the photon ring of the EB wormhole are brighter than those of the Schwarzschild BH. These differences arise from the absence of an event horizon in the EB wormhole, allowing the emission from the accreting matter around and beyond the throat to contribute to the observed intensity. We also compare the simulated images with the Event Horizon Telescope (EHT) observations of M87* and find that both the EB wormhole and the Schwarzschild BH are in reasonable agreement with the current EHT results.

gr-qc↗

arXiv2025

Probing higher curvature gravity via ringdown with overtones

We investigate metric perturbations of a spherically symmetric black hole in higher curvature gravity. We show that higher curvature corrections deform the near-horizon region of the effective potential, and that the deviations of the quasinormal mode (QNM) frequencies from their general relativity (GR) values become more pronounced for overtone modes. We find that, as the order of the higher curvature term increases, the deformations approach the horizon and the deviations of the overtone QNM frequencies grow progressively larger. We also analyze the ringdown waveforms in the higher curvature gravity model. We consider setups in which the deviations from the vacuum-GR QNMs remain mild for the fundamental mode and the first few overtones, and show that these shifted QNMs can be identified in the ringdown signal through waveform fitting.

gr-qc↗

arXiv2023

Black hole ringdown from physically sensible initial value problem in higher-order scalar-tensor theories

We study odd-parity perturbations about static and spherically symmetric black hole solutions with a linearly time-dependent scalar field in higher-order scalar-tensor theories. In particular, we consider stealth Schwarzschild and stealth Schwarzschild-de Sitter solutions, where the deviation from the general relativity case is controlled by a single parameter. We find that complex frequencies of quasinormal modes (QNMs) are given by a simple scaling of those in general relativity. We also show that there is a degeneracy between the parameter characterizing the modification from general relativity and the black hole mass. We then consider a physically sensible initial value problem by taking into account the fact that the effective metric for the odd-parity perturbations is in general different from the background metric. We confirm that damped oscillations appearing at late times are indeed dominated by the QNMs. Our analysis includes the case where the perturbations are superluminal, and we demonstrate in this case that the perturbations can escape from the region inside the horizon for the background metric.

gr-qc↗

arXiv2022

Black hole perturbations in higher-order scalar-tensor theories: initial value problem and dynamical stability

We propose a physically sensible formulation of initial value problem for black hole perturbations in higher-order scalar-tensor theories. As a first application, we study monopole perturbations around stealth Schwarzschild solutions in a shift- and reflection-symmetric subclass of degenerate higher-order scalar-tensor (DHOST) theories. In particular, we investigate the time evolution of the monopole perturbations by solving a two-dimensional wave equation and analyze the Vishveshwara's classical scattering experiment, i.e., the time evolution of a Gaussian wave packet. As a result, we confirm that stealth Schwarzschild solutions in the DHOST theory are dynamically stable against the monopole perturbations with the wavelength comparable or shorter than the size of the black hole horizon. We also find that the damped oscillations at the late time do not show up unlike the ringdown phase in the standard case of general relativity. Moreover, we investigate the characteristic curves of the monopole perturbations as well as a static spherically symmetric solution with monopole hair.

gr-qc↗

arXiv2022

Revisiting timelike geodesics in the Fisher-Janis-Newman-Winicour-Wyman spacetime

We investigate the timelike geodesics and the periapsis precession of orbits in the Fisher-Janis-Newman-Winicour-Wyman spacetime. This spacetime represents the naked singularity spacetime in the Einstein-massless scalar system. We revisit the results in the previous studies and relax the assumptions about the eccentricity of a bound orbit and the size of a semilatus. We find that the negative periapsis precession occurs when the spacetime sufficiently deviates from the Schwarzschild spacetime. In particular, for the small eccentric orbits, we show the negative periapsis precession occurs for $γ< 1/2$, where $γ$ is the deviation parameter from the Schwarzschild spacetime. We also obtain the analytical solutions for the special cases of $γ=0,1/2,1/4$. Then, we show that the negative precession never occurs for $γ=1/2$.

gr-qc↗

arXiv2021

Photon surfaces in less symmetric spacetimes

We investigate photon surfaces and their stability in a less symmetric spacetime, a general static warped product with a warping function acting on a Riemannian submanifold of codimension two. We find a one-dimensional pseudopotential that gives photon surfaces as its extrema regardless of the spatial symmetry of the submanifold. The maxima and minima correspond to unstable and stable photon surfaces, respectively. It is analogous to the potential giving null circular orbits in a spherically symmetric spacetime. We also see that photon surfaces indeed exist for the spacetimes which are solutions to the Einstein equation. The parameter values for which the photon surfaces exist are specified. As we show finally, the pseudopotential arises due to the separability of the null geodesic equation, and the separability comes from the existence of a Killing tensor in the spacetime. The result leads to the conclusion that photon surfaces may exist even in a less symmetric spacetime if the spacetime admits a Killing tensor.

gr-qc↗

arXiv2020

Towards rotating non-circular black holes in string-inspired gravity

We study stationary slowly rotating black holes, up to quadratic order in the spin angular momentum, in dynamical Chern-Simons gravity and shift symmetric Einstein scalar Gauss-Bonnet gravity, as models of string-inspired gravities. These gravity theories modify general relativity by introducing dynamical scalar fields coupled with curvature invariants. We show that the linear time dependence of the scalar fields is allowed from the stationarity of the effective stress energy tensors. However, these time dependent scalar fields yield singular behavior of the metric functions at the black hole horizons, or they are incompatible with the stationarity of the spacetimes. Thus, these gravity theories admit only known solutions as regular stationary solutions. Our results suggest the non-existence of rotating non-circular black holes in these gravity theories.

gr-qc↗

arXiv2020

Observability of the innermost stable circular orbit in a near-extremal Kerr black hole

We consider the escape probability of a photon emitted from the innermost stable circular orbit (ISCO) of a rapidly rotating black hole. As an isotropically emitting light source on a circular orbit reduces its orbital radius, the escape probability of a photon emitted from it decreases monotonically. The escape probability evaluated at the ISCO also decreases monotonically as the black hole spin increases. When the dimensionless Kerr parameter $a$ is at the Thorne limit $a=0.998$, the escape probability from the ISCO is $58.8\%$. In the extremal case $a=1$, even if the orbital radius of the light source is arbitrarily close to the ISCO radius, which coincides with the horizon radius, the escape probability remains at $54.6\%$. We also show that such photons that have escaped from the vicinity of the horizon reach infinity with sufficient energy to be potentially observed because Doppler blueshift due to relativistic beaming can overcome the gravitational redshift. Our findings indicate that signs of the near-horizon physics of a rapidly rotating black hole will be detectable on the edge of its shadow.

astro-ph.HE↗

arXiv2019

Effect of a second compact object on stable circular orbits

We investigate how stable circular orbits around a main compact object appear depending on the presence of a second one by using the Majumudar--Papapetrou dihole spacetime, which consists of the two extremal Reissner--Nordstr\" om black holes with different masses. While the parameter range of the separation of the two objects is divided due to the appearance of stable circular orbits, this division depends on its mass ratio. We show that the mass ratio range separates into four parts, and we find three critical values as the boundaries.

gr-qc↗

arXiv2019

Null Geodesics and Repulsive Behavior of Gravity in (2+1)-dimensional Massive Gravity

We study the null geodesics in a static circularly symmetric (SCS) black hole spacetime which is a solution in the $(2+1)$-dimensional massive gravity proposed by Bergshoeff, Hohm and Townsend (BHT massive gravity). We obtain analytic solutions for the null geodesic equation in the SCS black hole background and find the explicit form of deflection angles. We see that for various values of the impact parameter, the deflection angle can be positive, negative or even zero in this black hole spacetime. The negative deflection angle indicates the repulsive behavior of the gravity which comes from the gravitational hair parameter that is the most characteristic quantity of the BHT massive gravity.

gr-qc↗

arXiv2019

Innermost stable circular orbits in Majumdar--Papapetrou dihole spacetime

We investigate the positions of stable circular massive particle orbits in the Majumdar--Papapetrou dihole spacetime with equal mass. In terms of qualitative differences of their sequences, we classify the dihole separation into five ranges and find four critical values as the boundaries. When the separation is relatively large, the sequence on the symmetric plane bifurcates, and furthermore, they extend to each innermost stable circular orbit in the vicinity of each black hole. In a certain separation range, the sequence on the symmetric plane separates into two parts. On the basis of this phenomenon, we discuss the formation of double accretion disks with a common center. Finally, we clarify the dependence of the radii of marginally stable circular orbits and innermost stable circular orbits on the separation parameter. We find a discontinuous transition of the innermost stable circular orbit radius. We also find the separation range at which the radius of the innermost stable circular orbit can be smaller than that of the stable circular photon orbit.

gr-qc↗