arXiv subjects

Kostas D. Kokkotas

Publications and source records attributed to Kostas D. Kokkotas.

At least 19 recordsLinked to original sources

arXiv2026

Modeling X-ray reflection spectra from disk returning radiation: Application to 4U 1630-47

Returning radiation is thought to play a key role in disk illumination of black-hole X-ray binaries in the high-soft state, yet it has not been fully incorporated into XSPEC reflection models. We present a new table model for reflection spectroscopy that self-consistently accounts for the returning radiation of the thermal component. To isolate this effect, we adopted the standard disk-corona configuration but disabled the corona, allowing the reflection spectrum to be produced solely by the self-irradiation of the disk. We applied our model to three NICER and NuSTAR simultaneous observations of the black-hole X-ray binary 4U 1630-47 during the soft state. The model fits the data well, suggesting that returning radiation from the disk could potentially explain the reflection features observed in this source. Our results suggest a rapidly spinning black hole ($a_* \sim 0.99$), a disk inclination of $i \sim 43^\circ-54^\circ$, a mass accretion rate of $\dot{M}_{\rm BH} \sim (0.13-0.25)\,\dot{M}_{\rm Edd}$, and a source distance of $7.3-11.3 \,\rm{kpc}$, which is in broad agreement with previous studies.

astro-ph.HE↗

arXiv2026

Constraining the geometry of rotating black holes with eikonal QNMs

Quasinormal modes of black holes provide a central avenue for confronting general relativity with gravitational-wave observations from the ringdown stage of compact binary coalescence. However, inferring black-hole properties or the background spacetime requires theory-dependent input. In this work, we connect a calibrated eikonal method, which relates quasinormal mode shifts to local metric properties near the light ring, with the underlying rotating black hole metric. Using Bayesian inference, we compare a metric-specific model based on the global Konoplya-Rezzolla-Zhidenko parametrization with a metric-agnostic approach based on local values and radial derivatives of the metric functions at the light ring. For each model, we perform both one-parameter and simultaneous multi-parameter analyses and quantify how their priors map onto the same local quantities. We find that uniform priors on the metric-specific coefficients can induce strongly nonuniform and skewed priors on these local metric quantities. For an injection compatible with general relativity, both models remain consistent with Kerr and yield similar posteriors for the orbital frequency and Lyapunov exponent despite their different induced priors. For injections representing deviations from general relativity, one-parameter analyses can fail to recover the injected local deviations and, in the metric-specific case, can bias the global metric reconstruction. The multi-parameter analyses of both models yield consistent constraints, suggesting sufficient robustness. However, they introduce substantial degeneracies and differences in marginalized parameters, demonstrating practical limitations when considering more flexible models. Finally, we apply the framework to approximate posterior information from the ringdown analysis of GW250114 and find that both models are consistent with the Kerr metric in general relativity.

gr-qc↗

arXiv2026

Parametrized beyond-Teukolsky framework in the time domain

Modifications to General Relativity can significantly alter the perturbative response of black holes, leaving imprints on quasinormal-mode spectra, waveform amplitudes and phases, and late-time tails. The parametrized beyond-Teukolsky framework was introduced to capture possible deviations from Kerr dynamics, but the ringdown has so far only been explored as an eigenvalue problem. We present the first time-domain implementation of this framework and perform (2+1)-dimensional scattering experiments with Gaussian wave packets. This approach provides the full linear evolution of the perturbation, from the initial prompt response through the ringdown and into the late-time regime. Using frequency-domain eigenvalue results as benchmarks, we find excellent agreement for low azimuthal numbers, while higher azimuthal numbers are affected by mode mixing, which limits the precision of the extracted modes. We further use the time-domain waveforms to estimate quadratic and mixed coefficients within the linearized modified-potential model, providing a diagnostic for the regime of validity of the linear approximation. Beyond mode frequencies, we show that the deformation parameters can strongly affect the ringdown amplitude and phase, with trends that can be understood from the near-horizon structure of the modified potential. We also analyze the late- time behavior, finding that near-horizon deformations leave the tail exponent unchanged but can substantially shift the onset of the power-law decay. These results demonstrate that time-domain evolutions provide a complementary and flexible framework for testing parametrized deviations from General Relativity in black-hole perturbation theory.

gr-qc↗

arXiv2026

Tidally-enhanced resonances in extreme-mass-ratio inspirals: A tertiary path to chaos

Extreme-mass-ratio inspirals (EMRIs) provide a unique laboratory for probing strong-field gravity and complex relativistic dynamics. We study a tidally deformed EMRI composed of a stellar-mass secondary orbiting a supermassive (non-)rotating black hole embedded in an external, adiabatically varying tidal environment. These systems provide a restricted, yet astrophysically motivated, realization of the relativistic three-body problem, expected to appear in active galactic nuclei, with a clear hierarchy of masses and radiation-reaction timescales. The external tidal deformation breaks the axisymmetry of the Kerr spacetime, rendering the geodesic dynamics non-integrable and giving rise to chaotic motion. The resulting signature of non-integrability is characterized through Poincaré maps and rotation curves constructed from the ratios of the fundamental frequencies of bound radial, polar, and azimuthal motion. We identify two prominent plateaus whose widths increase with the tidal field amplitude, signaling a transition from weak to strong chaos. We then demonstrate the sensitivity of the chaotic dynamics to the orientation of the orbit relative to the tidal field. We further analyze the proper-time evolution of the action-angle variables, showing that the angle combinations associated with the dominant commensurabilities are phase locked, thereby allowing the associated tidal contributions to induce secular changes in the constants of motion, whereas off-plateau angle combinations circulate. These results clarify the dynamical significance of the prominent plateaus and provide a novel phase-space characterization of tidal resonances in EMRIs. Finally, we discuss the potential role of radiation-reaction effects in driving EMRIs through tidal island crossings and the implications for gravitational-wave inference with future detectors.

gr-qc↗

arXiv2026

Magnetorotational instabilities in solids: Application to neutron-star crusts

The magnetorotational instability can generate strong, turbulent substructure within magnetized shear flows. The efficacy of the mechanism as a function of microphysical aspects of the fluid, such as stratification and diffusivity, has been explored extensively. One aspect that has not been studied thus far, however, is whether the instability can also operate in solids. Motivated by the possibility that solid regions within planets or degenerate stars may rotate differentially with respect to liquid or gaseous layers during some phase of their life, we examine the extent to which elasticity suppresses the instability. A simplified, plane-parallel analysis reveals that only in cases where the flow is strongly sheared, such that the magnetic tension that would result from the instability in a liquid exceeds the shear modulus of the elastic cavity, can magnetic growth occur. In the context of dynamical tides in binary neutron-star mergers, this implies that the magnetic field can be amplified in the crust prior to coalescence only if the star boasts a spin frequency of $\gtrsim 300$Hz. If viscous heating weakens the crystalline structure prior to resonance, the required spin frequency is reduced.

astro-ph.HE↗

arXiv2026

Confronting eikonal and post-Kerr methods with numerical evolution of scalar field perturbations in spacetimes beyond Kerr

The accurate computation of quasinormal modes from rotating black holes beyond general relativity is crucial for testing fundamental physics with gravitational waves. In this study, we assess the accuracy of the eikonal and post-Kerr approximations in predicting the quasinormal mode spectrum of a scalar field on a deformed Kerr spacetime. To obtain benchmark results and to analyze the ringdown dynamics from generic perturbations, we further employ a 2+1-dimensional numerical time-evolution framework. This approach enables a systematic quantification of theoretical uncertainties across multiple angular harmonics, a broad range of spin parameters, and progressively stronger deviations from the Kerr geometry. We then confront these modeling errors with simple projections of statistical uncertainties in quasinormal mode frequencies as a function of the signal-to-noise ratio, thereby exploring the domain of validity of approximate methods for prospective high-precision black-hole spectroscopy. We also report that near-horizon deformations can affect prograde and retrograde modes differently and provide a geometrical explanation.

gr-qc↗

arXiv2026

Parameter estimation of gravitational wave echoes from exotic compact objects

Relativistic ultracompact objects without an event horizon may be able to form in nature and merge as binary systems, mimicking the coalescence of ordinary black holes. The postmerger phase of such processes presents characteristic signatures, which appear as repeated pulses within the emitted gravitational waveform, i.e., echoes with variable amplitudes and frequencies. Future detections of these signals can shed new light on the existence of horizonless geometries, and provide new information on the nature of gravity in a genuine strong-field regime. In this work we analyze phenomenological templates used to characterize echolike structures produced by exotic compact objects, and we investigate for the first time the ability of current and future interferometers to constrain their parameters. Using different models with an increasing level of accuracy, we determine the features that can be measured with the largest precision, and we span the parameter space to find the most favorable configurations to be detected. Our analysis shows that current detectors may already be able to extract all the parameters of the echoes with good accuracy, and that multiple interferometers can measure frequencies and damping factors of the signals at the level of percent.

gr-qc↗

arXiv2025

Perturbing the vortex: quasinormal and quasibound spectra of rotating acoustic geometries

Strong-field gravity simulators are laboratory experiments that can investigate a wide range of both classical and quantum phenomena occurring in nature. In this work, we introduce an effective geometry that captures most of the characteristics of the strong-field regime of astrophysical, rotating black holes. This geometry can represent a vortex made from a variety of fluid and superfluid profiles with zero viscosity, making it a promising finite-temperature quantum-field-theory simulator for rotating curved spacetimes. Our geometry includes not only the typical radial flow which gives rise to an acoustic horizon, but also azimuthal circulation of the fluid. We compute the quasinormal modes, semi-analytically, and the exact quasibound states of acoustic excitations interacting with this effective geometry. The resulting spectra can be identified for both co-rotating and counter-rotating surface acoustic waves. In particular, the behavior of our acoustic geometry with circulation aligns with the phenomenology observed in recent experiments that include superfluids.

gr-qc↗

arXiv2025

Probing Dark Star Parameters Through $f$-Mode Gravitational Wave Signals

Theoretical models of self-interacting dark matter offer a promising solution to several unresolved issues within the collisionless cold dark matter framework. For asymmetric dark matter, these self-interactions may encourage gravitational collapse, potentially leading to the creation of compact objects primarily composed of dark matter. By considering both fermionic and bosonic equations of state, we analyze the equilibrium structure of non-rotating dark stars, examining their bulk properties and comparing them with baryonic neutron stars. We show that the frequency and damping rate of $f$-mode oscillations of dark compact stars can be expressed in terms of universal functions of stellar mass, radius and moment of inertia. Finally, by employing the universality in the $f$-mode, we propose a scheme to infer accurate values of the physical parameters of dark compact stars from their $f$-mode gravitational wave signal.

gr-qc↗

arXiv2024

Premerger phenomena in neutron-star binary coalescences

A variety of high-energy events can take place in the seconds leading up to a binary neutron-star merger. Mechanisms involving tidal resonances, electrodynamic interactions, or shocks in mass-loaded wakes have been proposed as instigators of these precursors. With a view of gravitational-wave and multimessenger astrophysics more broadly, premerger observations and theory are reviewed emphasising how gamma-ray precursors and dynamical tides can constrain the neutron-star equation of state, thermodynamic microphysics, and evolutionary pathways. Connections to post-merger phenomena, notably gamma-ray bursts, are discussed together with how magnetic fields, spin and misalignment, crustal elasticity, and stratification gradients impact observables.

astro-ph.HE↗

arXiv2024

The dynamical response of viscous objects to gravitational waves

We study the dynamical response of viscous materials to gravitational waves, in the context of a fully relativistic theory of fluid dynamics. For the first time, we calculate oscillation modes and scattering properties of viscous stars. Viscous stars absorb high frequency radiation, following a dispersion relation introduced by Press. In the extremely large viscosity regime, stars would become reflectors of waves, but this regime appears to be forbidden by causality bounds. In the context of black hole mimickers, we show how maximally viscous stars on the threshold of stability mimic the absorption of a black hole with the same mass. Our results suggest that rotating viscous stars will amplify incoming radiation.

gr-qc↗

arXiv2024

Resonant excitation of eccentricity in spherical extreme-mass-ratio inspirals

Gravitational radiation reaction, has been one of the fundamental issues in general relativity. Over a span of decades, this process has been analyzed in the adiabatic limit, in order to comprehend how it drives extreme-mass-ratio binaries, that are prime targets for space-borne detectors. It has been shown that spherical orbits around Schwarzschild and Kerr black holes remain spherical (zero eccentricity) under the influence of gravitational radiation reaction. Here, we show that spherical orbits in non-Kerr black holes, that still preserve most of the good qualities and symmetries of Kerr spacetime, can access certain resonances in such a way that an initially spherical inspiral acquires non-zero eccentricity and becomes non-spherical. Therefore, the crossing of resonances under radiation reaction interrupts and even inverts, up to some small radius close to plunge, the process of circularization of orbits. The strength of resonant excitation of eccentricity depends on the initial position and inclination of the integrable extreme-mass-ratio system, as well as the integrability-breaking parameter introduced in the background spacetime that amplifies further the excitation. We find that the harmonics of gravitational waves emitted from these inspirals undergo a frequency modulation as the orbit `metamorphoses' from spherical to non-spherical, due to the effect of resonant eccentricity excitation. The gain that low-amplitude harmonics experience in these oligochromatic EMRIs, due to resonances, may be detectable with future spaceborne detectors and serves as an indicator of non-Kerrness of the background spacetime.

gr-qc↗

arXiv2024

Inverse problem of analog gravity systems II: Rotation and energy-dependent boundary conditions

In this work, we study the inverse problem of analog gravity systems which admit rotation and energy-dependent boundary conditions. By extending two recent results, we provide a recipe that allows one to relate resonant transmission spectra with effective potentials and even reconstruct the boundary condition at the core. Our methodology is based on the WKB method and the identification of universal features in the transmission. One of the main advantages of this method is that it is parameter-free, and relies only on general properties of the underlying potential, instead of specific models. While the reconstruction of underlying potentials is generally not uniquely possible, the inverse method provides effective potentials with similar spectral properties to the original one. To demonstrate the accuracy and scope of our method, we apply it to a rotating imperfect draining vortex, which has been proposed as an analog system to astrophysical extreme compact objects. We conclude that the capability to explore energy-dependent boundary conditions could be of interest for experimental studies of such systems.

gr-qc↗

arXiv2024

Couplings of torsional and shear oscillations in a neutron star crust

Mature neutron stars are thought to be sufficiently cold that nuclei in the outer layers freeze, solidifying a crust. Crustal elasticity allows the star to support a set of seismic modes, such as torsional oscillations. These axial-parity modes can couple to the polar sector in a number of ways, for example via rotation or a magnetic field. Even in a static, spherically-symmetric star however these modes can couple at non-linear order. In this study, such couplings in the crust are examined for the first time: we derive the axisymmetric perturbation equations for second-order axial eigenfunctions, which are sourced by axial-polar couplings at first-order, and solve the resulting equations in the time domain. Through our studies, we find that the second-order spectrum contains additional oscillation modes, not predicted by the linear analysis with either axial or polar perturbations, which can be excited to relatively large amplitudes.

astro-ph.HE↗

arXiv2024

Inverse problem in energy-dependent potentials using semiclassical methods

Wave equations with energy-dependent potentials appear in many areas of physics, ranging from nuclear physics to black hole perturbation theory. In this work, we use the semi-classical WKB method to first revisit the computation of bound states of potential wells and reflection/transmission coefficients in terms of the Bohr-Sommerfeld rule and the Gamow formula. We then discuss the inverse problem, in which the latter observables are used as a starting point to reconstruct the properties of the potentials. By extending known inversion techniques to energy-dependent potentials, we demonstrate that so-called width-equivalent or WKB-equivalent potentials are not isospectral anymore. Instead, we explicitly demonstrate that constructing quasi-isospectral potentials with the inverse techniques is still possible. Those reconstructed, energy-independent potentials share key properties with the width-equivalent potentials. We report that including energy-dependent terms allows for a rich phenomenology, particularly for the energy-independent equivalent potentials.

hep-ph↗

arXiv2024

Magnetic amplification in premerger neutron stars through resonance-induced magnetorotational instabilities

Tidal resonances in the final seconds of a binary neutron-star inspiral can excite oscillation modes in one or both of the constituents to large amplitudes. Under favorable circumstances, resonant pulsations can overstrain the stellar crust and unleash a torrent of magnetoelastic energy that manifests as a gamma-ray ``precursor flare.'' We show that for realistic, stratified stars rotating with a spin frequency of $\gtrsim30\,$Hz, the fundamental $g$ or its first overtone can also execute a differential rotation in the crust such that a magnetic field of strength $\gtrsim10^{13}\,$G is generated via magnetorotational instabilities. This may help to explain observed precursor rates and their luminosities. Premerger magnetic growth would also provide seed magnetic energy for the postmerger remnant.

astro-ph.HE↗

arXiv2024

Finding Universal Relations using Statistical Data Analysis

We present applications of statistical data analysis methods from both bi- and multivariate statistics to find suitable sets of neutron star features that can be leveraged for accurate and EoS independent -- or universal -- relations. To this end, we investigate the ability of various correlation measures such as Distance Correlation and Mutual Information in identifying universally related pairs of neutron star features. We also evaluate relations produced by methods of multivariate statistics such as Principal Component Analysis to assess their suitability for producing universal relations with multiple independent variables. As part of our analyses, we also put forward multiple entirely novel relations, including a multivariate relation for the $f$-mode frequency of neutron stars with a reduced average relative error of $0.010$, compared to an error of $0.015$ of existing, bivariate relations.

gr-qc↗

arXiv2024

Inverse problem of analog gravity systems

Analog gravity models of black holes and exotic compact objects provide a unique opportunity to study key properties of such systems in controlled laboratory environments. In contrast to astrophysical systems, analog gravity systems can be prepared carefully and their dynamical aspects thus investigated in unprecedented ways. While gravitational wave scattering properties of astrophysical compact objects are more connected to quasinormal modes, laboratory experiments can also access the transmission and reflection coefficients, which are otherwise mostly relevant for Hawking radiation related phenomena. In this work, we report two distinct results. First, we outline a semiclassical, nonparametric method that allows for the reconstruction of the effective perturbation potential from the knowledge of transmission and reflection coefficients for certain types of potentials in the Schrödinger wave equation admitting resonant tunneling. Second, we show how to use our method by applying it to an imperfect draining vortex, which has been suggested as an analog of extreme compact objects. Although the inverse problem is, in general, not unique, choosing physically motivated assumptions and requiring the validity of semiclassical theory, we demonstrate that the method provides efficient and accurate results.

gr-qc↗