arXiv subjects

Galin N. Gyulchev

Publications and source records attributed to Galin N. Gyulchev.

8 recordsLinked to original sources

arXiv2026

Polarized Equatorial Emission around Kerr Black Holes with Synchronized Scalar Hair. I. Direct images

We investigate the polarization properties of the direct images of a geometrically and optically thin accretion disk around fully self-consistent models of rotating Kerr black holes with synchronized bosonic hair. The presence of a massive scalar field alters the geodesic structure of the spacetime and thus leaves an imprint on the polarization of radiation emitted near the black hole horizon. To study this effect, we employ a simple analytical model of a geometrically thin accretion disk, orbiting in the equatorial plane and emitting synchrotron radiation. The main deviation from a corresponding Kerr black hole in general relativity is found to be a dephasing in the twist of the polarization vector, which is surprisingly larger for the least scalarized solutions we consider. This behavior suggests that polarization observables are primarily sensitive to local geometric and transport effects along photon trajectories rather than to the overall scalar field strength. Furthermore, our results demonstrate that while equatorial magnetic fields produce qualitatively similar polarization patterns to Kerr black holes in general relativity, vertical magnetic fields at high observer inclinations can lead to a characteristic reversal of the twist direction of the polarization vector.

gr-qc↗

arXiv2026

Images of the Thin Accretion Disk Around Kerr Black Holes coupled to time periodic scalar fields

We investigate the orbital structure and observable appearance of rotating Kerr black holes endowed with synchronized scalar hair described by two time-periodic scalar fields with a flat target-space geometry. The presence of scalar hair enriches the geodesic structure of the spacetime relative to the Kerr case and significantly modifies the emission properties of geometrically thin Novikov-Thorne accretion disks. Combining an analysis of timelike circular orbits with backward ray tracing, we show that the normalized scalar charge governs the morphology and luminosity of both prograde and counter-rotating disks. In the strongly scalarized regime, additional light rings and modified circular-orbit regions produce multiple inner emitting zones and strongly enhanced redshift patterns that depart markedly from the Kerr prediction. The most pronounced deviations occur in the counter-rotating sector, where scalar hair generates inner retrograde radiative rings with substantially enhanced luminosity and distinctive frequency-shift signatures. Even when the spacetime approaches the Kerr geometry at weaker scalarization, the retrograde disk remains highly sensitive to the presence of scalar hair. Our results demonstrate that geometrically thin accretion disks can provide robust observational diagnostics of synchronized scalar hair and may offer a promising avenue for testing tensor-multi-scalar gravity with future horizon-scale black-hole imaging observations.

gr-qc↗

arXiv2024

Testing disformal non-circular deformation of Kerr black holes with LISA

There is strong observational evidence that almost every large galaxy has a supermassive black hole at its center. It is of fundamental importance to know whether such black holes are described by the standard Kerr solution in General Relativity (GR) or by another black hole solution. An interesting alternative is the so-called disformal Kerr black holes which exist within the framework of degenerate higher-order scalar-tensor (DHOST) theories of gravity. The departure from the standard Kerr black hole spacetime is parametrized by a parameter $D$, called $\textit{disformal parameter}$. In the present work, we discuss the capability of LISA to detect the disformal parameter. For this purpose, we study Extreme Mass Ratio Inspirals (EMRI's) around disformal Kerr black holes within the framework of the quadrupole hybrid formalism. Even when the disformal parameter is very small, its effect on the globally accumulated phase of the gravitational waveform of an EMRI can be significant due to the large number of cycles in the LISA band made by the small compact object. We show that LISA will in principle be able to detect and measure extremely small values of the disformal parameter which in turn, can be seen as an assessment of LISA's ability to detect very small deviations from the Kerr geometry.

gr-qc↗

arXiv2024

Shadows of rotating hairy Kerr black holes coupled to time periodic scalar fields with non-flat target space

We study the shadows cast by rotating hairy black holes with two non-trivial time-periodic scalar fields having a non-flat Gaussian curvature of the target space spanned by the scalar fields. Such black holes are a viable alternative to the Kerr black hole, having a much more complicated geodesic structure and resulting shadows. We investigate how a nontrivial Gauss curvature alters the pictures for different amounts of scalar hair around the black holes, quantified by a normalized charge. Our results show that for high values of this charge, close to a boson star limit, chaotic shadows are observed with multiple small disconnected components for all considered Gaussian curvatures. For moderately large amounts of scalar hair and corresponding normalized charge, although the shadows still exhibit chaotic behavior, a dominant shadow component emerges, the size and shape of which are substantially influenced by the Gaussian curvature. For instance, highly chaotic shadows for flat target space, start developing a large central shadow region with the increase of the Gauss curvature even for black holes with substantially heavy scalar hair. For lower values of the normalized charge, the shadows resemble qualitatively the Kerr black hole while the Gaussian curvature has a small impact on their properties.

gr-qc↗

arXiv2013

Gravitational Lensing by Phantom Black holes

In some models dark energy is described by phantom scalar fields (scalar fields with "wrong" sign of the kinetic term in the lagrangian). In the current paper we study the effect of phantom scalar field and/or phantom electromagnetic field on gravitational lensing by black holes in the strong deflection regime. The black-hole solutions that we have studied have been obtained in the frame of the Einstein-(anti-)Maxwell-(anti-)dilaton theory. The obtained results show considerable effect of the phantom scalar and electromagnetic fields on the angular position, brightness and separation of the relativistic images.

gr-qc↗

arXiv2009

Analytical Kerr-Sen Dilaton-Axion Black Hole Lensing in the Weak Deflection Limit

We investigate analytically gravitational lensing by charged, stationary, axially symmetric Kerr-Sen dilaton-axion black hole in the weak deflection limit. Approximate solutions to the lightlike equations of motion are present up to and including third-order terms in $M/b$, $a/b$ and $r_α/b$, where $M$ is the black hole mass, $a$ is the angular momentum, $r_α=Q^2/M$, $Q$ being the charge and $b$ is the impact parameter of the light ray. We compute the positions of the two weak field images, the corresponding signed and absolute magnifications up to post-Newtonian order. It is shown that there are static post-Newtonian corrections to the signed magnification and their sum as well as to the critical curves, which are functions of the charge. The shift of the critical curves as a function of the lens angular momentum is found, and it is shown that they decrease slightly with the increase of the charge. The point-like caustics drift away from the optical axis and do not depend on the charge. All of the lensing quantities are compared to particular cases as Schwarzschild and Kerr black holes as well as the Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole.

gr-qc↗

arXiv2008

Gravitational Lensing by Rotating Naked Singularities

We model massive compact objects in galactic nuclei as stationary, axially-symmetric naked singularities in the Einstein-massless scalar field theory and study the resulting gravitational lensing. In the weak deflection limit we study analytically the position of the two weak field images, the corresponding signed and absolute magnifications as well as the centroid up to post-Newtonian order. We show that there are a static post-Newtonian corrections to the signed magnification and their sum as well as to the critical curves, which are function of the scalar charge. The shift of the critical curves as a function of the lens angular momentum is found, and it is shown that they decrease slightingly for the weakly naked and vastly for the strongly naked singularities with the increase of the scalar charge. The point-like caustics drift away from the optical axis and do not depend on the scalar charge. In the strong deflection limit approximation we compute numerically the position of the relativistic images and their separability for weakly naked singularities. All of the lensing quantities are compared to particular cases as Schwarzschild and Kerr black holes as well as Janis--Newman--Winicour naked singularities.

gr-qc↗

arXiv2007

Kerr-Sen dilaton-axion black hole lensing in the strong deflection limit

In the present work we study numerically quasi-equatorial lensing by the charged, stationary, axially-symmetric Kerr-Sen dilaton-axion black hole in the strong deflection limit. In this approximation we compute the magnification and the positions of the relativistic images. The most outstanding effect is that the Kerr-Sen black hole caustics drift away from the optical axis and shift in clockwise direction with respect to the Kerr caustics. The intersections of the critical curves on the equatorial plane as a function of the black hole angular momentum are found, and it is shown that they decrease with the increase of the parameter $Q^{2}/M$. All of the lensing quantities are compared to particular cases as Schwarzschild, Kerr and Gibbons-Maeda black holes.

gr-qc↗