# Galin N. GyulchevSofia University "St. Kliment Ohridski" · Department of Theoretical Physics

Galin N. Gyulchev

PhD

## About

38

Publications

2,608

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811

Citations

Citations since 2017

Introduction

Additional affiliations

December 2010 - present

November 2008 - present

## Publications

Publications (38)

We study the linear polarization from the accretion disk around a class of static traversable wormholes. Applying the simplified model of a magnetized fluid ring orbiting in the equatorial plane, we search for characteristic signatures, which could distinguish wormhole from black hole spacetimes by their polarization properties. For the purpose we...

In this article, we conduct a sequential study of possible observable images of black hole simulators described by two recently obtained rotating geometries in Einstein gravity, minimally coupled to a scalar field. One of them, “Kerr-like,” can be seen as a legitimate alternative to the rotating Fisher-Janis-Newman-Winicour solution, and the other...

We study the linear polarization from the accretion disk around a class of static traversable wormholes. Applying the simplified model of a magnetized fluid ring orbiting in the equatorial plane, we search for characteristic signatures, which could distinguish wormhole from black hole spacetimes by their polarization properties. For the purpose we...

In this article, we conduct a sequential study of possible observable images of black hole simulators described by two recently obtained rotating geometries in Einstein gravity, minimally coupled to a scalar field. One of them, "Kerr-like" (KL), can be seen as a legitimate alternative to the rotating Fisher-Janis-Newman-Winicour (FJNW) solution, an...

We describe and illustrate light deflection near neutron stars possessing a photon sphere motivated by modern confirmations of general relativity and its significance for relativistic astrophysics. In this paper, we model the spacetime outside the star with the Schwarzschild metric and summarize the properties of the photon orbits. More especially...

We investigate the potential appearance of qualitatively new observational features of accretion disk images around compact objects. More specifcally we focus on a particular class of traversable wormholes, possessing a photon sphere. We discover that for certain solution parameters the shape of the images produced possesses distinct features that...

We study the optical appearance of a thin accretion disk around compact objects within the Einstein–Gauss–Bonnet gravity. Considering static spherically symmetric black holes and naked singularities we search for characteristic signatures which can arise in the observable images due to the modification of general relativity. While the images of the...

We study the optical appearance of a thin accretion disk around compact objects within the Einstein-Gauss-Bonnet gravity. Considering static spherically symmetric black holes and naked singularities we search for characteristic signatures which can arise in the observable images due to the modification of general relativity. While the images of the...

We study the optical appearance of a thin accretion disk around the strongly naked static Janis–Newman–Winicour singularity. The solution does not possess a photon sphere, which results in the formation of a complex structure of bright rings in the central region of the disk image. Such structure is absent in the case of the Schwarzschild black hol...

We study the optical appearance of a thin accretion disk around the strongly naked static Janis-Newman-Winicour singularity. The solution does not possess a photon sphere, which results in the formation of a complex structure of bright rings in the central region of the disk image. Such structure is absent in the case of the Schwarzschild black hol...

We study the optical appearance and the apparent radiation flux of a geometrically thin and optically thick accretion disk around the static Janis-Newman-Winicour naked singularity. We confine ourselves to the astrophysically most relevant case, when the solution possesses a photon sphere, assuming that the radiation emitted by the disk is describe...

We study the optical appearance and the apparent radiation flux of a thin accretion disk around the static Janis-Newman-Winicour naked singularity. We confine ourselves to the astrophysically most relevant case, when the solution possesses a photon sphere, assuming that the radiation emitted by the disk is described by the Novikov-Thorne model. The...

In this paper, we study the deflection of light by a class of phantom black hole and wormhole solutions in the weak limit approximation. More specifically, in the first part of this work we study the deflection of light by Garfinkle-Horowitz-Str\"{o}minger black hole and Einstein-Maxwell anti-dilaton black hole using the optical geometry and the Ga...

We investigate the shadows of a certain class of rotating traversable wormhole. We consider the unstable spherical orbits and reveal their crucial role over the cusp formation in the shadows. For relatively small values of the wormhole angular momentum the cusps structure changes significantly. A set of critical values of the wormhole angular momen...

We study the gravitational deflection of relativistic massive particles by Janis-Newman-Winicour (JNW) spacetimes (also known as a rotating source with a surface-like naked singularity), and a rotating Kerr-like wormholes. Based on the recent article [K. Jusufi, Phys. Rev. D 98, 064017 (2018)], we extend some of these results by exploring the effec...

Degeneration of the human aorta can occur in the thoracic, abdominal and arch region. Rarely this kind of disease happen in the arch region of aorta closer to the semilunar valve. By definition, an aneurysm is a localized dilation of an artery with a diameter at least 50% greater than normal size. The main objective of the study is to recreate the...

We study the gravitational deflection of relativistic massive particles by Janis-Newman-Winicour (JNW) spacetimes (also known as a rotating source with a surface-like naked singularity), and a rotating Kerr-like wormholes. Based on the recent article [arXiv: 1806.01256], we extend some of these results by exploring the effects of naked singularity...

We revisit the shadow of rotating traversable wormholes discussing the role of the wormhole throat in the shadow formation. For certain classes of wormholes the throat serves as a potential barrier for light rays with particular impact parameters, thus modifying the shadow shape. We consider a couple of wormhole solutions and examine the structure...

In this paper, we study the deflection of light by a class of phantom black hole and wormhole solutions in the weak limit approximation. More specifically, in the first part of this work we study the deflection of light by Garfinkle-Horowitz-Str\"{o}minger black hole and Einstein-Maxwell anti-dilaton black hole using the optical geometry and the Ga...

We revisit the shadow of rotating traversable wormholes discussing the role of the wormhole throat in the shadow formation. For certain classes of wormholes the throat serves as a potential barrier for light rays with particular impact parameters, thus modifying the shadow shape. We consider a couple of wormhole solutions and examine the structure...

We study the shadows of disformal black holes in vector-tensor Galileons modified gravity. Our analysis shows that the apparent image of the black hole in the observer's sky is non-spherical and cuspy, which is in contrast to the Kerr and Kerr-Newman cases. The non-trivial silhouette of the apparent image of the black hole provides novel templates...

We model the spacetime of low-mass X-ray binaries with the Tomimatsu-Sato
{\delta} = 2 (TS2) metric and study the properties of the orbital and the
epicyclic frequencies. The numerical analysis shows that the properties of the
characteristic frequencies of oscillation do not differ qualitatively from
those of the Kerr black hole. Estimates for the...

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...

We estimate the focal length F of the gravitational lens Coma galaxy cluster and obtain that an observer from Earth cannot see the lensing effect of the cluster in the case of a background source. The main result from our calculations is that the dark matter content in the Coma galaxy cluster lens changes its focal length 34 times. We also estimate...

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 rα/b, where M is the black hole mass, a is the
angular momentum, rα =...

The characteristic modes of oscillation of black holes-the quasi-normal
modes-are extensively studied since they carry information about the
physical properties of the black holes and there is hope that they can
be measured in near future gravitational wave experiments. In the
current report an alternative method for the measurement of the
frequenc...

The purpose of the current Letter is to give some relations between gravitational lensing in the strong-deflection limit and the frequencies of the quasinormal modes of spherically symmetric, asymptotically flat black holes. On the one side, the relations obtained can give a physical interpretation of the strong-deflection limit parameters. On the...

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 $r_{\alpha}/b$, where $M$ is the black hole mass, $a$ is the angu...

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 wel...

We model massive compact objects in galactic nuclei as stationary, axially‐symmetric naked singularities in the massless Einstein scalar field theory and study the resulting gravitational lensing in the strong deflection limit. In this approximation we compute the position of the relativistic images and the flux ratio of the outermost and the inner...

In the present work we study strong field limit, quasi-equatorial
lensing by the charged, stationary, axially-symmetric Kerr-Sen
dilaton-axion black hole. The most outstanding effect is that the
Kerr-Sen black hole caustics drift away from the optical axis and shift
in clockwise direction. The intersections of the critical curves on the
equatorial...

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...

In the present work we study numerically equatorial lensing by the charged, static, spherically symmetric Garfinkle-Horowitz-Strominger dilaton black hole in the strong deflection limit. In this approximation we compute the strong deflection limit coefficients and the bending angle of the light ray and it is shown that the last one decreases with t...

We model massive compact objects in galactic nuclei as stationary, axially-symmetric naked singularities in the Einstein-massless scalar field theory and study numerically the resulting grav-itational lensing. In the strong deflection limit approximation we compute the positions of the relativistic images and compare the results to the case of stro...

We investigate analytically gravitational lensing by charged, sta-tionary, axially symmetric Kerr–Sen dilaton-axion black hole in the weak de-flection limit. Approximate solutions to the light-like equations of motion are present up to and including third-order terms in M/b, a/b and rα/b, where M is the black hole mass, a is the angular momentum, r...

The purpose of the current paper is to give some relations between gravi-tational lensing in the strong-deflection limit and the frequencies of the quasi-normal modes of spherically symmetric, asymptotically flat black holes. On the one side, the obtained relations can give physical interpretation of the strong-deflection limit parameters. On the o...

## Projects

Project (1)

The phenomenon of gravitational lensing, predicted by Einstein’s theory of general relativity, is a side effect of light moving along the curvature of spacetime, where the light passing nearby a massive object is deflected slightly toward the mass. First time, in 1919, it was observed by Eddington and Dyson during a solar eclipse . Since then, the gravitational lensing is one of the important tools to be used in astronomy and astrophysics. We study the gravitational lensing problem for various types of black holes. For this purpose, we follow the geometrical method of Gibbons and Werner [Class.Quant.Grav.25:235009,2008 arXiv:0807.0854 [gr-qc]) to study the deflection of light moving in the IR region of the black holes.