arXiv subjects

Fatih Dogan

Publications and source records attributed to Fatih Dogan.

14 recordsLinked to original sources

arXiv2025

Spherically Symmetric Quantum-Corrected Black Holes with String Clouds: A Multi-Observable Analysis

We present an investigation of quantum-corrected black hole spacetimes coupled with clouds of strings, examining two distinct theoretical models that incorporate quantum gravitational effects through different implementations of correction terms. Our study explores the geodesic structure, focusing on photon sphere properties, black hole shadows, and innermost stable circular orbits of test particles around these exotic geometries. The analysis reveals fundamental modifications to particle trajectories that distinguish quantum-corrected solutions from their classical counterparts, with observable implications for high-energy astrophysics. We investigate quasi-periodic oscillations arising from test particle motion, deriving frequency relationships that could serve as observational probes of quantum gravity effects in accreting black hole systems. Through rigorous gravitational lensing analysis using the Gauss-Bonnet theorem, we calculate weak-field deflection angles and identify distinctive signatures that enable discrimination between the two quantum correction models. The gravitational lensing study reveals opposite dependencies on quantum parameters between the models, providing unambiguous observational discriminants. Additionally, we analyze the thermodynamic properties including temperature, entropy, and heat capacity modifications, exploring topological characteristics and phase transition behavior in these quantum-corrected systems. The investigation reveals that the two models exhibit contrasting behaviors across multiple observational channels, from gravitational lensing deflection angles to quasi-periodic oscillation frequencies, providing remarkable discriminants for testing quantum gravity theories.

astro-ph.HE↗

arXiv2025

Analytic and Numerical Constraints on QPOs in EHT and XRB Sources Using Quantum-Corrected Black Holes

This investigation examines QPOs in two quantum-corrected BH spacetimes that preserve general covariance while incorporating quantum gravitational effects through a dimensionless parameter \zeta. We combine analytical derivations of epicyclic frequencies with comprehensive numerical simulations of BHL accretion to explore how quantum corrections manifest in observable astrophysical phenomena. Using a fiducial BH mass of M=10M_\odot representative of stellar-mass X-ray binaries, we demonstrate that the two models exhibit fundamentally different behaviors: Model-I modifies both temporal and radial metric components, leading to innermost stable circular orbit migration proportional to \zeta^4 and dramatic stagnation point evolution from 27M to 5M as quantum corrections strengthen. Model-II preserves the classical temporal component while altering only spatial geometry, maintaining constant stagnation points and stable cavity structures throughout the parameter range. Our numerical simulations reveal distinct QPO generation mechanisms, with Model-I showing systematic frequency evolution and cavity shrinkage that suppresses oscillations for \zeta \geq 3M, while Model-II maintains stable low-frequency modes up to \zeta \geq 5M. Power spectral density analyzes demonstrate characteristic frequency ratios (3:2, 2:1, 5:3) consistent with observations from X-ray binaries, providing specific targets for discriminating between quantum correction scenarios. The hydrodynamically derived constraints (\zeta \lesssim 4M) show remarkable agreement with independent EHT limits for M87* and Sgr A*, validating our theoretical framework through multiple observational channels. These results establish QPO frequency analysis as a probe for detecting quantum gravitational effects in astrophysical BHs and demonstrate the complementary nature of timing and imaging observations in constraining fundamental physics.

astro-ph.HE↗

arXiv2024

Estimating the possible QPOs of M87* from the parameters of a hairy Kerr black hole

In this paper, we study the dynamics of the shock cone formed around the hairy Kerr black hole due to BHL accretion and investigate the QPO behaviors resulting from the black hole-cone interaction, aiming to predict the QPO frequencies that may occur around the M87* black hole. To achieve this, we use the hairy Kerr black hole parameters consistent with the observed shadow parameters of M87* as initial conditions in numerical simulations, revealing the structure of the resulting shock cone and QPOs in a strong gravitational field. Numerical calculations show that the deviation of the hairy Kerr black hole from the Kerr metric and the hair parameters significantly influence the complex behavior of the resulting QPOs. It is found that the Lense-Thirring effect and the pressure modes trapped within the cone lead to the excitation of QPOs. The hair parameter has been observed to suppress the resulting QPO frequencies. The Lense-Thirring effect, in a strong gravitational field with a black hole spin parameter of a/M > 0.7, also suppresses other modes and generates high-frequency QPOs. It is predicted that the QPO frequencies observed around the M87* black hole could span a wide range from nHz to mHz. Using both Kerr and hairy Kerr gravities, the QPO frequencies formed around the M87* black hole can be explained. Especially in cases where the black hole is spinning rapidly, PSD analyses have shown very distinct low-frequency QPOs and resonance conditions in both gravities. By comparing the results obtained from numerical calculations with observational and analytical results, we discuss the observability of the QPO frequencies that may occur around the M87* black hole.

gr-qc↗

arXiv2022

Study of Asymptotic Velocity in the Bondi-Hoyle Accretion Flows in the Domain of Kerr and 4-D Einstein-Gauss-Bonnet Gravities

Understanding the physical structures of the accreated matter very close to the black hole in quasars and active galactic nucleus (AGNs) is an important milestone to constrain the activities occurring in their centers. In this paper, we numerically investigate the effects of the asymptotic velocities on the physical structures of the accretion disk around the Kerr and Einstein-Gauss-Bonnet (EGB) rapidly rotating black holes. The Bondi-Hoyle accretion is considered with a falling gas towards the black hole in upstream region of the computational domain. The shock cones are naturally produced in the downstream part of the flow around both black holes. It is found that the structure of the cones and the amount of the accreated matter depend on asymptotic velocity $V_{\infty}$ (Mach number) and the types of the gravities (Kerr or EGB). Increasing the Mach number of the inflowing matter in the supersonic region causes the shock opening angle and accretion rates getting smaller because of the rapidly falling gas towards the black hole. The EGB gravity leads to an increase in the shock opening angle of the shock cones while the mass accretion rates $\dot{M}$ are decreasing in EGB gravity with a Gauss-Bonnet (GB) coupling constant $\alpha$. It is also confirmed that accretion rates and drag forces are significantly altered in the EGB gravity. Our numerical simulation results could be used to identify the accreation mechanism and physical properties of the accretion disk and black hole in the observed $X-$ rays such as NGC $1313$ $X-1$ and $1313$ $X-2$ and MAXI $J1803-298$.

astro-ph.HE↗

arXiv2018

Isophote curves on spacelike surfaces in Lorentz-Minkowski space E31

Isophote curve consists of a locus of surface points whose normal vectors make a constant angle with a fixed vector (the axis). In this paper, we define an isophote curve on a spacelike surface in Lorentz-Minkowski space and then find its axis as timelike and spacelike vectors via the Darboux frame. Besides, we give some characterizations concerning isophote curve and its axis.

math.DG↗

arXiv2015

On characteristic curves of developable surfaces in Euclidean 3-space

We investigate the relationship among characteristic curves on developable surfaces. In case parameter curves coincide with these curves, we show that the base curve of a developable surface could be either a plane curve, a circular helix, a general helix or a slant helix.

math.DG↗

arXiv2013

Tailoring spin-orbit torque in diluted magnetic semiconductors

We study the spin orbit torque arising from an intrinsic linear Dresselhaus spin-orbit coupling in a single layer III-V diluted magnetic semiconductor. We investigate the transport properties and spin torque using the linear response theory and we report here : (1) a strong correlation exists between the angular dependence of the torque and the anisotropy of the Fermi surface; (2) the spin orbit torque depends nonlinearly on the exchange coupling. Our findings suggest the possibility to tailor the spin orbit torque magnitude and angular dependence by structural design.

cond-mat.mes-hall↗

arXiv2012

The detailed proof of theorem which characterizes a slant helix

In this paper, firstly the axis of a slant helix is found with a method. Secondly, the theorem which characterizes a unit speed curve to be a slant helix is proved in detail. The importance of this theorem is stemed from that it has led to many papers regarding slant helices in the differential geometry literature.

math.DG↗

arXiv2012

The relation between parameter curves and lines of curvature on canal surfaces

A canal surface is the envelope of a moving sphere with varying radius, defined by the trajectory C(t) (spine curve) of its center and a radius function r(t). In this paper, we investigate when parameter curves of the canal surface are also lines of curvature. Last of all, for special spine curves we obtain the radius function of canal surfaces.

math.DG↗

arXiv2012

Isophote curves on timelike surfaces in Minkowski 3-space

Isophote comprises a locus of the surface points whose normal vectors make a constant angle with a fixed vector. In this paper, isophote curves are studied on timelike surfaces in Minkowski 3-space E31. The axises of spacelike and timelike isophote curves are found via their Darboux frames. Subsequently, the relationship between isophotes and slant helices is shown on timelike surfaces.

math.DG↗

arXiv2012

On isophote curve and its characterizations

Isophote comprises a locus of the surface points whose normal vectors make a constant angle with a fixed vector. Main objective of this paper is to find the axis of an isophote curve via its Darboux frame and afterwards to give some characterizations about isophote and its axis. Particularly, for isophotes lying on a canal surface will be obtained other characterizations again.

math.DG↗

arXiv2009

Emerging nonequilibrium bound state in spin-current-local-spin scattering

Magnetization reversal is a well-studied problem with obvious applicability in computer hard-drives. One can accomplish a magnetization reversal in at least one of two ways: application of a magnetic field, or through a spin current. The latter is more amenable to a fully quantum mechanical analysis. We formulate and solve the problem whereby a spin current interacts with a ferromagnetic Heisenberg spin chain, to eventually reverse the magnetization of the chain. Spin-flips are accomplished through both elastic and inelastic scattering. A consequence of the inelastic scattering channel, when it is no longer energetically possible, is the occurrence of a new entity: a non-equilibrium bound state (NEBS), which is an emergent property of the coupled local plus itinerant spin system. For certain definite parameter values the itinerant spin lingers near the local spins for some time, before eventually leaking out as an outwardly diffusing state. This phenomenon results in novel spin-flip dynamics and filtering properties for this type of system.

cond-mat.mes-hall↗

arXiv2008

Electron and Spin Transport in the Presence of Complex Absorbing Potential

We examine the impact of a complex absorbing potential on electron transport, both in the continuum and on a lattice. This requires the use of non-Hermitian Hamiltonians; the required formalism is briefly outlined. The lattice formulation allows us to study the interesting problem of an electron interacting with a stationary spin, and the subsequent time evolution of the electron and spin properties as the electron is absorbed after the initial interaction. Remarkably, the properties of the localized spin are affected 'at-a-distance' by the interaction of the (now entangled) electron with a complex potential.

cond-mat.mes-hall↗

arXiv2003

Self-Consistent Modification To The Electron Density Of States Due To Electron-Phonon Coupling In Metals

The "standard" theory of a normal metal consists of an effective electron band which interacts with phonons and impurities. The effects due to the electron-phonon interaction are often delineated within the Migdal approximation; the properties of many simple metals are reasonably well described with such a description. On the other hand, if the electron-phonon interaction is sufficiently strong, a polaron approach is more appropriate. The purpose of this paper is to examine to what degree the Migdal approximation is self-consistent, as the coupling strength increases. We find that changes in the electron density of states become significant for very large values of the coupling strength; however, there is no critical value, nor even a crossover regime where the Migdal approximation has become inconsistent. Moreover, the extent to which the electron band collapses is strongly dependent on the detailed characteristics of the phonon spectrum.

cond-mat.str-el↗