arXiv subjects

Orhan Donmez

Publications and source records attributed to Orhan Donmez.

At least 37 records · Page 2Linked to original sources

arXiv2024

From Low- to High-Frequency QPOs around the Non-Rotating Hairy Horndeski Black Hole: Microquasar GRS 1915+105

Research on the Horndeski black hole, associated with the scalar hairy parameter, offers insights into enigmatic cosmic phenomena such as dark matter. Additionally, the numerical study of the GRS 1915+105 source, which exhibits continuous variability in X-ray observations, along with its physical properties and mechanisms behind Quasi-periodic oscillations (QPOs) frequencies, can contribute to observational studies. Motivated by this, we examine the variations in physical mechanisms around the non-rotating Horndeski black hole with Bondi-Hoyle-Lyttleton (BHL) accretion related to the scalar hair parameter and the resulting QPO frequencies. With a decrease in the scalar hair parameter, the shock cone opening angle narrows due to the influence of the scalar field potential, and the stagnation point within the cone moves closer to the black hole horizon. With the changing scalar hair parameter, the simultaneous formation of the shock cone and bow shock is observed. Due to the intense increase in scalar potential, both the shock cone and bow shock disappeared, and a cavity surrounding the black hole forms in the area where the shock cone was. Additionally, QPO oscillations induced by the physical mechanisms observed in relation to the hair parameter are revealed through numerical simulations. A broad range of QPO frequencies is observed, from low to high frequencies, with resonance states like 3:2 occurring. Lastly, we define the potential range of the spin parameter for the GRS 1915+105 source based on the agreement between observational and numerical results. It has also been found that for most of the QPOs obtained from numerical calculations to be consistent with observations, h/M should be greater than -0.5.

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↗

arXiv2024

Possible wormholes in $f(R)$ gravity sourced by solitonic quantum wave and cold dark matter halos and their repulsive gravity effect

In this paper, we present new generalized wormhole (WH) solutions within the context of $f(R)$ gravity. Specifically, we focus on $f(R)$ gravitational theories formulated in the metric formalism, with our investigation centered on a power-law form represented by $f(R) = εR^χ$. Here, $ε$ is an arbitrary constant, and $χ$ is a real number. Notably, this form possesses the advantageous property of reducing to Einstein gravity when $ε=1$ and $χ=1$. To obtain these novel WH solutions, we establish the general field equations for any $f(R)$ theory within the framework of Morris-Thorne spacetime, assuming metric coefficients that are independent of time. By utilizing an anisotropic matter source and a specific type of energy density associated with solitonic quantum wave (SQW) and cold dark matter (CDM) halos, we calculate two distinct WH solutions. We thoroughly investigate the properties of the exotic matter (ExoM) residing within the WH geometry and analyze the matter contents through energy conditions (ECs). Both analytical and graphical methods are employed in this analysis to examine the validity of different regions. Notably, the calculated shape functions for the WH geometry satisfy the necessary conditions in both scenarios, emphasizing their reliability. This ExoM is characterized by an energy-momentum tensor that violates the null energy condition (NEC) and, consequently, the weak energy condition as well, in the vicinity of the WH throats. Furthermore, we investigated the repulsive effect of gravity and discovered that its presence results in a negative deflection angle for photons following null geodesics. Importantly, we observed that the deflection angle consistently exhibits negative values across all $r_0$ values in both scenarios, indicating the manifestation of the repulsive gravity effect.

gr-qc↗

arXiv2024

Bondi-Hoyle-Lyttleton Accretion around the Rotating Hairy Horndeski Black Hole

Modeling of the shock cone formed around a static, hairy Horndeski black hole with Bondi-Hoyle-Lyttleton (BHL) accretion has been conducted. We model the dynamical changes of the shock cone resulting from the interaction of matter with the Horndeski black hole. The effects of the scalar hair, the black hole rotation parameter, and the impacts of the asymptotic speed have been examined. As the absolute value of the hair parameter increases, the matter in the region of the shock cone is observed to move away from the black hole horizon. After h/M<-0.6, a visible change in the physical structure of the shock cone occurs. On the other hand, it has been revealed that the asymptotic speed significantly affects the formation of the shock cone. As h/M increases in the negative direction and the asymptotic speed increases, the stagnation point moves closer to the black hole horizon. When the value of the hair parameter changes, the rest-mass density of the matter inside the cone decreases, whereas the opposite is observed with the asymptotic speed. Additionally, the formed shock cone has excited QPO modes. The deformation of the cone due to the hair parameter has led to a change or complete disappearance of the QPOs. Meanwhile, at asymptotic speeds of V_{\infty}/c< 0.4, all fundamental frequency modes are formed, while at V_{\infty}/c=0.4, only the azimuthal mode is excited, and 1:2:3:4:... resonance conditions occur. No QPOs have formed at V_{\infty}/c = 0.6. The results obtained from numerical calculations have been compared with theoretical studies for M87*, and it has been observed that the possible values of h/M found in the numerical simulations are consistent with the theory. Additionally, the results have been compared with those for the GRS 1915+105 black hole, and the hair parameters corresponding to the observed frequencies have been determined.

astro-ph.HE↗

arXiv2024

Perturbing the Stable Accretion Disk in Kerr and 4-D Einstein-Gauss-Bonnet Gravities: Comprehensive Analysis of Instabilities and Dynamics

The study of a disturbed accretion disk holds great significance in the realm of astrophysics, as such events play a crucial role in revealing the nature of disk structure, the release of energy, and the generation of shock waves. Consequently, they can help explain the causes of X-ray emissions observed in black hole accretion disk systems. In this paper, we perturb the stable disk formed by spherical accretion around Kerr and EGB black holes. This perturbation reveals one- and two-armed spiral shock waves around the black hole. We find a strong connection between these waves and the black hole spin parameter (a/M) and the EGB coupling constant (α). Specifically, we found that as αincreases in the negative direction, the dynamics of the disk and the waves become more chaotic. Additionally, we observe that the angular momentum of the perturbing matter significantly affects mass accretion and the oscillation of the arising shock waves. This allows us to observe changes in QPO frequencies. Particularly, perturbations with angular momentum matching the observed $C-$ type low-frequency QPOs of the GRS 1915 + 105 source. Thus, we conclude that the possibility of the occurrence of shock waves within the vicinity of GRS 1915 + 105 is substantial.

astro-ph.HE↗

arXiv2024

Unraveling the mysteries of wormhole formation in Rastall-Rainbow gravity: A comprehensive study using the embedding approach

The present work looks for the possible existence of static and spherically symmetric wormhole geometries in Rastall-Rainbow gravity. Since, the Rastall-Rainbow gravity model has been constructed with the combination of Rastall theory and the gravity's rainbow formalism. Taking advantage of the Karmarkar condition for embedding class one metrics, we solve the modified field equations analytically that describe wormholes for specific choice of redshift function. For specific parameter ranges, the solution represents a traversable wormhole that exhibits the violation of null energy condition and consequently the weak energy condition also. Furthermore, we focus on the wormhole stability via adiabatic sound velocity analysis. This model establishes a strong connection between two model parameters, namely, the Rastall parameters and the Rainbow functions, and how it affects the wormhole solution.

gr-qc↗

arXiv2024

Proposing a Physical Mechanism to Explain Various Observed Sources of QPOs by Simulating the Dynamics of Accretion Disks around the Black Holes

We propose a mechanism to explain the low-frequency QPOs observed in X-ray binary systems and AGNs. To achieve this, we perturbed stable accretion disks around Kerr and EGB black holes at different angular velocities, revealing characteristics of shock waves and oscillations on the disk. By applying this perturbation to scenarios with varying alpha values for EGB black holes and different spin parameters for Kerr black holes, we numerically observed changes in the disk dynamic structure and its oscillations. Through various numerical models, we found that the formation of one- and two-armed spiral shock waves on the disk serves as a mechanism for generating QPOs. We compared the QPOs obtained from numerical calculations with the low-frequency QPOs observed in $X$-ray binary systems and AGN sources, finding high consistency with observations. We observed that the shock mechanism, leading to QPOs, explains the X-ray binaries and AGNs studied in this article. Our numerical findings indicate that QPOs are more strongly dependent on the EGB constant than on the black hole spin parameter. However, we highlighted that the primary impact on oscillations and QPOs is driven by the perturbation angular velocity. The results from the models showed that the perturbation asymptotic speed at V_{\infty}=0.2 independently generates QPO frequencies, regardless of the black hole spin parameter and the EGB coupling constant. Therefore, for the moderate value of V_{\infty}, a two-armed spiral shock wave formed around the black hole is suggested as a decisive mechanism in explaining low-frequency QPOs.

astro-ph.HE↗

arXiv2024

The comparison of alternative spacetimes using the spherical accretion around the black hole

In the region where the gravitational field is strong, we have examined the influence of different gravities on the accretion disk formed due to spherical accretion. To achieve this, we obtain numerical solutions of the GRH equations, utilizing Schwarzschild, Kerr, Einstein-Gauss-Bonnet, and Hartle-Thorne spacetime metrics. We investigate the impact of the rotation parameter of a black hole (a/M), the EGB coupling constant ($α$), and the quadrupole moment of the rotating black hole (q) on the accretion disk formed in a strong field. The formation of the disk for the slowly and rapidly rotating black hole models is separately examined, and comparisons are made. Our numerical simulations reveal that, under the specific conditions, the solution derived from Hartle-Thorne gravity converges towards solutions obtained from Kerr and other gravitational models. In the context of the slowly rotating black hole with $a/M=0.28$, we observe a favorable agreement between the Hartle-Thorne result and the Kerr result within the range of 0 < q < 0.5. Conversely, in the scenario of the rapidly rotating black hole, a more pronounced alignment with the value of q=1 is evident within the range of 0.5 < q < 1. Nevertheless, for q > 1, it becomes apparent that the Hartle-Thorne solution diverges from solutions provided by all gravitational models. Our motivation here is to utilize the Hartle-Thorne spacetime metric for the first time in the numerical solutions of the GRH equations for the black holes, compare the results with those obtained using other gravities, and identify under which conditions the Hartle-Thorne solution is compatible with known black hole spacetime metric solutions. This may allow us to provide an alternative perspective in explaining observed X-ray data.

gr-qc↗

arXiv2024

Exploring accelerated expansion in the universe: A study of $f(Q,T)$ gravity with parameterized EoS and cosmological constraints

The study conducted in this research paper utilizes the $f(Q,T)$ gravity, where $Q$ represents non-metricity and $T$ represents the trace of the energy-momentum tensor, to investigate the accelerated expansion of the universe. To complete the study, an effective EoS with one parameter $α$, is parameterized as $ω_{eff}=-\frac{3}{α(1+z)^{3}+3}$. The linear version of $f(Q,T)=-Q+σT$ is also considered, where $σ$ is a constant. By constraining the model with six BAO points, 57 Hubble points, and 1048 Pantheon sample datasets, the parameters $α$ and $σ$ are determined to best match the data. The cosmological parameters and energy conditions for the model are derived and examined. The results show that the model is in good agreement with observations, and can serve as a valuable starting point for analyzing FLRW models in the $f(Q,T)$ theory of gravity.

astro-ph.CO↗

arXiv2023

The gravitational collapse of the dust toward the newly formed rotating black holes in Kerr and 4-D Einstein-Gauss-Bonnet Gravities

Studying the gravitational collapse of dust particles toward newly formed black holes has gained popularity following the observation of gravitational waves resulting from the merger of black holes. In this paper, we focus on modelling the descent of dust debris toward a black hole using a numerical code that incorporates relativistic hydrodynamics in the framework of General and Einstein-Gauss Bonnet gravity. We explore the influence of various parameters, such as the black hole's rotation parameter a and the EGB coupling constant alpha, on the curvature effects observed. Both parameters significantly impact the dynamics of the accretion disk formed around the black holes. Furthermore, we discuss the gravitational collapsing process in two distinct scenarios. It is also observed that the mass accretion rate is significantly influenced by these two parameters. The rate at which mass is accreted toward a black hole directly impacts the black hole's growth and evolutionary trajectory.

astro-ph.HE↗

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 $α$. 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↗

arXiv2021

On the dynamics of the torus around the kicked black hole

There is a special interest to understand the dynamical properties of the accretion disk created around the newly formed black hole due to the supermassive black hole binaries which merge inside the gaseous disk. The newly formed black hole would have a kick velocity at up to thousands of km/s that drives a perturbation on a newly accreted torus around the black hole. Some of the observed supermassive black holes at the center of the Active Galactic Nucleus (AGN) move with a certain velocity relative to its broader accretion disk. In this paper, the effects of the kicked black holes onto the infinitesimally thin accreted torus are studied by using the general relativistic hydrodynamical code, focusing on changing the dynamics of the accretion disk during the accretion disk-black hole interaction. We have found the non-axisymmetric global mode m=1 inhomogeneity, which causes a spiral-wave structure, is excited on the torus due to kicked black hole. The higher the perturbation velocity produced by the kicked black hole, the longer time the torus takes to reach the saturation point. The created spiral density waves which rapidly evolve into the spiral shocks are also observed from the numerical simulations. The spiral shock is responsible for accreting matter toward the black hole. Firstly, the spiral-wave structure is developed and the accretion through the spiral arms is stopped around the black hole. At the later time of simulation, the formed spiral shocks partly cause the angular momentum loss across the torus.

astro-ph.HE↗

arXiv2021

Dynamical Evolution of the Shock Cone around $4D$ Einstein-Gauss Bonnet Rotating Black Hole

In this paper, a Bondi-Hoyle accretion onto the rotating black hole in Einstein-Gauss Bonnet gravity is studied. By injecting the gas from the upstream region of the computational domain, we have found the occurrence of the stable shock cones in the downstream region. The dynamical structures and oscillation properties of these shock cones strongly depend on the black hole spin parameter $a$ and Gauss-Bonnet coupling constant alpha. It is found that the various values of alpha can lead the different amounts of matter to pile up close to the black hole horizon, higher alpha causes bigger oscillation amplitude in the mass accretion rate, and the required time to reach the steady-state is getting smaller with the increase in alpha. Moreover, increasing alpha in the negative direction causes a decrease in the shock opening angle and this angle slightly increases with the increasing $α$ in the positive direction. We found that the negative values of Gauss-Bonnet coupling constant are more favored to have interesting physical outcomes such as accretion rate and oscillation. In addition, the higher the black hole rotation parameter (a) emerges the higher the accretion rate. It is also confirmed that, for alpha \rightarrow 0, the black hole solution in EGB gravity converges to Kerr in general relativity. Furthermore, Gauss-Bonnet coupling constant could be used to constrain the size of the observed shadow of M87* radius for various values of the black hole rotation parameter.

astro-ph.HE↗

arXiv2021

Bondi-Hoyle Accretion around the Non-rotating Black Hole in 4D Einstein-Gauss-Bonnet Gravity

In this paper, the numerical investigation of a Bondi-Hoyle accretion around a non-rotating black hole in a novel four dimensional Einstein-Gauss-Bonnet gravity is investigated by solving the general relativistic hydrodynamical equations using the high resolution shock capturing scheme. For this purpose, the accreated matter from the wind-accreating X-ray binaries falls towards the black hole from the far upstream side of the domain, suphersonically. We study the effects of Gauss-Bonnet coupling constant $α$ in 4D EGB gravity on the accreated matter and shock cones created in the downstream region in detail. The required time having the shock cone in downstream region is getting smaller for alpha > 0 while it is increasing for alpha < 0. It is found that increases in alpha leads violent oscillations inside the shock cone and increases the accretion efficiency. The violent oscillations would cause increase in the energy flux, temperature, and spectrum of X- rays. So the quasi-periodic oscillations (QPOs) are naturally produced inside the shock cone when -5 \leq alpha \leq 0.8. It is also confirmed that EGB black hole solution converges to the Schwarzschild one in general relativity when alpha \rightarrow 0. Besides, the negative coupling constants also give reasonable physical solutions and increase of alpha in negative directions suppresses the possible oscillation observed in the shock cone.

gr-qc↗

arXiv2017

Angular Velocity Perturbations Inducing the Papaloizou-Pringle Instability and QPOs in the Torus around the Black Hole

A numerical study of the dynamic of the nonselfgravitating, unmagnetized, nonaxisymmetric, and rotating the torus around the non-rotating black hole is presented. We investigate the instability of the rotating torus subject to perturbations presented by increasing or decreasing the angular velocity of the stable torus. We have done, for the first time, an extensive analysis of the torus dynamic response to the perturbation of the angular velocity of the stable torus. We show how the high, moderate, and low values of the perturbations affect the torus dynamic and help us to understand the properties of the instability and Quasi-Periodic Oscillation (QPO). Our numerical simulations indicate the presence of Papaloizou-Pringle instability (PPI) with global m=1 mode and QPOs for the moderate and lower values of the perturbations on the angular velocity of the stable torus. Furthermore, with the lower values of the perturbations, the torus can lead to a wiggling initially and then PPI is produced in it. Finally, the matter of the torus would be dissipated due to the presence of a strong torque.

astro-ph.HE↗

arXiv2013

On the development of the Papaloizou-Pringle instability of the black hole-torus systems and quasi-periodic oscillations

We present the numerical study of dynamical instability of a pressure-supported relativistic torus, rotating around the black hole with a constant specific angular momentum on a fixed space-time background, in case of perturbation by a matter coming from the outer boundary. Two dimensional hydrodynamical equations are solved at equatorial plane using the HRSCS to study the effect of perturbation on the stable systems. We have found that the perturbed torus creates an instability which causes the gas falling into the black hole in a certain dynamical time. All the models indicate an oscillating torus with certain frequency around their instant equilibrium. The dynamic of the accreted torus varies with the size of initial stable torus, black hole spin and other variables, such as Mach number, sound speed, cusp location of the torus, etc. The mass accretion rate is slightly proportional to the torus-to-hole mass ratio in the black hole-torus system, but it strongly depends on the cusp location of the torus. The cusp located in the equipotential surfaces of the effective potential moves outwards into the torus. The dynamical change of the torus increases the mass accretion rate and triggers the Papaloizou-Pringle instability. It is also observed that the growth of the $m=1$ mode of the Papaloizou-Pringle instability occurs for a wide range of fluid and hydrodynamical parameters and a black hole spin. We have also computed the QPOs from the oscillating relativistic torus.

astro-ph.HE↗

arXiv2011

On the development of QPOs in Bondi-Hoyle accretion flows

The numerical investigation of Bondi-Hoyle accretion onto a moving black hole has a long history, both in Newtonian and in general-relativistic physics. By performing new two-dimensional and general-relativistic simulations onto a rotating black hole, we point out a novel feature, namely, that quasi-periodic oscillations (QPOs) are naturally produced in the shock cone that develops in the downstream part of the flow. Because the shock cone in the downstream part of the flow acts as a cavity trapping pressure perturbations, modes with frequencies in the integer ratios 2:1 and 3:1 are easily produced. The frequencies of these modes depend on the black-hole spin and on the properties of the flow, and scale linearly with the inverse of the black-hole mass. Our results may be relevant for explaining the detection of QPOs in Sagittarius A*, once such detection is confirmed by further observations. Finally, we report on the development of the flip-flop instability, which can affect the shock cone under suitable conditions; such an instability has been discussed before in Newtonian simulations but was never found in a relativistic regime.

astro-ph.HE↗

arXiv2006

Quasi Periodic Oscillations (QPOs) and frequencies in an accretion disk and comparison with the numerical results from non-rotating black hole computed by the GRH code

The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in $X-$ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and non-rotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from our papers Refs.\refcite{Donmez2} and \refcite{Donmez3} using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.

astro-ph↗