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.Comment: 14 Pages, 14 Figures, High resolution version of Figs.1 and 7 can be found in the original manuscript. Substantial revision has been made. The title, abstract and whole text are revise

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 $\alpha$ 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.Comment: 9 pages, 10 figures, Resubmitted to EPJC after referee repor

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. This is because such events play a crucial role in revealing the nature of disk structure, the release of energy, and the generated shock waves. Thus, 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 on the disk's surface. We find a strong connection between these waves and the black hole's spin parameter (a/M) and the EGB coupling constant ($\alpha$). Specifically, we found that as alpha increases 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 QPOs frequencies. Particularly, perturbations with angular momentum matches the observed C-type QPOs frequencies of GRS 1915 + 105 source. Thus, we conclude that the possibility of the shock waves occurring within the vicinity of GRS 1915 + 105 is substantial.Comment: 19 pages, 11 figure

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 do this, we perturbed stable accretion disks around Kerr and EGB black holes at different angular velocities, revealing the characteristics of shock waves and oscillations presented on the disk. Applying this perturbation to scenarios with different alpha values for EGB black holes and different spin parameters for Kerr black holes, we numerically observed changes in the dynamic structure of the disk and oscillations. Through various numerical modeling, we found that the formation of one- and two-armed spiral shock waves on the disk serves as a mechanism for the generation of QPOs. We compared the QPOs obtained from numerical calculations with the low-frequency QPOs observed in $X-$ray binary systems and AGN sources. We found that the results obtained are highly consistent with observations. We observed that the shock mechanism on the disk, which leads to quasi-periodic oscillations, explains the X-ray binaries and AGNs studied in this article. As a result of the numerical findings, we find that QPOs are more strongly dependent on the EGB constant rather than the black hole's spin parameter However, we highlighted that the primary impact on oscillations and QPOs is driven by the perturbation's angular velocity. According to the results obtained from the models, it has been observed that the perturbation's asymptotic speed at V_{\infty}=0.2 is responsible for generating QPO frequencies independently of the black hole's spin parameter and the EGB coupling constant. Therefore, for the moderate value of V_{\infty}, a two-armed spiral shock wave formed on the disk is suggested as a decisive mechanism in explaining low-frequency QPOs.Comment: 20 pages, 9 figure

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.Comment: 12 pages, 6 figure

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.Comment: 13 figures, added reference, accepted for publication in Modern Physics Letters

Solution of the 1D Special Relativistic Hydrodynamics(SRH) Equations Using Different Numerical Method and Results from Different Test Problems

In this paper, we have solved 1D special relativistic hydrodynamical equations using different numerical method in computational gas dynamics. The numerical solutions of these equations for smooth wave cases give better solution when we use $Non-TVD$(Total Variable Diminishing) but solution of discontinuity wave produces some oscillation behind the shock. On the other hand, $TVD$ type schemes give good approximation at discontinuity cases. Because $TVD$ schemes completely remove the oscillations, they reduce locally the accuracy of the solution around the extrema.Comment: 8 figure