Numerical Simulation of Rotating Accretion Disk Around the Schwarzschild Black Hole Using GRH Code

The 2D time dependent solution of thin accretion disk in a close binary system have been presented on the equatorial plane around the Schwarzschild black hole. To do that, the special part of the General Relativistic Hydrodynamical(GRH) equations are solved using High Resolution Shock Capturing (HRSC) schemes. The spiral shock waves on the accretion disk are modeled using perfect fluid equation of state with adiabatic indices $\gamma = 1.05, 1.2$ and 5/3. The results show that the spiral shock waves are created for gammas except the case $\gamma=5/3$. These results consistent with results from Newtonian hydrodynamic code except close to black hole. Newtonian approximation does not give good solution while matter closes to black hole. Our simulations illustrate that the spiral shock waves are created close to black hole and the location of inner radius of spiral shock wave is around $10M$ and it depends on the specific heat rates. We also find that the smaller $\gamma$ is the more tightly the spiral winds.Comment: 19 pages 11 figure

Numerical simulation of small perturbation on an accretion disk due to the collision of a star with the disk near the black hole

In this paper, perturbations of an accretion disk by a star orbiting around a black hole are studied. We report on a numerical experiment, which has been carried out by using a parallel-machine code originally developed by D\"{o}nmez (2004). An initially steady state accretion disk near a non-rotating (Schwarzschild) black hole interacts with a "star", modeled as an initially circular region of increased density. Part of the disk is affected by the interaction. In some cases, a gap develops and shock wave propagates through the disk. We follow the evolution for order of one dynamical period and we show how the non-axisymetric density perturbation further evolves and moves downwards where the material of the disk and the star become eventually accreted onto the central body. When the star perturbs the steady state accretion disk, the disk around the black hole is destroyed by the effect of perturbation. The perturbed accretion disk creates a shock wave during the evolution and it loses angular momentum when the gas hits on the shock waves. Colliding gas with the shock wave is the one of the basic mechanism of emitting the $X-$rays in the accretion disk. The series of supernovae occurring in the inner disk could entirely destroy the disk in that region which leaves a more massive black hole behind, at the center of galaxies.Comment: 20pages, 8 figures, accepted for publication in Astrophysics and Space Scienc

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

Influence of self-gravity on the runaway instability of black hole-torus systems

Results from the first fully general relativistic numerical simulations in axisymmetry of a system formed by a black hole surrounded by a self-gravitating torus in equilibrium are presented, aiming to assess the influence of the torus self-gravity on the onset of the runaway instability. We consider several models with varying torus-to-black hole mass ratio and angular momentum distribution orbiting in equilibrium around a non-rotating black hole. The tori are perturbed to induce the mass transfer towards the black hole. Our numerical simulations show that all models exhibit a persistent phase of axisymmetric oscillations around their equilibria for several dynamical timescales without the appearance of the runaway instability, indicating that the self-gravity of the torus does not play a critical role favoring the onset of the instability, at least during the first few dynamical timescales.Comment: To appear on Phys.Rev.Let

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

Extragalactic sources of rapidly variable high energy gamma radiation

The atmosphere is an intrinsic part of any imaging atmospheric Čerenkov telescope and the telescope response is therefore sensitive to unpredictable changes in the atmosphere. A lot of observational data taken during non-ideal atmospheric transparency have not been analyzed because of a lack of an appropriate analysis method that would be able to provide corrections for the imperfect transparency. On the other hand, extragalactic sources of high-energy cosmic gamma-rays (e.g. active galactic nuclei and gamma-ray bursts) are usually highly variable and the temporal characteristics of their light curves are key to the understanding of the physics of their sources. It is therefore very important to extend the observation time for variable sources as much as possible. In order to significantly extend the effective observational time of variable gamma-ray sources, we have developed a new data analysis method, which first determines the actual variable atmospheric transparency from the gamma-ray measurements, and then corrects those measurements according to the estimated atmospheric effect. To learn how the clouds influence gamma-ray measurements, we have implemented the simulation of clouds into the Monte Carlo simulation chain. Simulations starts with program Corsika, which simulates the development of particle showers in the atmosphere, and traces the Čerenkov light emitted by charged particles to the telescope. This method may extend the effective observation time in ground-based gamma-ray astronomy in general. Here we report on the application of the method in the first analysis of a particularly important data set from 2001 that includes a period of very strong activity of the blazar Mkn 421. In the near future, modern lidars will be able to precisely measure the distribution of the cloud density along the line of sight of the telescope, and thus provide significantly more information for our correction method. We also developed a new approach to correlation study based on MC simulations and the Fourier convolution. We applied the new method to analyze our CT1 data from 2003 of Mkn 421. We also reanalyzed correlation of some recent MAGIC data on blazars Mkn 421, Mkn 501 and newly discovered Mkn 180