37 research outputs found
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 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 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 (). 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 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 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 (Total Variable Diminishing) but solution of
discontinuity wave produces some oscillation behind the shock. On the other
hand, type schemes give good approximation at discontinuity cases.
Because schemes completely remove the oscillations, they reduce locally
the accuracy of the solution around the extrema.Comment: 8 figure