arXiv subjects

Jamshid Ghanbari

Publications and source records attributed to Jamshid Ghanbari.

16 recordsLinked to original sources

arXiv2026

Poloidal Field Amplification through Compression-Shear Dynamics in Schwarzschild Accretion: Pathways to MAD States

The amplification of magnetic fields in black hole accretion flows governs key high-energy phenomena such as magnetically arrested disks and relativistic jets. We develop a semi-analytical general relativistic framework that extends classical compressional amplification models by incorporating rotational shear, and apply it to large-scale poloidal magnetic field evolution in accretion flows around a Schwarzschild black hole. By parameterizing the azimuthal velocity as a fraction of the Keplerian value ($ξ\in [0,1]$), from purely radial infall ($ξ=0$) to Keplerian rotation ($ξ=1$), we examine the combined effects of radial compression and shear. Purely radial flows maximize amplification of both $B_r$ and $B_θ$ due to strong compression. In rotating flows, a distinct dichotomy emerges: sub-Keplerian regimes ($ξ<1$) preferentially enhance $B_r$, whereas Keplerian rotation strengthens $B_θ$ via shear. The transition from subsonic outer regions to supersonic relativistic inner regions further accelerates magnetic growth, revealing effects absent in earlier analytical treatments. These results show that rotational support controls both amplification efficiency and magnetic geometry, with sub-Keplerian phases particularly favorable for advecting the radial flux required for MAD formation. This work provides an analytical bridge between classical accretion theory and modern GRMHD simulations, with implications for X-ray binaries, AGNs, and EHT-scale systems.

astro-ph.HE↗

arXiv2025

A Study of thin relativistic magnetic accretion disk around a distorted black hole

Accretion disks, swirling structures of matter spiraling into black holes, play a pivotal role in our understanding of binary star systems and their intricate evolutionary processes. While current models often simplify these complex phenomena by neglecting the influence of powerful magnetic fields, particularly within warped or distorted black hole geometries, this study delves into the crucial impact of such fields. Focusing on a thin accretion disk encircling a Schwarzschild black hole, we meticulously investigate how the presence of a quadrupole moment, an inherent distortion in the black hole's shape, affects its spectral characteristics. By analyzing key parameters like total pressure, magnetic pressure, temperature, height scale, surface density, and radiative flux (the energy emitted by the disk) we reveal significant alterations induced by incorporating both magnetic fields and a quadrupole moment. Notably, our findings demonstrate that negative quadrupoles exert a more pronounced influence on these disk properties, highlighting the intricate interplay between these factors. This comprehensive study provides invaluable insights into the dynamics of accretion disks surrounding distorted black holes with magnetic fields, paving the way for a more accurate and nuanced understanding of these fascinating astrophysical systems.

astro-ph.HE↗

arXiv2025

A Study of thin relativistic viscose accretion disk around a distorted kerr black hole (DKB)

In this paper, we analyze a thin disk around the distorted Kerr black hole (DKB) within the framework of general relativity using an axisymmetric solution of the Einstein equations. We consider this accretion disk around the Kerr black hole in an external gravitational field up to the quadrupole moment and discuss the key aspects of black hole accretion disk theory. Our findings indicate that the presence of a quadrupole moment significantly influences the radiation emitted from the accretion disk. While the location of the innermost stable circular orbit (ISCO) remains largely unchanged, the magnitude of the radiation flux, as well as the shape, orientation, and energy distribution of the accretion disk, are affected. The direction of distortion of the event horizon determines whether the disk becomes more oblate or prolate, impacting observed variations in maximum height, position, and temperature. Furthermore, the quadrupole moment alters the geometry of the black hole's spacetime, which can influence the efficiency of energy extraction from the black hole's spin, an important factor in powering emissions from accretion disks. We obtain dynamical quantities around a distorted rotating black hole disk. Additionally, we examine how rotation influences the dynamics of the DKB. We also investigate the effects of varying the viscosity coefficient on the behavior of the DKB.

gr-qc↗

arXiv2024

Magnetic Field Evolution for Keplerian and Sub-Keplerian Flows around Non Rotating Black Hole

The exact two-dimensional non-stationary solution for the evolution of the magnetic field during accretion with nearly spherical symmetry, using Newton's solution and considering both Keplerian and sub-Keplerian flows around the black hole (BH), has been derived using the Schwarzschild metric. In this paper, we also discuss the possible origins of large-scale magnetic field production around black holes (BHs). For example, the origin of this strong large-scale magnetic field could be the interstellar medium or a companion star outside the accretion disk, which is drawn in by the accretion plasma and grows over time. The findings show that the uniform magnetic field of a subsonic flow, which is weak at infinity, increases with time and evolves into a quasi-radial field. This is true for both types of flows. We have also observed that the growth of the radial component of the magnetic field is more pronounced in the sub-Keplerian flow than in the Keplerian flow. However, it is worth noting that, for both types of flows, the magnetic field does not reach its saturation value in the regions $r> r_{c}$; instead, the process of strengthening and growing the magnetic field progresses to a point where the disk evolves from its initial condition to a state close to the magnetically arrested disk (MAD) state, forming a sub-MAD state.

astro-ph.HE↗

arXiv2012

Shear tensor and dynamics of relativistic accretion disks around rotating black holes

In this paper we solve the hydrodynamical equations of optically thin, steady state accretion disks around Kerr black holes. Here, fully general relativistic equations are used. We use a new method to calculate the shear tensor in the LNRF (Locally Non-Rotating Frame), BLF (Boyer-Lindquist Frame) and FRF (Fluid Rest Frame). We show that two components of shear tensor in the FRF are nonzero (in previous works only one nonzero component was assumed). We can use these tensors in usual transonic solutions and usual causal viscosity, but we derive solutions analytically by some simplifications. Then we can calculate the four velocity and density in all frames such as the LNRF, BLF and FRF.

astro-ph.GA↗

arXiv2012

Bipolar jets launched from magnetically diffusive accretion disks. I. Ejection efficiency vs field strength and diffusivity

We investigate the launching of jets and outflows from magnetically diffusive accretion disks. Using the PLUTO code we solve the time-dependent resistive MHD equations taking into account the disk and jet evolution simultaneously. The main question we address is which kind of disks do launch jets and which kind of disks do not? In particular, we study how the magnitude and distribution of the (turbulent) magnetic diffusivity affect mass loading and jet acceleration. We have applied a turbulent magnetic diffusivity based on α-prescription, but have also investigate examples where the scale height of diffusivity is larger than that of the disk gas pressure. We further investigate how the ejection efficiency is governed by the magnetic field strength. Our simulations last for up to 5000 dynamical time scales corresponding to 900 orbital periods of the inner disk. As a general result we observe a continuous and robust outflow launched from the inner part of the disk, expanding into a collimated jet of super fast magneto-sonic speed. For long time scales the disk internal dynamics changes, as due to outflow ejection and disk accretion the disk mass decreases. For magneto-centrifugally driven jets we find that for i) less diffusive disks, ii) a stronger magnetic field, iii) a low poloidal diffusivity, or a iv) lower numerical diffusivity (resolution), the mass loading of the outflow is increased - resulting in more powerful jets with high mass flux. For weak magnetization the (weak) outflow is driven by the magnetic pressure gradient. We further investigate the jet asymptotic velocity and the jet rotational velocity in respect of the different launching scenarios. We find a lower degree of jet collimation than previous studies, most probably due to our revised outflow boundary condition.

astro-ph.HE↗

arXiv2011

An Accretion-Disc Model for the Algol-Type Eclipsing Binary System AV Del

This study inspects the light and radial-velocity curves of the eclipsing binary AV Del. In comparison with other studies already done, the study shows that the absolute elements, fundamental orbital and physical parameters of the system can be determined using the Wilson-Devinney code. Using these parameters, the configuration of the system is presented. Then, an accretion disc model for the system is introduced by using the SHELLSPEC code. The results indicate that AV Del is a semi-detached system in which an optically thick accretion disc is surrounding the primary star. The outer radius of the disc is 8.0 R_sun}, corresponding to a distance of 1.1 R_sun} from the surface of the secondary. Also, the temperature of the disc is calculated to be T=5700 K.

astro-ph↗

arXiv2010

A statistical study of the luminosity gap in galaxy groups

The luminosity gap between the two brightest members of galaxy groups and clusters is thought to offer a strong test for the models of galaxy formation and evolution. This study focuses on the statistics of the luminosity gap in galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an analogy with the same in a cosmological simulation. We use spectroscopic legacy data of seventh data release (DR7) of SDSS, to extract a volume limited sample of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm. Attention is paid to galaxy groups with the brightest group galaxy (BGG) more luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical fossil groups, where the luminosity gap exceeds 2 magnitude, were identified. We compare the statistics of the luminosity gap in galaxy groups at low mass range from the SDSS with the same in the Millennium simulations where galaxies are modeled semi-analytically. We show that the BGGs residing in galaxy groups with large luminosity gap, i.e. fossil groups, are on average brighter and live in lower mass halos with respect to their counter parts in non-fossil systems. Although low mass galaxy groups are thought to have recently formed, we show that in galaxy groups with 15 galaxies brighter than $M_r\ge -19.5$, evolutionary process are most likely to be responsible for the large luminosity gap. We also examine a new probe of finding fossil group. In addition we extend the recently introduced observational probe based on the luminosity gap, the butterfly diagram, to galaxy groups and study the probe as a function of halo mass. This probe can, in conjunction with the luminosity function, help to fine tune the semi-analytic models of galaxies employed in the cosmological simulations.

astro-ph.CO↗

arXiv2008

Reanalysis of two eclipsing binaries: EE Aqr and Z Vul

We study the radial-velocity and light curves of the two eclipsing binaries EE Aqr and Z Vul. Using the latest version of the Wilson & Van Hamme (2003) model, absolute parameters for the systems are determined. We find that EE Aqr and Z Vul are near-contact and semi-detached systems, respectively. The primary component of EE Aqr fills about 96% of its 'Roche lobe', while its secondary one appears close to completely filling this limiting volume. In a similar way, we find fill-out proportions of about 72 and 100% of these volumes for the primary and secondary components of Z Vul respectively. We compare our results with those of previous authors.

astro-ph↗

arXiv2006

Self-Similar Evolutionary Solutions of Self-Gravitating, Polytropic $β$-Viscous Disks

We carry out the effect of $β$-prescription for viscosity which introduced by Duschel et al. 2000 & Hure, Richard & Zhan 2001, in a standard self-gravitating thin disks. We were predicted in a self-gravitating thin disk the $β$-model will have different dynamical behavior compare the well known $α$-prescriptions. We used self-similar methods for solving the integrated equations which govern the dynamical behavior of the thin disks. We present the results of self-similar solutions of the time evolution of axisymmetric, polytropic, self-gravitating viscous disks around a new born central object. We apply a $β$-viscosity prescription which has been derived from rotating shear flow experiments ($ν=βr^2Ω$). Using reduced equations in a slow accretion limit, we demonstrate inside-out self-similar solutions after core formation in the center. Some physical quantities for $β$-disks are determined numerically.We have compared our results with $α$-disks under the same initial conditions. It has been found that the accretion rate onto the central object for $β$-disks more than $α$-disks at least in the outer regions where $β$-disks are more efficient. Our results show that Toomre instability parameter is less than one everywhere on the $β$-disk which means that in such disks gravitational instabilities can be occurred, so the $β$-disk model can be a good candidate for the origin of planetary systems. Our results show that the $β$-disks will decouple in the outer part of the disk where the self-gravity plays an important role which is in agreement with Duschl predictions.

astro-ph↗

arXiv2006

Formation of Small-Scale Condensations in the Molecular Clouds via Thermal Instability

A systematic study of the linear thermal instability of a self-gravitating magnetic molecular cloud is carried out for the case when the unperturbed background is subject to local expansion or contraction. We consider the ambipolar diffusion, or ion-neutral friction on the perturbed states. In this way, we obtain a non-dimensional characteristic equation that reduces to the prior characteristic equation in the non-gravitating stationary background. By parametric manipulation of this characteristic equation, we conclude that there are, not only oblate condensation forming solutions, but also prolate solutions according to local expansion or contraction of the background. We obtain the conditions for existence of the Field lengths that thermal instability in the molecular clouds can occur. If these conditions establish, small-scale condensations in the form of spherical, oblate, or prolate may be produced via thermal instability.

astro-ph↗

arXiv2005

Linear Thermal Instability and Fluctuations in Molecular Clouds

Evidence of small-scale condensations in the magnetic molecular clouds has been accumulating over the past decades through radio and optical/ultraviolet observations. The origin and shape of these small-scale condensations is a disputable issue. Nejad-Asghar & Ghanbari (2004 hereafter NG) have recently studied the effect of the linear thermal instability on the formation of fluctuations in molecular clouds. The authors inferred that under certain conditions (e.g., depending on expansion or contraction of the background) thermal instability and ambipolar diffusion can produce spherical, oblate, or prolate condensations.

astro-ph↗

arXiv2003

Linear Thermal Instability and Formation of Clumpy Gas Clouds Including the Ambipolar Diffusion

Thermal instability is one of the most important processes in the formation of clumpy substructure in magnetic molecular clouds. On the other hand, ambipolar diffusion, or ion-neutral friction, has long been thought to be an important energy dissipation mechanism in these clouds. Thus, we would interested to investigate the effect of ambipolar diffusion on the thermal instability and formation of clumps in the magnetic molecular clouds. For this purpose, in the first step, we turn our attention to the linear perturbation stage. In this way, we obtain a non-dimensional characteristic equation which reduces to the prior characteristic equation in the absence of the magnetic field and ambipolar diffusion. With numerical manipulation of this characteristic equation, we conclude that there are solutions where the thermal instability allows compression along the magnetic field but not perpendicular to it. We infer that this aspect might be an evidence in formation of observed disc-like (oblate) clumps in magnetic molecular clouds.

astro-ph↗

arXiv2002

Cooling Flows of Self-Gravitating, Rotating, Viscous Systems

We obtain self-similar solutions that describe the dynamics of a self-gravitating, rotating, viscous system. We use simplifying assumptions; but explicitly include viscosity and the cooling due to the dissipation of energy. By assuming that the turbulent dissipation of energy is as power law of the density and the speed v_{rms} and for a power-law dependence of viscosity on the density, pressure, and rotational velocity, we investigate turbulent cooling flows. It has been shown that for the cylindrically and the spherically cooling flows the similarity indices are the same, and they depend only on the exponents of the dissipation rate and the viscosity model. Depending on the values of the exponents, which the mechanisms of the dissipation and viscosity determine them, we may have solutions with different general physical properties. The conservation of the total mass and the angular momentum of the system strongly depends on the mechanisms of energy dissipation and the viscosity model.

astro-ph↗

arXiv2001

Self-Similar Equilibria of Self-Gravitating, Magnetized, Rotating, Isothermal Systems

The self-similar equilibrium models of self-gravitating, rotating, isothermal systems are investigated analytically. In these models the rotation velocity is constant and the density varies as $\frac{f(θ, ϕ)}{r^2}$, where $r$ and $θ$ are the spherical radius and the co-latitude, respectively. The nonaxisymmetric solutions contain three free parameters, one of the parameters depends on the rotation velocity. These parameters determine the overall shape of the density distribution. By assuming that the dominant component of the magnetic field is purely toroidal and the ratio of the purely toroidal magnetic pressure to the gas pressure, $α$, is spatially constant, the axisymmetric solutions generalized so as the effect of magnetic field could be studied. We find that the equilibria of axially symmetric systems yield ellipsoids or spheres only when the ratio of rotation velocity to the sound speed is taken to be $\sqrt{2α}$.

astro-ph↗

arXiv2001

Radiative Cooling Flows of Self-Gravitating Filamentary Clouds

We study the dynamics of a self-gravitating cooling filamentary cloud using a simplified model. We concentrate on the radial distribution and restrict ourselves to quasi-hydrostatic, cylindrically symmetric cooling flows. For a power-law dependence of cooling function on the temperature, self-similar solutions which describe quasi-hydrostatic cooling flows are derived. We consider obtically thin filaments with a constant mass per unit length and the solutions are parameterized by their line masses. There is no polytropic relation between the density and the pressure. The filament experiences radiative condensation, irrespective of the γ, the gas specific heat ratio. So, the filament becomes denser due to the quasi-hydrostatic flows and the density at the center increases in proportion to (t_{0}-t)^{-1}, where t denotes the time. The term, t_{0}, denotes an epoch at which the central density increases infinitely. We also found that the radius of the filament decreases in proportion to (t_{0}-t)^{0.5}.

astro-ph↗