Shahram Abbassi

Shahram Abbassi
Ferdowsi University Of Mashhad | FUM · Department of Physics (Faculty of Science)

PhD

About

75
Publications
3,766
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
487
Citations
Citations since 2017
35 Research Items
296 Citations
20172018201920202021202220230102030405060
20172018201920202021202220230102030405060
20172018201920202021202220230102030405060
20172018201920202021202220230102030405060
Additional affiliations
September 2012 - January 2016
Ferdowsi University Of Mashhad
Position
  • Research Assistant
September 2008 - present
Institute for Research in Fundamental Sciences (IPM)
Position
  • Part time researcher
September 2005 - September 2012
Damghan University
Position
  • Research Assistant

Publications

Publications (75)
Article
We investigate the dynamics of clumps that coexisted with/in advection-dominated accretion flows by considering thermal conductivity. Thermal conduction can be one of the effective factors in the energy transportation of ADAFs; hence it may indirectly affect the dynamics of clumps by means of a contact force between them and their host medium. We f...
Preprint
We investigate the dynamics of clumps that coexisted with/in advection-dominated accretion flows by considering thermal conductivity. Thermal conduction can be one of the effective factors in the energy transportation of ADAFs; hence it may indirectly affect the dynamics of clumps by means of a contact force between them and their host medium. We f...
Article
We study the global solutions of slowly rotating accretion flows around the supermassive black hole in the nucleus of an elliptical galaxy. The velocity of accreted gas surrounding the black hole is initially subsonic and then falls on to the black hole supersonically, so accretion flow must be transonic. We numerically solve equations from the Bon...
Preprint
We study the global solutions of slowly rotating accretion flows around the supermassive black hole in the nucleus of an elliptical galaxy. The velocity of accreted gas surrounding the black hole is initially subsonic and then falls onto the black hole supersonically, so accretion flow must be transonic. We numerically solve equations from the Bond...
Article
The present work proposes a new formalism for the inner regions of a neutrino-dominated accretion flows (NDAFs) by considering the self-gravity, where the neutrino opacity is high enough to make neutrinos trapped becoming a dominant factor in the transportation of energy and angular momentum over the magneto rotational instability. We investigate t...
Preprint
The present work proposes a new formalism for the inner regions of a neutrino-dominated accretion flows (NDAFs) by considering the self-gravity, where the neutrino opacity is high enough to make neutrinos trapped becoming a dominant factor in the transportation of energy and angular momentum over the magneto rotational instability. We investigate t...
Article
Full-text available
Filamentary molecular clouds are omnipresent in the cold interstellar medium. Observational evidences show that the non-isothermal equations of state describe the filaments properties better than the isothermal one. In this paper we use the logatropic and the polytropic equations of state to study the gravitational instability of the pressure-confi...
Preprint
Full-text available
Filamentary molecular clouds are omnipresent in the cold interstellar medium. Observational evidences show that the non-isothermal equations of state describe the filaments properties better than the isothermal one. In this paper we use the logatropic and the polytropic equations of state to study the gravitational instability of the pressure-confi...
Article
The main purpose of this paper is to obtain analytical solutions for radiative transfer equations related to the vertical structure of accretion discs with finite optical depth. In the non-grey atmosphere, we employ the optical-depth dependent Eddington factor to define the relationship between the mean intensity and radiation stress tensor. Analyt...
Preprint
The main purpose of this paper is to obtain analytical solutions for radiative transfer equations related to the vertical structure of accretion discs with finite optical depth. In the non-gray atmosphere, we employ the optical-depth dependent Eddington factor to define the relationship between the mean intensity and radiation stress tensor. Analyt...
Article
Full-text available
The evolution of disk galaxies in modified gravity is studied by using high-resolution N-body simulations. More specifically, we use the weak field limit of two modified gravity theories, i.e., nonlocal gravity (NLG) and scalar-tensor-vector gravity known as MOG, and ignore the existence of dark matter halo. On the other hand, we construct the same...
Preprint
Full-text available
The evolution of disk galaxies in modified gravity is studied by using high-resolution N-body simulations. More specifically, we use the weak field limit of two modified gravity theories, i.e., nonlocal gravity (NLG) and scalar-tensor-vector gravity known as MOG, and ignore the existence of dark matter halo. On the other hand, we construct the same...
Preprint
Full-text available
X-ray flares can be accounted for a hint to the late time activity of Gamma-ray bursts (GRBs) central engines. Such a long term activity has been described through some models, one of which is the viscous evolution of the outer disc fragments that proposed by Perna et al. (2006), and developed quantitatively by Dall Osso et al. (2017). Here, we rec...
Preprint
Full-text available
We study the local thermal stability of thin accretion disks. We present a full stability analysis in the presence of a magnetic field and more importantly wind. For wind, we use a general model suitable for adequately describing several kinds of winds. First, we explicitly show that the magnetic field, irrespective of the type of wind, has a stabi...
Article
The classical Bondi model is adopted to study accretion onto the finite luminous region around the central massive black hole (MBH) in an elliptical galaxy. Unlike Bondi (1952), we define the boundary conditions at a certain finite radius (rf) instead of at the infinity and examine the variation of solutions for a simple case. In the following, we...
Preprint
The classical Bondi model is adopted to study accretion onto the finite luminous region around the central massive black hole (MBH) in an elliptical galaxy. Unlike Bondi (1952), we define the boundary conditions at a certain finite radius ($r_f$) instead of at the infinity and examine the variation of solutions for a simple case. In the following,...
Article
By taking into account photon absorption, we investigate the vertical structure of accretion flows with comparable radiation and gas pressures. We consider two separate energy equations for matter and radiation in the diffusion limit. In order to solve the set of radiation hydrodynamic equations in steady state and axisymmetric configuration, we em...
Preprint
By taking into account photon absorption, we investigate the vertical structure of accretion flows with comparable radiation and gas pressures. We consider two separate energy equations for matter and radiation in the diffusion limit. In order to solve the set of radiation hydrodynamic equations in steady state and axisymmetric configuration, we em...
Article
We implement a new semi-analytical approach to investigate radially self-similar solutions for the steady-state advection-dominated accretion flows (ADAFs). We employ the usual α-prescription for the viscosity, and all components of the energy-momentum tensor are considered. In this case, in the spherical coordinate, the problem reduces to a set of...
Conference Paper
Full-text available
The relativistic version of the gravitational instability criterion is studied in the first order of the post-Newtonian limit. In other words, we investigate the post-Newtonian Jeans analysis for a self-gravitating system whose characteristic velocity and corresponding gravitational field limit to the post-Newtonian regime. We find the first relati...
Poster
Full-text available
The relativistic generalization of the Jeans analysis is studied in the first post-Newtonian approximation. In fact, we investigate the local gravitational instability for a self-gravitating fluid in which characteristic velocity of the system and corresponding gravitational field can be relativistic and valid beyond Newtonian limit. To do so, the...
Preprint
Full-text available
We implement a new semi-analytical approach to investigate radially self-similar solutions for the steady-state advection-dominated accretion flows (ADAFs). We employ the usual $\alpha$-prescription for the viscosity and all the components of the energy-momentum tensor are considered. In this case, in the spherical coordinate, the problem reduces t...
Article
Full-text available
The existence of outflow in the advection dominated accretion flows has been confirmed by both numerical simulations and observations. The outow models for ADAF have been investigated by several groups with a simple self similar solution. But this solution is inaccurate at the inner regions and can not explain the emitted spectrum of the flow; so,...
Article
We solved one-dimensional hydrodynamic (HD) equations of the slim disks in the presence of the wind and in cylindrical. coordinates using self-similar solution. We considered power-law function of for mass loss rate due to wind. Then, we considered mass loss, angular-momentum loss and energy loss in equations. Then, we treated the influence of wind...
Article
Full-text available
Recent observations of the filamentary molecular clouds show that their properties deviate from the isothermal equation of state. Theoretical investigations proposed that the logatropic and the polytropic equations of state with negative indexes can provide a better description for these filamentary structures. Here, we aim to compare the effects o...
Article
Full-text available
We solved the set of two-dimensional magnetohydrodynamic (MHD) equations for optically thin black hole accretion flows incorporating toroidal component of magnetic field. Following global and local MHD simulations of black hole accretion disks, the magnetic field inside the disk is decomposed into a large scale field and a fluctuating field. The ef...
Article
We investigate the properties of a hot accretion flow bathed in a poloidal magnetic field. We consider an axisymmetric viscous resistive flow in the steady state configuration. We assume the dominant mechanism of energy dissipation is due to turbulence viscosity and magnetic diffusivity. A certain fraction of that energy can be advected towards the...
Article
Full-text available
What is conducted in the present work is the study of self-gravity effects on vertical structure of magnetized neutrino-dominated accretion disc (termed NDAF and considered as a central engine for Gamma-ray bursts (GRBs)). Some of the disc physical time scales such as viscous, cooling and diffusion time scales, that are supposed to play a pivotal r...
Article
The recent observations of rippled structures on the surface of the Orion molecular cloud (Bern\'{e} et al. 2010), have been attributed to the Kelvin-Helmholtz (KH) instability. The wavelike structures which have mainly seen near star-forming regions taking place at the interface between the hot diffuse gas, which is ionized by massive stars, and t...
Article
Full-text available
We present the effects of ordered large scale magnetic field on the structure of supercritical accretion flow in the presence of outflow. In the cylindrical coordinates ($r, \varphi,z$), We write the 1.5 dimensional, the steady state ($\frac{\partial}{\partial t}=0$) and axisymmetric ($\frac{\partial}{\partial \varphi}=0$) inflow-outflow equations...
Article
Full-text available
This paper explores the effects of large-scale magnetic fields in hot accretion flows for asymmetric configurations with respect to the equatorial plane. The solutions that we have found show that the large-scale asymmetric magnetic field can significantly affect the dynamics of the flow and also cause notable outflows in the outer parts. Previousl...
Article
Full-text available
The Jeans analysis is studied in the first post-Newtonian limit. In other words, the relativistic effects on the local gravitational instability are considered for systems where characteristic velocity of the system and corresponding gravitational field are higher than what permitted in Newtonian limit. The dispersion relation for propagation of sm...
Article
Full-text available
We examine the effects of a global magnetic field and outflow on radiatively inefficient accretion flow (RIAF) in the presence of magnetic resistivity. We find self-similar solutions for the height integrated equations that govern the behavior of the flow. We use the mixing length mechanism for studying the convection parameter. We adopt the radius...
Article
Full-text available
The gravitational instability of a filamentary molecular cloud in non-ideal magnetohydrodynamics is investigated. The filament is assumed to be in hydrostatic equilibrium. We add the effect of ambipolar diffusion to the filament which is threaded by an initial uniform axial magnetic field along its axis. We write down the fluid equations in cylindr...
Article
We obtain two-dimensional exact analytic solutions for the structure of the hot accretion flows without wind. We assume that the only non-zero component of the stress tensor is $T_{r\varphi}$. Furthermore we assume that the value of viscosity coefficient $\alpha$ varies with $\theta$. We find radially self-similar solutions and compare them with th...
Article
Full-text available
We study the global gravitational stability of a gaseous self-gravitating Maclaurin disk in the absence of a halo. Further, we replace Newtonian gravity with the specific Modified gravity theory known as MOG in the relevant literature. MOG is an alternative theory for addressing the dark matter problem without invoking exotic dark matter particles,...
Article
We solve the radiation-hydrodynamic (RHD) equations of supercritical accretion flows in the presence of radiation force and outflow by using self similar solutions. Compare with the pioneer works, in this paper we consider power-law function for mass inflow rate as $ \dot{M} \propto r^{s} $. We found that $ s = 1 $ when the radiative cooling term i...
Article
Full-text available
We study Active Galactic Nucleus (AGN) activity in the fossil galaxy cluster, RX J1416.4+2315. Radio observations were carried out using Giant Metrewave Radio Telescope (GMRT) at two frequencies, 1420 MHz and 610 MHz. A weak radio lobe that extends from the central nucleus is detected in 610 MHz map. Assuming the radio lobe originated from the cent...
Article
Full-text available
We present the effect of a hydrodynamical wind on the structure and the surface temperature of a vertically self-gravitating magnetized ADAFs using self-similar solutions. Also a model for an axisymmetric, steady-state, vertically self-gravitating hot accretion flow threaded by a toroidal magnetic field has been formulated. The model is based on $\...
Article
Full-text available
We construct self-similar inflow–outflow solutions for a hot viscous–resistive accretion flow with large-scale magnetic fields that have odd symmetry with respect to the equatorial plane in Bθ and even symmetry in Br and Bϕ. Following previous authors, we also assume that the polar velocity vθ is non-zero. We focus on four parameters: βr0, βϕ0 (the...
Article
Full-text available
We solve the set of hydrodynamic (HD) equations for optically thin Advection Dominated Accretion Flows (ADAFs) by assuming radially self-similar in spherical coordinate system $ (r, \theta, \phi) $. The disk is considered to be steady state and axi-symmetric. We define the boundary conditions at the pole and the equator of the disk and to avoid sin...
Article
Full-text available
In the framework of metric $f(R)$ gravity, we find the dispersion relation for the propagation of tightly wound spiral density waves in the surface of rotating, self-gravitating disks. Also, new Toomre-like stability criteria for differentially rotating disks has been derived for both fluid and stellar disks.
Article
Full-text available
We find the dispersion relation for tightly wound spiral density waves in the surface of rotating, self-gravitating disks in the framework of Modified Gravity (MOG). Also, the Toomre-like stability criterion for differentially rotating disks has been derived for both fluid and stellar disks. More specifically, the stability criterion can be express...
Article
Full-text available
MOdified Gravity (MOG) is a covariant modification of Einstein's general relativity. This theory is one of the current alternatives to dark matter models. We describe dynamics of collisionless self-gravitating systems in the context of MOG. By studying the weak field approximation of this theory, we derive the equations governing the dynamics of th...
Article
Full-text available
Dynamics of clumps within a magnetized advection dominated accretion flow is investigated by solving the collisionless Boltzmann equation and considering the drag force due to the relative velocity between the clumps and the gas. Toroidal component of the magnetic field is assumed to be dominant. Dynamical properties of the hot gaseous component su...
Article
We investigate the Hall effect in a standard magnetized accretion disk which is accompanied by dissipation due to viscosity and magnetic resistivity. By considering an initial magnetic field, using the PLUTO code, we perform a numerical magnetohydrodynamic simulation in order to study the effect of Hall diffusion on the physical structure of the di...
Article
Full-text available
By taking into account the effect of a toroidal magnetic field and its corresponding heating, we determine the thickness of advection-dominated accretion flows. We consider an axisymmetric, rotating, steady viscous–resistive, magnetized accretion flow under an advection-dominated stage. The dominant mechanisms of energy dissipation are assumed to b...
Article
Full-text available
The main aim of this paper is studying the effect of toroidal magnetic field on the structure of Advection-Dominated Accretion Flows (ADAF) in the presence of the turbulence viscosity and diffusivity due to viscosity and magnetic field respectively. We use self-similar assumption in radial direction to solve the magnetohydrodynamic (MHD) equations...
Article
Full-text available
We examine the effect of thermal conduction on the observational properties of a super critical hot magnetized flow. We obtained self-similar solution of a magnetized disc when the thermal conduction plays an important role. Follow of our first paper (Ghasemnezhad et al. 2012 (hereafter GKA12)) we have extended our solution on the observational app...
Article
Full-text available
Given the fact that accretion discs are associated with their parent molecular cloud, we studied its effects as a constraint on the outer boundary of the viscous polytropic self-gravitating accretion flows subject to the mass and angular momentum loss.
Article
Full-text available
Self-similar and semi-analytical solutions are found for the height-averaged equations govern the dynamical behavior of a polytropic, self-gravitating disk under the effects of winds, around the nascent object. In order to describe time evolution of the system, we adopt a radius dependent mass loss rate, then highlight its importance on both the tr...
Article
Full-text available
We have explored the structure of hot flow bathed in a general large-scale magnetic field. The importance of outflow and thermal conduction on the self-similar structure of a hot accretion flows has been investigated. We consider the additional magnetic parameters $ \beta_{r,\varphi,z}\big[= c^2_{r,\varphi,z}/(2 c^2_{s}) \big] $, where $ c^2_{r,\va...
Article
Full-text available
The existence of outflow and magnetic field in the inner region of hot accretion flows have been confirmed by observations and numerical magnetohydrodynamic simulations (MHD). We present self-similar solutions for radiation inefficiently accretion flows (RIAF) around black holes in the presence of outflow and global magnetic field. The influence of...
Article
Full-text available
We have studied the structure of hot accretion flow bathed in a general large-scale magnetic field. We have considered magnetic parameters \(\beta_{r,\varphi,z}[=c^{2}_{r,\varphi,z}/(2c^{2}_{s})] \), where \(c^{2}_{r, \varphi, z} \) are the Alfvén sound speeds in three direction of cylindrical coordinate (r,φ,z). The dominant mechanism of energy di...
Article
Full-text available
We have explored the structure of hot magnetized accretion flow with thermal conduction. The importance of thermal conduction in hot accretion flows has been confirmed by observations of the hot gas surrounding Sgr $A^*$ and a few other nearby galactic nuclei. For a steady state structure of such accretion flows a set of self similar solutions are...
Article
Full-text available
Force due to the self-gravity of the disc in the vertical direction is considered to study its possible effects on the structure of a magnetized advection-dominated accretion disc. We present steady-sate self similar solutions for the dynamical structure of such a type of the accretion flows. Our solutions imply reduced thickness of the disc becaus...
Article
The combined impact of rotation and magnetic fields on oscillations of stellar fluids is still not well known theoretically. It mixes Alfvén and inertial waves. Neutron stars are a place where both effects may be at work. We aim to solve this problem in the context of the r-mode instability in neutron stars, as it appears when these modes are coupl...
Article
Full-text available
In this paper, the effect of troidal magnetic field is studied in standard self-gravitating thin disks with β priscription. By applying the magnetic field, we expect to see different behaviors compared to a non-magnetic field case. The study reveals self-similar solutions for radial in fall velocity, rotation velocity, surface density and mass accr...
Article
Full-text available
The ability to predict the future behavior of solar activity has become of extreme importance due to its effect on the near Earth environment. Predictions of both the amplitude and timing of the next solar cycle will assist in estimating the various consequences of Space Weather. The level of solar activity is usually expressed by international sun...
Article
Observations of the hot gas, which is surrounding Sgr A* and a few other nearby galactic nuclei, imply that mean free paths of electron and proton are comparable to gas capture radius. So, hot accretion flows likely proceed under week collision conditions. As a result thermal conduction by ions has a considerable contribution in transfer of the rea...
Article
We examine the effects of a hydrodynamical wind on advection dominated accretion flows with thermal conduction in the presence of a toroidal magnetic field under a self-similar treatment. The disk gas is assumed to be isothermal. For a steady state structure of such accretion flows a set of self similar solutions are presented. The mass-accretion r...
Article
Observations of the hot gas that surrounds Sgr A* and a few other nearby galactic nuclei imply that the mean free paths of electrons and protons are comparable to the gas capture radius. Hot accretion flows therefore likely proceed under weak collision conditions. As a result, thermal conduction by ions has a considerable contribution to the transf...
Article
Full-text available
Observations of the hot gas, which is surrounding Sgr A* and a few other nearby galactic nuclei, imply that mean free paths of electron and proton are comparable to gas capture radius. So, hot accretion flows likely proceed under week collision conditions. As a result thermal conduction by ions has a considerable contribution in transfer of the rea...
Article
Full-text available
Duschl et al. (2000) have shown that the standard model for geometrically thin accretion disks ($\alpha$-disks) leads to inconsistency if self-gravity play a role. This problem arise from parametrization of viscosity in terms of local sound velocity and vertical disks scale hight. The $\beta$-viscosity prescription was introduced by Duschl et al. (...
Article
The observation of the hot gas surrounding Sgr $A^*$ and a few other nearby galactic nuclei imply that electron and proton mean free paths are comparable to the gas capture radius. So, the hot accretion flows is likely to proceed under week-collision conditions. Hence, thermal conduction has been suggested as a possible mechanism by which the suffi...
Article
Full-text available
We investigate the effect of self-gravity on the equilibrium structure of a thick non-rotating disk around a central object by the self-similar method. We introduce three dimensionless variables C sg , C t and C k that indicate the relative importance of self-gravity, thermal energy and kinetic energy, respectively. We study the effect of each of t...
Article
We carry out self-similar solutions of viscous–resistive accretion flows around a magnetized compact object. We consider an axisymmetric, rotating, isothermal steady accretion flow, which contains a poloidal magnetic field of the central star. The dominant mechanism of energy dissipation is assumed to be the turbulence viscosity and magnetic diffus...
Article
We carry out the self-similar solutions of viscous-resistive accretion flows around a magnetized compact object. We consider an axisymmetric, rotating, isotheral steady accretion flow which contains a poloidal magnetic field of the central star. The dominant
Article
Full-text available
We carry out the effect of $\beta$-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 $\beta$-model will have different dynamical behavior compare the well known $\alpha$-prescriptions. We used self-similar m...
Article
We examine the effect of self-gravity in a rotating thick-disc equilibrium in the presence of a dipolar magnetic field. First, we find a self-similar solution for non-self-gravitating discs. The solution that we have found shows that the pressure and density equilibrium profiles are strongly modified by a self-consistent toroidal magnetic field. We...
Article
Both theoretical studies and observational features of Type-I X-ray bursts reveal that weakly-magnetic (B˜ 108-109 G) but high rate accreting (>10-9 M&sun; yr-1) neutron star in low mass X-ray binaries (LMXBs) burns the accreting hydrogen and helium to mid-weight elements like C O Ne etc. and accumulates a massive degenerate liquid ocean with densi...

Network

Cited By

Projects

Project (1)
Project
In this project we investigate effect of non-ideal MHD (currently ambipolar diffusion) on stability of filamentary molecular clouds with different equation of states.