arXiv subjects

Shahram Abbassi

Publications and source records attributed to Shahram Abbassi.

At least 37 records · Page 2Linked to original sources

arXiv2020

Analytical Solutions of Radiative Transfer Equations in Accretion Discs with Finite Optical Depth

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. Analytical solutions are achieved for two cases: (i) radiative equilibrium, and (ii) a disc with uniform internal heating and both cases are assumed to be in local thermodynamical equilibrium (LTE), too. These solutions enable us to study the probable role of scattering and disc optical depth on the emergent intensity and other radiative quantities. Our results show that for the first case, the surface value of mean intensity with constant Eddington factor is three times larger than that with a variable factor. Moreover, scattering has no role in the vertical radiative structure of discs with the assumptions of the first case. On the other hand, for the second case, we encounter reductions in all radiative quantities as the photon destruction probability decreases (which is equivalent to increasing scattering). Furthermore, for both cases with total optical depth less than unity, the outward intensity towards the polar direction becomes less than that from the edges of disc which is contrary to limb-darkening. In the end, we apply our results to find the spectrum from accretion systems, based on two dynamical models. Consequently, we can see how the total optical depth varies with frequency and causes remarkable changes in the emergent spectra.

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arXiv2020

Does modified gravity predict fast stellar bars in spiral galaxies?

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 models in the standard dark matter model and compare their dynamics with the galactic models in modified gravity. It turns out that there are serious differences between galactic models in these different viewpoints. For example, we explicitly show that the galactic models in modified gravity, host faster bars compared to the dark matter case. On the other hand, final stellar bars are weaker in modified gravity. These facts are not new and have already been reported in our previous simulations for exponential galactic models. Therefore, our main purpose in this study is to show that the above-mentioned differences, with emphasis on the speed of the bars, are independent of the initial density profile of the adopted disk/halo. To do so, we employ different profiles for the disk and halo and show that the results remain qualitatively independent of the initial galactic models. Moreover, a more accurate method has been used to quantify the kinematic properties of the stellar bar. Our results imply that contrary to the dark matter models, bars in modified gravity are fast rotators which never leave the fast-bar region until the end of the simulation.

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arXiv2019

Viscous Evolution of Magnetized Clumps: a Source for X-ray Flares in Gamma-ray Bursts

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 reconstruct Dall Osso et al. (2017) framework through taking both small and large scale effects of magnetic field into account. To consider the magnetic barrier as a possible mechanism that might govern the accretion process of each magnetized clump, we make a simple pattern in boundary condition through which this mechanism might happen. Regarding various model parameters, we probe for their influence and proceed some key analogies between our model predictions and previous phenomenological estimates, for two different choices of boundary conditions (with and without a magnetic barrier). Our model is remarkably capable of matching flare bolometric and X-ray light-curves, as well as reproducing their statistical properties, such as the ratios between rise and decay time, width parameter and peak time, and the power-law correlation between peak luminosity and peak time. Combining our results with the conclusions of previous studies, we are led to interpret magnetic barrier as a less probable mechanism that might control the evolution of these clumps, especially the later created (or viscously evolved) ones.

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arXiv2019

Thermal instability of thin accretion disks in the presence of wind and toroidal magnetic field

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 stabilizing effect. This is also true when there is no wind. In this case, we confirm the other works already presented in the literature. However, our main objective is to investigate the local thermal stability of the disk in the presence of the wind. In this case, interestingly, the response of disk is directly related to the type of wind. In other words, in some cases, the wind can stabilize the disk. On the other hand, in some cases, it can destabilize the disk. We found that in some thin disk models where the magnetic pressure cannot explain the stability of the disk by including a typical contribution for magnetic pressure, the wind can provide a viable explanation for the thermal stability.

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arXiv2019

Accretion flows with comparable radiation and gas pressures

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 employ self-similar technique in the radial direction. We need the reflection symmetry about the mid-plane to find gas density at the equator. For a typical solution, we assume that the gas pressure has 10-50\% portion of the total pressure. In this paper, since the radiation energy is involved directly, we are able to estimate how much energy of viscous heating is transported in the radial direction and advected towards the central object. Our results show that although the mass accretion rate does not approach the Eddington limit, the energy advection is rather high. Moreover, in a disc with greater accretion rate and less portion of gas pressure at the total pressure, more energy is advected to its center. In addition, as we expect the accretion flow becomes thicker with greater values of gas pressure. Based on Solberg-HÃžiland conditions, we notice that the flow is convectively stable in all parts of such a disc.

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arXiv2018

Fourier analysis of advection-dominated accretion flows

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 the components of the energy-momentum tensor are considered. In this case, in the spherical coordinate, the problem reduces to a set of eighth-order, nonlinear differential equations with respect to the latitudinal angle $θ$. Using the Fourier expansions for all the flow quantities, we convert the governing differential equations to a large set of nonlinear algebraic equations for the Fourier coefficients. Using the Newton-Raphson method we solve the algebraic equations and ADAF properties are investigated over a wide range of the model parameters. We also show that the implemented series are truly convergent. The main advantage of our numerical method is that it does not suffer from the usual technical restrictions which may arise for solving ADAF differential equations near to the polar axis. In order to check the reliability of the approach, we recover some widely studied solutions. We also introduce a new varying $α$ viscosity model and new outflow and inflow solutions for ADAFs are presented using Fourier expansion series.

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arXiv2017

Gravitational instability of filamentary molecular clouds, including ambipolar diffusion; Non-isothermal filament

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 of these softer non-isothermal equation of states with their isothermal counterpart on the global gravitational instability of a filamentary molecular cloud. By incorporating the ambipolar diffusion, we use the non-ideal magnetohydrodynamics framework for a filament that is threaded by a uniform axial magnetic field. We perturb the fluid and obtain the dispersion relation both for the logatropic and polytropic equations of state by taking the effects of magnetic field and ambipolar diffusion into account. Our results suggest that, in absence of the magnetic field, a softer equation of state makes the system more prone to gravitational instability. We also observed that a moderate magnetic field is able to enhance the stability of the filament in a way that is sensitive to the equation of state in general. However, when the magnetic field is strong, this effect is suppressed and all the equations of state have almost the same stability properties. Moreover, we find that for all the considered equations of state, the ambipolar diffusion has destabilizing effects on the filament.

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arXiv2017

The effects of toroidal magnetic field on the vertical structure of hot accretion flows

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 effects of the fluctuating magnetic field in transferring the angular momentum and dissipating the energy are described through the usual $ α$ description. We solved the MHD equations by assuming steady state and radially self-similar approximation in $ r-θ$ plane of spherical coordinate system. We found that as the amount of magnetic field at the equatorial plane increases, the heating by the viscosity decreases. In addition, the maximum amount of the heating by the viscous dissipation is produced at the mid-plane of the disk, while that of the heating by the magnetic field dissipation is produced at the surface of the disk. Our main conclusion is that in terms of the no-outflow solution, thermal equilibrium still exists for the strong magnetic filed at the equatorial plane of the disk.

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arXiv2017

Anchoring polar magnetic field in a stationary thick accretion disk

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 central compact object. We employ self-similar method in the radial direction to find a system of ODEs with just one varible, $θ$ in the spherical coordinates. For the existence and maintaining of a purely poloidal magnetic in a rotating thick disk, we find the necessary condition is a constant value of angular velocity along a magnetic field line. We obtain an analytical solution for the poloidal magnetic flux. We explore possible changes in the vertical structure of the disk under the influences of even symmetric and asymmetric magnetic fields. Our results reveal that a polar magnetic field with even symmetry about the equatorial plane makes the disk vertically thin. Moreover, the accretion rate decreases when we consider a strong magnetic field. Finally, we notice hot magnetized accretion flows can be fully advected even in a slim shape.

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arXiv2017

Self-Gravity in Magnetized Neutrino-Dominated Accretion Discs

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 role in the late time evolutions of the disc, have been studied. It is of our interest to investigate the possibility of the X-ray flares occurrence, observed in late time GRB's extended emission through the "magnetic barrier" and "fragmentation" processes in our model. The results lead us to interpret self-gravity as an amplifier for Blandford-Payne luminosity (BP power) and the generated magnetic field, but a suppressor for neutrino luminosity and magnetic barrier process via highlighting fragmentation mechanism in the outer disc, especially for the higher mass accretion rates.

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arXiv2017

How does an asymmetric magnetic field change the vertical structure of a hot accretion flow?

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. Previously, we treated a viscous resistive accreting disc in the presence of an odd symmetric $\textbf{B-}$ field about the equatorial plane. Now we extend our earlier work by taking into account another configuration of large-scale magnetic field which is no longer symmetric. We provide asymmetric field structures with small deviations from even and odd symmetric $\textbf{B-}$field. Our results show that the disc's dynamics and appearance become different above and below the equatorial plane. The set of solutions also predicts that even a small deviation in a symmetric field causes the disc to compress on one side and expand on the other. In some cases, our solution represents a very strong outflow from just one side of the disc. Therefore, the solution may potentially explain the origin of one-sided jets in radio galaxies.

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arXiv2017

The Kelvin-Helmholtz instability in the Orion nebula: The effect of radiation pressure

The recent observations of rippled structures on the surface of the Orion molecular cloud (Berné 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 the cold dense molecular clouds. The radiation pressure of massive stars and stellar clusters is one of the important issues that has been considered frequently in the dynamics of clouds. Here, we investigate the influence of radiation pressure, from well-known Trapezium cluster in the Orion nebula, on the evolution of KH instability. The stability of the interface between HII region and molecular clouds in the presence of the radiation pressure, has been studied using the linear perturbation analysis for the certain range of the wavelengths. The linear analysis show that consideration of the radiation pressure intensifies the growth rate of KH modes and consequently decreases the e-fold time-scale of the instability. On the other hand the domain of the instability is extended and includes the more wavelengths, consisting of smaller ones rather than the case when the effect of the radiation pressure is not considered. Our results shows that for $λ_{\rm KH}>0.15\rm pc$, the growth rate of KH instability dose not depend to the radiation pressure. Based on our results, the radiation pressure is a triggering mechanism in development of the KH instability and subsequently formation of turbulent sub-structures in the molecular clouds near massive stars. The role of magnetic fields in the presence of the radiation pressure is also investigated and it is resulted that the magnetic field suppresses the effects induced by the radiation pressure.

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arXiv2017

The influence of outflow and global magnetic field on the structure and spectrum of resistive CDAFs

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 a 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 a radius dependent mass accretion rate as $\dot{M}=\dot{M}_{out}{(\frac{r}{r_{out}})^{s}}$ with $s> 0$ to investigate the influence of outflow on the structure of inflow where $s$ is a constant and indication the effect of wind. Also, we have studied the radiation spectrum and temperature of CDAFs. The thermal bermsstrahlung emission as a radiation mechanism is taken into account for calculating the spectra emitted by the CDAFs. The energy that powers bremsstrahlung emission at large radii is provided by convective transport from small radii and viscous and resistivity dissipation. Our results indicate that the disc rotates slower and accretes faster, it becomes hotter and thicker for stronger wind. By increasing all component of magnetic field, the disc rotates faster and accretes slower while it becomes hotter and thicker. We show that the outflow parameter and all component of magnetic field have the same effects on the luminosity of the disc. We compare the dynamical structure of the disc in two different solutions (with and without resistivity parameter). We show that only the radial infall velocity and the surface density could changed by resistivity parameter obviously. Increasing the effect of wind increases the disc's temperature and luminosity of the disc. The effect of magnetic field is similar to the effect of wind in the disc's temperature and luminosity of the disc, but the influence of resistivity on the observational properties is not evident.

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arXiv2017

The influence of large scale magnetic field in the structure of supercritical accretion flow with outflow

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 ($r, φ,z$), We write the 1.5 dimensional, the steady state ($\frac{\partial}{\partial t}=0$) and axisymmetric ($\frac{\partial}{\partial φ}=0$) inflow-outflow equations by using the self similar solutions. Also a model for radiation pressure supported accretion flow threaded by both toroidal and vertical components of magnetic field has been formulated. For studying the outflows, we adopt a radius dependent mass accretion rate as $\dot{M}=\dot{M}_{out}{(\frac{r}{r_{out}})^{s+\frac{1}{2}}}$ with $s=\frac{1}{2}$. Also by following the previous works, we have considered the interchange of mass, radial and angular momentum and the energy between inflow and outflow. we have found numerically that two components of magnetic field have the opposite effects on the thickness of the disc and similar effects on the radial and angular velocities of the flow. We have found that the existence of the toroidal component of magnetic field will lead to increasing of radial and azimuthal velocities as well as the relative thickness of the disc, which is increased . Moreover, the thickness of the disc will decrease when the vertical component of magnetic field becomes important in magnetized flow. The solutions indicated that the mass inflow rate, the specific energy of outflow affect strongly on the advection parameter.We have shown that by increasing the two components of magnetic field, the temperature of the accretion flow will decrease significantly. On the other hand we have shown the bolometric luminosity of the slim discs for high values of $\dot{m} (\dot{m}>>1)$ is not sensitive to mass accretion rate and is kept constant ($L \approx 10 L_{E}$).

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arXiv2017

Post-Newtonian Jeans analysis

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 small perturbations is found in the post-Newtonian approximation using two different techniques. A new Jeans mass is derived and compared to the standard Jeans mass. In this limit, the relativistic effects make the new Jeans mass to be smaller than the Newtonian Jeans mass. Furthermore, the fractional difference between these two masses increases when temperature/pressure of the system increases. Interestingly, in this limit pressure can help the gravitational instability instead of preventing it. Finally the results are applied to high temperature astrophysical systems and the possibility of local fragmentations in some relativistic systems is investigated.

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arXiv2016

Gravitational instability of filamentary molecular clouds, including ambipolar diffusion

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 cylindrical coordinates and perform linear perturbation analysis. We integrate the resultant differential equations and then derive the numerical dispersion relation. We find that, a more efficient ambipolar diffusion leads to an enhancement of the growth of the most unstable mode, and to increase of the fragmentation scale of the filament.

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arXiv2016

Exact analytical solutions for ADAFs

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 $T_{rφ}$. Furthermore we assume that the value of viscosity coefficient $α$ varies with $θ$. We find radially self-similar solutions and compare them with the numerical and the analytical solutions already studied in the literature. The no-wind solution obtained in this paper may be applied to the nuclei of some cool-core clusters.

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arXiv2016

On the gravitational stability of the Maclaurin disk

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, and possesses two free parameters $α$ and $μ_0$ in the weak field limit. We derive the equilibrium gravitational potential of the Maclaurin disk in MOG and develop a semi-analytic method for studying the response of the disk to linear non-axysymmetric perturbations. The eigenvalue spectrum of the normal modes of the disk is obtained and its physical meaning has been explored. We show that Maclaurin disks are less stable in MOG than in Newtonian gravity. In fact both parameters $(α,μ_0)$ have destabilizing effects on the disk. Interestingly $μ_0$ excites only the bar mode $m=2$ while $α$ affects all the modes. More specifically, when $α>1$, the bar mode is strongly unstable and unlike in Newtonian gravity can not be avoided, at least in the weak field limit, with increasing the pressure support of the disk.

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