arXiv subjects

Maryam Samadi

Publications and source records attributed to Maryam Samadi.

7 recordsLinked to original sources

arXiv2022

Thermal Conduction in Clumpy Disks and BLR clouds

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 first study the ensemble of clumps by assuming them as collision-less particles and secondly we find the orbital motion of these clouds as individuals. For both parts, clumps are subject to the gravity of the central object and a drag force. The strong coupling between clumps and ADAF leads to equality between the average treatment of the clumps and the dynamics of their background. By employing the collision-less Boltzmann equation we calculate the velocity dispersion of the clumps which turns out approximately one order of magnitude higher than the ADAF. In fact, involving drag force in such a system causes the angular momentum of the clumps can be transported outwards by the ADAF, and hence the clouds eventually will be captured at the tidal radius. The results show that the presence of thermal conduction increases the root of the averaged radial velocity square and this in turn speeds up the process of capturing the clouds through the tidal force. In the end, we focus on a typical individual cloud, the spiral orbits appear thanks to only the toroidal component of friction force. The parametric study again proves that the operation of thermal conduction helps for decreasing the lifetime of clumps.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2015

The Effect of Large Scale Magnetic Field on Outflow in ADAFs: an Odd Symmetry Configuration

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 $B_θ$, and even symmetry in $B_r$ and $B_ϕ$. Following previous authors, we also assume that the polar velocity $v_θ$ is nonzero. We focus on four parameters: $β_{r0}$, $β_{\phi0}$ (the plasma beta parameters for associated with magnetic field components at the equatorial plane), the magnetic resistivity $η_0$, and the density index $n=-d\lnρ/d\ln r$. The resulting flow solutions are divided into two parts consisting of an inflow region with a negative radial velocity ($v_r<0$) and an outflow region with $v_r>0$. Our results show that stronger outflows emerge for smaller $β_{r0}$ ($\le10^{-2}$ for $n>1$) and larger values of $β_{\phi0}$, $η_0$ and $n$.

astro-ph.HE↗

arXiv2013

The Effect of Toroidal Magnetic Field on Thickness of a Viscose-Resistive Hot Accreting Flow

By taking into account the effect of toroidal magnetic field and its correspond heating, we determine the thickness of advection-dominated accretion flows. We consider an axisymetric, rotating, steady viscous-resistive, magnetized accretion flow under an advection dominated stage. The dominant mechanism of energy dissipation is assumed to be turbulence viscosity and magnetic diffusivity. We adopt a self-similar assumption in the radial direction to obtain the dynamical quantities, that is, radial, azimuthal, sound and Alfv\' en velocities. Our results show the vertical component of magnetic force acts in the opposite direction of gravity and compresses the disc, thus compared with the non-magnetic case, in general the disc half-thikness, $Δθ$, significantly is reduced. On the other hand, two parameters, appearing due to action of magnetic field and reaction of the flow, affect the disc thickness. The first one, $β_0$, showing the magnetic field strength at the equatorial plane, decreases $Δθ$. The other one, $η_0$ is the magnetic resistivity parameter and when it increases, $Δθ$ increases, too.

astro-ph.HE↗