arXiv subjects

Fahimeh Habibi

Publications and source records attributed to Fahimeh Habibi.

4 recordsLinked to original sources

arXiv2026

Angle-dependent emission from evolving accretion layers

Photon propagation through an evolving atmosphere can change the angular distribution of emerging radiation even when its integrated optical column remains constant. We quantify this effect in an irradiated, internally heated, expanding plane-parallel layer using grey, special-relativistic similarity solutions. The full angular radiation field determines both the source function and the acceleration of a pressureless flow in prompt radiative balance. We compare each evolving solution with two frozen transport calculations on the same density and velocity profiles, holding the source fixed in one and allowing it to relax in the other. Heating produces the familiar transition from limb darkening to limb brightening, while the temporal correction to the angular contrast changes sign with optical column. For an upper boundary moving at $0.04c$, an injection speed of $0.06c$, and heating four times the reference incident flux, the intensity ratio between direction cosines $0.1$ and $1$ exceeds the frozen, fixed-source result by $18.8$ per cent at column $τ_0=0.15$, but is smaller by $2.53$ per cent at $τ_0=0.55$. Source relaxation preserves this reversal. The corresponding direct photon flight times are one third and three per cent of the flow age in the oblique and normal directions, respectively. These solutions provide a controlled local benchmark for temporal angular transfer in evolving disc atmospheres and reprocessing layers; application to a specific accreting system requires thermal, spectral, and geometrical physics beyond the present model.

astro-ph.HE↗

arXiv2025

The Role of Outflows in dynamic of Advection Dominated Accretion Flows: a Self-Similar Solution

The effects of outflow on the behavior of a viscous gaseous disc around a compact object in an advection-dominated state are examined in this paper. We suppose that the flow is steady, axisymmetric, and rotating. Also, we focus on the model in which the mass, the angular momentum, and the energy can be transported outward by outflow. Similar to the pioneering studies, we consider a power-law function for mass inflow rate as $\dot{M} \propto r^s$. We assume that the power index $s$ is proportional to the dimensionless thickness $H/R$ of disc. To analyze such a system, the hydrodynamic equations have extracted in cylindrical coordinates $(r,φ,z)$. Then, the flow equations were vertically integrated, and a set of self-similar solutions was got in the radial direction. Our solutions include three essential parameters: $λ$, $f$ and $ζ$. The influence of the outflow on the dynamics of the disc is investigated by the $λ$ parameter. The degree of advection of flow is shown by the advection parameter $f$. Also, energy extraction from the disc by the outflow is showed by $ζ$ parameter. Our findings demonstrate a significant correlation between the outflow parameters, flow advection parameter, and the temperature, thickness, and inflow-outflow rate of the disc. In addition, we explored the influence of these parameters on the power index $s$, too. The results of our study demonstrate that enhancing the outflow parameter or flow advection degree increases power index $s$, while extracting more energy through outflow decreases index $s$.

astro-ph.HE↗

arXiv2020

Time dependence of advection dominated accretion flow around a rotating compact object

Time evolution of advection-dominated accretion flow (ADAF) around a rotating compact object is presented. The equations of time-dependent of fluid including the Coriolis force along with the centrifugal and pressure gradient forces are derived. In this research, it is assumed that angular momentum transport is due to viscous turbulence and the α-prescription is used for the kinematic coefficient of viscosity. Moreover, the general relativistic effects are neglected. In order to solve the equations, we have used a self-similar solution. The solutions show that the behaviour of the physical quantities in a dynamical ADAF is different from that for a steady accretion flow. Our results indicate that the physical quantities are dependent of rotation parameter which is defined as the ratio of the intrinsic angular velocity of the central body to the angular velocity of disc. Also, the effect of rotation parameter on these quantities is different for co and counter-rotating flows. The solution shows that by increasing the rotation parameter a, inflow-outflow region approaches the central object for co-rotating flow and moves outward for counter-rotating flow. We find that when flow is fully advection dominated (f ! 1), the entire gas has positive Bernoulli function. Also, we suggest that the Bernoulli function becomes more positive when the effect of rotation on the structure of disc decreases.

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↗