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A. Hujeirat

Publications and source records attributed to A. Hujeirat.

9 recordsLinked to original sources
arXiv2008

Angular momentum transport during X-ray bursts on neutron stars: a numerical general relativistic hydrodynamical study

The distribution of angular momentum of the matter during X-ray bursts on neutron stars is studied by means of 3D axi-symmetric general relativistic hydrodynamics. The set of fully general relativistic Navier-Stokes equations is solved implicitly using the implicit solver GR-I-RMHD in combination with a third order spatial and second order temporal advection scheme. The viscous operators are formulated using a Kerr-like metric in the fixed background of a slowly rotating neutron star whose radius coincides with the corresponding last stable orbit. The importance of these operators and their possible simplifications are discussed as well. In the rotating case and depending on the viscosity parameter, $α_\mm{tur}$, it is found that the viscously-initiated fronts at the center of bursts propagate at much faster speed than the fluid motion. These fast fronts act to decouple angular momentum from matter: angular momentum is transported outwards while matter sinks inwards into the deep gravitational well of the neutron star, thereby enhancing the compression of matter necessary for initiating ignition, that subsequently spreads over the whole surface of the neutron star on the viscous time scale. Based on the numerical simulations, we find that a viscosity parameter $α_\mm{tur} = \mathcal{O}(0.1)$ is most suitable for fitting observations of neutron stars during X-ray bursts. It is argued that the spin up observed in the cooling tails of X-ray bursts is a transient phase, which eventually should be followed by a spin down phase. This delay can be attributed to a significant lengthening of the viscous time scale due to rapid cooling of matter in the outer layers.

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arXiv2007

Compressed low Mach number flows in astrophysics: a nonlinear Newtonian numerical solver

Internal flows inside gravitationally stable astrophysical objects, such as the Sun, stars and compact stars are compressed and extremely subsonic. Such low Mach number flows are usually encountered when studying for example dynamo action in stars, planets, the hydro-thermodynamics of X-ray bursts on neutron stars and dwarf novae. Treating such flows is numerically complicated and challenging task. We aim to present a robust numerical tool that enables modeling the time-evolution or quasi-stationary of stratified low Mach number flows under astrophysical conditions. It is argued that astrophysical low Mach number flows cannot be considered as an asymptotic limit of incompressible flows, but rather as highly compressed flows with extremely stiff pressure terms. Unlike the pseudo-pressure in incompressible fluids, a Possion-like treatment for the pressure would smooth unnecessarily the physically induced acoustic perturbations, thereby violating the conservation character of the compressible equations. Moreover, classical dimensional splitting techniques, such as ADI or Line-Gauss-Seidel methods are found to be unsuited for modeling compressible flows with low Mach numbers. In this paper we present a nonlinear Newton-type solver that is based on the defect-correction iteration procedure and in which the Approximate Factorization Method (AFM) is used as a preconditioner. This solver is found to be sufficiently robust and is capable of capturing stationary solutions for viscous rotating flows with Mach number as small as $\mcal{M} \approx 10^{-3},$ i.e., near the incompressibility limit.

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arXiv2007

Advanced numerical methods in astrophysical fluid dynamics

Computational gas dynamics has become a prominent research field both in astrophysics and cosmology. In the first part of this review we intend to briefly describe several of the numerical methods used in this field, discuss their range of application and present strategies for converting conditionally-stable numerical methods into unconditionally-stable solution procedures. The underlying aim of the conversion is to enhance the robustness and unification of numerical methods and subsequently enlarge their range of applications considerably. In the second part Fabian Heitsch presents and discusses the implementation of a time-explicit MHD Boltzmann solver.

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arXiv2003

A method for relaxing the CFL-condition in time explicit schemes

A method for relaxing the CFL-condition, which limits the time step size in explicit methods in computational fluid dynamics, is presented. The method is based on re-formulating explicit methods in matrix form, and considering them as a special-Jacobi iteration scheme that converge efficiently if the CFL- number is less than unity. By adopting this formulation, one can design various solution methods in arbitrary dimensions that range from explicit to unconditionally stable implicit methods in which CFL-number could reach arbitrary large values. In addition, we find that adopting a specially varying time stepping scheme accelerates convergence toward steady state solutions and improves the efficiently of the solution procedure.

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arXiv2003

A model for electromagnetic extraction of rotational energy and formation of accretion-powered jets in radio galaxies

A self-similar solution for the 3D axi-symmetric radiative MHD equations, which revisits the formation and acceleration of accretion-powered jets in AGNs and microquasars, is presented. The model relies primarily on electromagnetic extraction of rotational energy from the disk plasma and forming a geometrically thin super-Keplerian layer between the disk and the overlying corona. The outflowing plasma in this layer is dissipative, two-temperature, virial-hot, advective and electron-proton dominated. The innermost part of the disk in this model is turbulent-free, sub-Keplerian rotating and advective-dominated. This part ceases to radiate as a standard disk, and most of the accretion energy is converted into magnetic and kinetic energies that go into powering the jet. The corresponding luminosities of these turbulent-truncated disks are discussed.

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arXiv2003

A model for the jet-disk connection in BH accreting systems

Based on theoretical arguments and quasi-stationary radiative MHD calculations, a model for an accretion-powered jet is presented. It is argued that accretion disks around BHs consist of 1) a cold, Keplerian-rotating and weakly magnetized mediumin the outer part, 2) a highly advective and turbulent-free plasma inside $r_\mm{tr}=10-20$ Schwarzschild radii, where magnetic fields are predominantly of large scale topology and in excess of thermal equipartition, and 3) an ion-dominated torus in the vicinity of the hole, where magnetic fields undergo a topological change into a monopole like-configuration. The action of magnetic fields interior to $r_\mm{tr}$ is to initiate torsional {\Alfven} waves that extract angular momentum from the disk-plasma and deposit it into the transition layer (-TL) above the disk, where the plasma is dissipative and tenuous. A significant fraction of the shear-generated toroidal magnetic field reconnects in the TL, thereby heating the plasma up to the virial-temperature and forming a super-Keplerian rotating, and hence centrifugally accelerated outflow. The strong magnetic field in the TL forces the electrons to cool rapidly yielding thereby an ion-dominated outflow. The toroidal magnetic field in the TL is in thermal equipartition with the ions, whereas the poloidal component is in equipartition with the electrons. Such a strong toroidal magnetic field is essential for increasing the jet-disk luminosity in the radio regime.

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arXiv2003

Jet-formation in accreting black hole systems and in GRB

Most powerful jets are observed to emanate from accreting black hole systems. Recent quasi-stationary radiative MHD calculations reveal that jet-launching start in the innermost region of a transition layer (:TL), which is located between the disk and the corona. The plasma in the TL is dominated by virial-hot protons, advective, highly magnetic-diffusive and rotates super-Keplerian. The amplified toroidal magnetic field (TMF) in the TL reach values beyond equipartition with respect to the electron thermal energy, thereby considerably enhancing the Poynting energy flux and yielding gravitationally unbound electron-proton outflows with a positive Bernoulli number. We speculate that beaming jets in GRBs may possess similar properties, provided that the GRB-progenitors have rotational energy sufficient for forming a nearly Keplerian and massive disk.

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arXiv2002

Ion-dominated plasma and the origin of jets in quasars

Low cooling plasmas associated with large kinetic energies are likely to be the origin of the kpc-extended and well collimated extra-galactic jets. It is proposed that jets are launched from a layer, governed by a highly diffusive, super-Keplerian rotating and thermally dominated by virial-hot and magnetized ion-plasma. The launching layer is located between the accretion disk and the corona surrounding the nucleus. The matter in the layer is causally connected to both the disk and to the central engine. Moreover we find that coronae, in the absence of heating from below, are dynamically unstable to thermal ion-conduction, and that accretion disks become intrinsically advection-dominated. We confirm the capability of this multi-layer model to form jets by carrying out 3D axisymmetric quasi-stationary MHD calculations with high spatial resolution, and taking into account turbulent and magnetic diffusion. The new multi-layer topology accommodates several previously proposed elements for jet-initiation, in particular the ion-torus, the magneto-centrifugal and the truncated disk - advective tori models.

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