arXiv subjects

Rachid Ouyed

Publications and source records attributed to Rachid Ouyed.

85 records · Page 5Linked to original sources

arXiv2006

Magnetic Field Decay and Period Evolution of Anomalous X-Ray Pulsars in the Context of Quark Stars

We discuss a model wherein soft gamma-ray repeaters (SGRs), anomalous X-ray pulsars (AXPs), and radio quiet isolated neutron stars (RQINSs) are all compact objects exhibiting superconductivity, namely color-flavor locked quark stars. In particular we calculate the magnetic field decay due to the expulsion of spin-induced vortices from the star's superfluid-superconducting interior, and the resultant spin-down rate. We find that, for initial parameters characteristic of AXPs/SGRs ($10^{13}<B<10^{14}~\rm{G}$; $3<P<12~\rm{s}$), the magnetic field strengths and periods remain unchanged within a factor of two for timescales of the order of $5\times 10^{5} - 5\times 10^{7}~\rm{yrs}$ given a quark star of radius $10~\rm{km}$. Within these timescales, we show that the observed period clustering in RQINSs can be explained by compactness, as well as calculate how the magnetic field and period evolve in a manner concurrent with RQINS observations.

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arXiv2006

Quark deconfinement in neutron star cores: The effects of spin-down

We study the role of spin-down in driving quark deconfinement in the high density core of isolated neutron stars. Assuming spin-down to be solely due to magnetic braking, we obtain typical timescales to quark deconfinement for neutron stars that are born with Keplerian frequencies. Employing different equations of state (EOS), we determine the minimum and maximum neutron star masses that will allow for deconfinement via spin-down only. We find that the time to reach deconfinement is strongly dependent on the magnetic field and that this time is least for EOS that support the largest minimum mass at zero spin, unless rotational effects on stellar structure are large. For a fiducial critical density of $5ρ_0$ for the transition to the quark phase ($ρ_0=2.5\times10^{14}$g/cm$^3$ is the saturation density of nuclear matter), we find that neutron stars lighter than $1.5M_{\odot}$ cannot reach a deconfined phase. Depending on the EOS, neutron stars of more than $1.5M_{\odot}$ can enter a quark phase only if they are spinning faster than about 3 milliseconds as observed now, whereas larger spin periods imply that they are either already quark stars or will never become one.

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arXiv2006

Disk Winds, Jets, and Outflows: Theoretical and Computational Foundations

We review advances in the theoretical and computational studies of disk winds, jets and outflows including: the connection between accretion and jets, the launch of jets from magnetized disks, the coupled evolution of jets and disks, the interaction of magnetized young stellar objects with their surrounding disks and the relevance to outflows, and finally, the link between jet formation and gravitational collapse. We also address the predictions that the theory makes about jet kinematics, collimation, and rotation, that have recently been confirmed by high spatial and spectral resolution observations. Disk winds have a universal character that may account for jets and outflows during the formation of massive stars as well as brown dwarfs.

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arXiv2005

Cannonballs in the context of Gamma Ray Bursts: Formation sites ?

We investigate possible formation sites of the cannonballs (in the gamma ray bursts context) by calculating their physical parameters, such as density, magnetic field and temperature close to the origin. Our results favor scenarios where the cannonballs form as instabilities (knots) within magnetized jets from hyperaccreting disks. These instabilities would most likely set in beyond the light cylinder where flow velocity with Lorentz factors as high as 2000 can be achieved. Our findings challenge the cannonball model of gamma ray bursts if these indeed form inside core-collapse supernovae (SNe) as suggested in the literature; unless hyperaccreting disks and the corresponding jets are common occurrences in core-collapse SNe.

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arXiv2005

Skyrmion Stars: Astrophysical motivations and implications

We study mass-radius relations for compact stars employing an equation of state (EOS) of dense matter based on a Skyrme fluid. The zero-temperature mean-field model is based on mesonic excitations, incorporates the scale breaking of QCD, and accommodates baryons (nucleons) which arise as a solitonic configuration of mesonic fields. Stable configurations are obtained for central densities $ρ_c/ρ_n\leq 5.0$ where $ρ_n=2.575\times 10^{14}$ g/cc is the nuclear saturation density. These {\it Skyrmion Stars} are mostly fluid, with a crust which we describe by the EOS of Baym, Pethick and Sutherland. Their masses and radii are in the range $0.4 \le M/M_{\odot} \le 3.6$ and $13 {\rm km} \le R \le 23 {\rm km}$, respectively. The minimum spin period is computed to be between $0.7 {\rm ms}\le P \le 2.1 {\rm ms}$. They appear to have a mass-radius curve quite different from either neutron or quark stars, and provide a suitable description of the heavier mass neutron stars discovered recently due to the inherently stiff EOS. Within the same model, we compute the dominant neutrino emissivity in neutron-rich $β$-equilibrated matter, and determine the cooling behaviour of Skyrmion stars.

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arXiv2005

3-Dimensional Simulations of the Reorganization of a Quark Star's Magnetic Field as Induced by the Meissner Effect

In a previous paper (Ouyed et al. 2004) we presented a new model for soft gamma-ray repeaters (SGR), based on the onset of colour superconductivity in quark stars. In this model, the bursts result from the reorganization of the exterior magnetic field following the formation of vortices that confine the internal magnetic field (the \mz effect). Here we extend the model by presenting full 3-dimensional simulations of the evolution of the inclined exterior magnetic field immediately following vortex formation. The simulations capture the violent reconnection events in the entangled surface magnetic field as it evolves into a smooth, more stable, configuration which consists of a dipole field aligned with the star's rotation axis. The total magnetic energy dissipated in this process is found to be of the order of $10^{44} \mathrm{erg}$ and, if it is emitted as synchrotron radiation, peaks typically at $280 \mathrm{keV}$. The intensity decays temporally in a way resembling SGRs and AXPs (anomalous X-ray pulsars), with a tail lasting from a few to a few hundred times the rotation period of the star, depending on the initial inclination between the rotation and dipole axis. One of the obvious consequences of our model's final state (aligned rotator) is the suppression of radio-emission in SGRs and AXPs following their bursting era. We suggest that magnetar-like magnetic field strength alone cannot be responsible for the properties of SGRs and AXPs, while a quark star entering the ``Meissner phase'' is compatible with the observational facts. We compare our model to observations and highlight our predictions.

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arXiv2005

Controlling the collimation and rotation of hydromagnetic disk winds

(Abriged) We present a comprehensive set of axisymmetric, time-dependent simulations of jets from Keplerian disks whose mass loading as a function of disk radius is systematically changed. For a reasonable model for the density structure and injection speed of the underlying accretion disk, mass loading is determined by the radial structure of the disk's magnetic field structure. We vary this structure by using four different magnetic field configurations, ranging from the "potential" configuration (Ouyed&Pudritz 1997), to the increasingly more steeply falling Blandford&Payne (1982) and Pelletier&Pudritz (1992) models, and ending with a quite steeply raked configuration that bears similarities to the Shu X-wind model. We find that the radial distribution of the mass load has a profound effect on both the rotational profile of the underlying jet as well as the degree of collimation of its outflow velocity and magnetic field lines. We show analytically, and confirm by our simulations, that the collimation of a jet depends on its radial current distribution, which in turn is prescribed by the mass load. Models with steeply descending mass loads have strong toroidal fields, and these collimate to cylinders (this includes the Ouyed-Pudritz and Blandford-Payne outflows). On the other hand, the more gradually descending mass load profiles (the PP92 and monopolar distributions) have weaker toroidal fields, and these result in wide-angle outflows with parabolic collimation. We also present detailed structural information about jets such as their radial profiles of jet density, toroidal magnetic field, and poloidal jet speed, as well as an analysis of the bulk energetics of our different simulations.

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arXiv2005

GRMHD Simulations of Disk/Jet Systems: Application to the Inner Engines of Collapsars

We have carried out 2D and 3D general relativistic magnetohydrodynamic simulations of jets launched self-consistently from accretion disks orbiting Kerr black holes and applied the results to the inner engine of the collapsar model of gamma-ray bursts. The accretion flow launches energetic jets in the axial funnel region of the disk/jet system, as well as a substantial coronal wind. The jets feature knot-like structures of extremely hot, ultra-relativistic gas; the gas in these knots begins at moderate velocities near the inner engine, and is accelerated to ultra-relativistic velocities (Lorentz factors of 50, and higher) by the Lorentz force in the axial funnel. The increase in jet velocity takes place in an acceleration zone extending to at least a few hundred gravitational radii from the inner engine. The overall energetics of the jets are strongly spin-dependent, with high-spin black holes producing the highest energy and mass fluxes. In addition, with high-spin black holes, the ultra-relativistic outflow is cylindrically collimated within a few hundred gravitational radii of the black hole, whereas in the zero- spin case the jet retains a constant opening angle of approximately 16 degrees. The simulations also show that the coronal wind, though considerably slower and colder than the jets, also carries a significant amount of mass and energy. When simulation data is scaled to the physical dimensions of a collapsar the jets operate for a period ranging from 0.1 to 1.4 seconds, until the accretion disk is depleted, delivering $10^{48}$ to $10^{49}$ erg.

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arXiv2004

Neutrino Emission and Mass Ejection in Quark Novae

We explore the role of neutrinos in a Quark Nova explosion. We study production of neutrinos during this event, their propagation and their interactions with the surrounding quark matter and neutron-rich envelope. We address relevant physical issues such as the timescale for the initial core collapse, the total energy emitted in neutrinos and their effect on the low density matter surrounding the core. We find that it is feasible that the neutrino burst can lead to significant mass ejection of the nuclear envelope.

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arXiv2004

JETGET - An analysis and visualization tool for (magneto-)hydrodynamic jet simulations

In simulations of (magnetized-)fluid dynamics in physics and astrophysics, the visualization techniques are so frequently applied to analyse data that they have become a fundamental part of the research. Data produced is often a multi-dimensional set with several physical quantities, that are usually complex to manage and analyse. JETGET is a visualization and analysis tool we developed for accessing data stored in Hierarchical Data Format (HDF) and ASCII files. Although JETGET has been optimized to handle data output from jet simulations using the Zeus code from NCSA, it is also capable of analysing other data output from simulations using other codes. JETGET can select variables from the data files, render both two- and three-dimensional graphics and analyse and plot important physical quantities. Graphics can be saved in encapsulated Postscript, JPEG, VRML or saved into an MPEG for later visualization and/or presentations. An example of use of JETGET in analysing a 3-dimensional simulation of jets emanating from accretion disks surrounding a protostar is shown. The strength of JETGET in extracting the physics underlying such phenomena is demonstrated as well as its capabilities in visualizing the 3-dimensional features of the simulated magneto-hydrodynamic jets. The JETGET tool is written in Interactive Data Language (IDL) and uses a graphical user interface to manipulate the data. The tool was developed on a LINUX platform and can be run on any platform that supports IDL. JETGET can be downloaded (including more information about its utilities) from http://www.capca.ucalgary.ca/software.

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arXiv2004

From the Skyrme model to hypothetical Skyrmion stars: Astrophysical implications

We discuss the Skyrme model for strong interactions and the concept of a Skyrmion fluid. The corresponding compact objects, namely Skyrmion stars, constructed from the equation of state describing such a fluid are presented and compared to models of neutron stars based on modern equation of states. We suggest plausible Skyrmion star candidates in the 4U 1636-53 and 4U 1820-30 low mass X-ray binary systems where the suggested masses of the accreting compact companion (\sim 2.0M_{\odot}) remain a challenge for neutron star models.

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arXiv2003

Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts

We present a detailed numerical study of the dynamics and evolution of ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under extreme magnetization conditions. We find that Lorentz factors of up to 3000 are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows for a wide variation in the flow Lorentz factor. Pending contamination induced by mass-entrainment, the linear Michel scaling links modulations in the ultrarelativistic wind to variations in mass accretion in the disk for a given magnetization. The jet is asymptotically dominated by the toroidal magnetic field allowing for efficient collimation. We discuss our solutions (jets) in the context of Gamma ray bursts and describe the relevant features such as the high variability in the Lorentz factor and how high collimation angles (~ 0-5 degrees), or cylindrical jets, can be achieved. We isolate a jet instability mechanism we refer to as the "bottle-neck" instability which essentially relies on a high magnetization and a recollimation of the magnetic flux surfaces. The instability occurs at large radii where any dissipation of the magnetic energy into radiation would in principle result in an optically thin emission.

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arXiv2001

On Jupiter's inertial mode oscillations

Properties of inertial modes of Jupiter are investigated for an n=1 polytropic description of the planet interior. We use the anelastic approximation to overcome the usual handicap of a severe spherical harmonics truncation. A powerful iterative solver then allows us to compute the frequencies of the most promising low-order modes using as many spherical harmonics as necessary. The induced O(1%) errors of our model are now within observational limits. A plausible seismological model of Jupiter might thus be at hand provided the use of a more realistic description of the planet interior.

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