arXiv subjects

Rachid Ouyed

Publications and source records attributed to Rachid Ouyed.

At least 73 records · Page 4Linked to original sources

arXiv2008

Dynamical and thermal evolution of the quark-nova ejecta

We explore the dynamical and thermal evolution of the ejected neutron star crust in a Quark-Nova explosion. Typical explosion energies and ejected crust masses result in relativistic ejection with Lorentz factors of a few to a few hundred. The ejecta undergoes a rapid cooling and stretching resulting in break up into many small pieces (clumps) when the ejecta is only ~ 100 km from the explosion site. The number and size of the clumps depends on whether the breakup occurs in the liquid or solid phase. For these two cases, the clump number is ~ 10^3 (liquid phase) or ~ 10^7 (solid phase) and, at break up, are spherical (size ~ 10^4 cm; liquid phase) or needle shaped (~ 10^4x10^2 cm; solid phase).

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arXiv2008

The role of jets in the formation of planets, stars, and galaxies

Astrophysical jets are associated with the formation of young stars of all masses, stellar and massive black holes, and perhaps even with the formation of massive planets. Their role in the formation of planets, stars, and galaxies is increasingly appreciated and probably reflects a deep connection between the accretion flows - by which stars and black holes may be formed - and the efficiency by which magnetic torques can remove angular momentum from such flows. We compare the properties and physics of jets in both non-relativistic and relativistic systems and trace, by means of theoretical argument and numerical simulations, the physical connections between these different phenomena. We discuss the properties of jets from young stars and black holes, give some basic theoretical results that underpin the origin of jets in these systems, and then show results of recent simulations on jet production in collapsing star-forming cores as well as from jets around rotating Kerr black holes.

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arXiv2008

3-d resistive MHD simulations of magnetic reconnection and the tearing mode instability in current sheets

Magnetic reconnection plays a critical role in many astrophysical processes where high energy emission is observed, e.g. particle acceleration, relativistic accretion powered outflows, pulsar winds and probably in dissipation of Poynting flux in GRBs. The magnetic field acts as a reservoir of energy and can dissipate its energy to thermal and kinetic energy via the tearing mode instability. We have performed 3d nonlinear MHD simulations of the tearing mode instability in a current sheet. Results from a temporal stability analysis in both the linear regime and weakly nonlinear (Rutherford) regime are compared to the numerical simulations. We observe magnetic island formation, island merging and oscillation once the instability has saturated. The growth in the linear regime is exponential in agreement with linear theory. In the second, Rutherford regime the island width grows linearly with time. We find that thermal energy produced in the current sheet strongly dominates the kinetic energy. Finally preliminary analysis indicates a P(k) 4.8 power law for the power spectral density which suggests that the tearing mode vortices play a role in setting up an energy cascade.

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arXiv2008

Supernova SN2006gy as a first ever Quark Nova?

The most luminous Supernova SN2006gy (more than a 100 times brighter than a typical supernova) has been a challenge to explain by standard models. For example, pair instability supernovae which are luminous enough seem to have too slow a rise, and core collapse supernovae do not seem to be luminous enough. We present an alternative scenario involving the quark-nova phenomenon (an explosive transition of the newly born neutron star to a quark star) in which a second explosion (delayed) occurs inside the ejecta of a normal supernova. The reheated supernova ejecta can radiate at higher levels for longer periods of time primarily due to reduced adiabatic expansion losses, unlike the standard supernova case. We find an encouraging match between the resulting lightcurve and that observed in the case of SN2006gy suggesting that we might have at hand the first ever signature of a quark-nova. Successful application of our model to SN2005gj and SN2005ap is also presented.

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arXiv2008

Quark-nova remnants III:Application to RRATs

This is the third paper of a series of papers where we explore the evolution of iron-rich ejecta from quark-novae. In the first paper, we explored the case where a quark-nova ejecta forms a degenerate shell, supported by the star's magnetic field, with applications to SGRs. In the second paper we considered the case where the ejecta would have sufficient angular momentum to form a degenerate Keplerian torus and applied such a system to two AXPs, namely 1E2259 + 586 and 4U0142 + 615. Here we explore the late evolution of the degenerate torus and find that it can remain unchanged for $\sim 10^{6}$ years before it becomes non-degenerate. This transition from a degenerate torus (accretion dominated) to a non-degenerate disk (no accretion), occurs about $10^{6}$ years following the quark-nova, and exhibits features that are reminiscent of observed properties of RRATs. Using this model, we can account for the duration of both the radio bursts and the quiet phase, as well as the observed radio flux from RRATs. The unique on and off activity of the radio pulsar PSR B1931 + 24 is similar to that of "old RRATs" in our model. For old RRATs, in our model, the spin-down rate during the radio loud phase is about a factor 1.6 larger than when it is quiet, remarkably similar to what has been measured for PSR B1931 + 24. We discuss a connection between XDINs and RRATs and argue that some XDINs may be ``dead RRATs'' that have already consumed their non-degenerate disk.

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arXiv2008

High-density Skyrmion matter and Neutron Stars

We examine neutron star properties based on a model of dense matter composed of B=1 skyrmions immersed in a mesonic mean field background. The model realizes spontaneous chiral symmetry breaking non-linearly and incorporates scale-breaking of QCD through a dilaton VEV that also affects the mean fields. Quartic self-interactions among the vector mesons are introduced on grounds of naturalness in the corresponding effective field theory. Within a plausible range of the quartic couplings, the model generates neutron star masses and radii that are consistent with a preponderance of observational constraints, including recent ones that point to the existence of relatively massive neutron stars with mass M 1.7 Msun and radius R (12-14) km. If the existence of neutron stars with such dimensions is confirmed, matter at supra-nuclear density is stiffer than extrapolations of most microscopic models suggest.

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arXiv2008

Chromo-thermal oscillations and collapse of strange stars to black holes : Astrophysical Implications

The effects of temperature on strange stars are studied and it is found that the maximum mass of the star decreases with the increase of temperature since at high temperatures the equations of state become softer. Moreover, if the temperature of a strange star increases, keeping its baryon number fixed, its gravitational mass increases and radius decreases. This leads to a limiting temperature where it turns into a black hole. These features are due to a combined effect of the change of gluon mass and the quark distribution with temperature. We report a new kind of radial oscillations of strange stars driven by what we call chromo-thermal instability. We also discuss the relevance of our findings in the astrophysics of core collapse supernovae and gamma ray bursts.

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arXiv2007

A three stage model for the inner engine of GRBs: Prompt emission and early afterglow

We describe a model within the ``Quark-nova'' scenario to interpret the recent observations of early X-ray afterglows of long Gamma-Ray Bursts (GRB) with the Swift satellite. This is a three-stage model within the context of a core-collapse supernova. STAGE 1 is an accreting (proto-) neutron star leading to a possible delay between the core collapse and the GRB. STAGE 2 is accretion onto a quark-star, launching an ultrarelativistic jet generating the prompt GRB. This jet also creates the afterglow as the jet interacts with the surrounding medium creating an external shock. Slower shells ejected from the quark star (during accretion), can re-energize the external shock leading to a flatter segment in the X-ray afterglow. STAGE 3, which occurs only if the quark-star collapses to form a black-hole, consists of an accreting black-hole. The jet launched in this accretion process interacts with the preceding quark star jet, and could generate the flaring activity frequently seen in early X-ray afterglows. Alternatively, a STAGE 2b can occur in our model if the quark star does not collapse to a black hole. The quark star in this case can then spin down due to magnetic braking, and the spin down energy may lead to flattening in the X-ray afterglow as well. This model seems to account for both the energies and the timescales of GRBs, in addition to the newly discovered early X-ray afterglow features.

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arXiv2007

Birth rates of SGRs and AXPs: delayed amplification of magnetic field

We present new determination of the birth rate of AXPs and SGRS and their associated SNRs. We find a high birth rate of 1/(500 yr) for AXPs/SGRs and 1/(1700 yr) for associated SNRs. These high rates suggest that all massive stars (greater than ~ 25 M_sun) give rise to remnants with magnetar-like fields. Recent observations indicate that fossil fields cannot explain such high fields in the progenitor stars. Dynamo mechanisms during the birth of the neutron stars require spin rates much faster than either observations or theory indicate. Here, we propose the neutron stars form with normal (~ 10^{12} G) magnetic fields, which are then amplified to 10^{14}-10^{15} G after a delay of a few hundred years. The amplification is speculated to be a consequence of color ferromagnetism and occurs after the neutron star core reaches quark-deconfinement density. This delayed amplification alleviates many difficulties in interpreting simultaneously the high birth rate and high magnetic fields of AXPs/SGRs and their link to massive stars.

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arXiv2007

Gamma Ray Burst engine activity within the quark nova scenario: Prompt emission, X-ray Plateau, and sharp drop-off

We present a three-stage model for a long GRB inner engine to explain the prompt gamma ray emission, and interpret recent Swift satellite observations of early X-ray afterglow plateaus followed by a sharp drop off or a shallow power law decay. The three stages involves a neutron star phase, a quark star (QS) and a black hole phase as described in Staff et al. (2007). We find that the QS stage allows for more energy to be extracted from neutron star to QS conversion as well as from ensuing accretion onto the QS. The QS accretion phase naturally extends the engine activity and can account for both the prompt emission and irregular early X-ray afterglow activity. Following the accretion phase, the QS can spin-down by emission of a baryon-free outflow. The magnetar-like magnetic field strengths resulting from the NS to QS transition provide enough spin-down energy, for the correct amount of time, to account for the plateau in the X-ray afterglow. In our model, a sharp drop-off following the plateau occurs when the QS collapses to a BH during the spin-down, thus shutting-off the secondary outflow. We applied our model to GRB 070110 and GRB 060607A and found that we can consistently account for the energetics and duration during the prompt and plateau phases.

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arXiv2007

Fall-back crust around a quark-nova compact remnant II: The degenerate torus case with applications to AXPs

In the first paper of this series we explored the case where a quark-nova ejecta forms a degenerate shell, supported by the star's magnetic field. Herein, we consider the case where the ejecta has sufficient angular momentum to form a Keplerian torus, and we show that the density and temperature of the torus are such that it will remain degenerate throughout it's lifetime. We go on to discuss the evolution of such a torus and apply it to two AXPs, namely 1E2259$+$586 and 4U0142$+$615. As it turns out, using our model we can account for many of the observations of these objects including the quiescent phase luminosity, and blackbody temperatures during quiescence and bursting phases. Furthermore, for 1E2259$+$586 our model explains the steep and slow decay components seen in the burst lightcurve, as well as the rotation period glitches and enhanced spin-down rate. We also estimate the mass of the degenerate torus to be of the order of $10^{-6}M_{\odot}$, and speculate that the observed optical/infrared emission from 4U0142$+$615 might be a signature of the thin degenerate torus we describe here.

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arXiv2007

Fall-back crust around a quark-nova compact remnant I: The degenerate shell case with applications to SGRs, AXPs and XDINs

We explore the formation and evolution of debris ejected around quark stars in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters (SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the fall-back (crust) material. Our model can account for many of the observed features of SGRs and AXPs such as: (i) the two types of bursts (giant and regular); (ii) the spin-up and spin-down episodes during and following the bursts with associated persistant increases in $\dot{P}$; (iii) the energetics of the boxing day burst, SGR1806$+$20; (iv) the presence of an Iron line as observed in SGR1900$+$14; (v) the correlation between the far-Infrared and the X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard X-ray component observed in SGRs during the giant bursts, and (vii) the discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also find a natural evolutionary relationship between SGRs and AXPs in our model which predicts that only the youngest SGRs/AXPs are most likely to exhibit strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are also accounted for in our model such as, (i) the two-component blackbody spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical emission.

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arXiv2007

Inductive acceleration of UHECRs in sheared relativistic jets

Relativistic outflows carrying large scale magnetic fields have large inductive potential and may accelerate protons to ultra high energies. We discuss a novel scheme of Ultra-High Energy Cosmic Ray (UHECR) acceleration due to drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (with jet luminosity $\geq 10^{46}$ erg s$^{-1}$). A positively charged particle carried by such a plasma is in an unstable equilibrium if ${\bf B} \cdot \nabla \times {\bf v}< 0$, so that kinetic drift along the velocity shear would lead to fast, regular energy gain. The highest rigidity particles are accelerated most efficiently implying the dominance of light nuclei for energies above the ankle in our model: from a mixed population of pre-accelerated particle the drift mechanism picks up and boosts protons preferably.

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arXiv2007

SGRs and AXPs proposed as ancestors of the Magnificent seven

The recently suggested correlation between the surface temperature and the magnetic field in isolated neutron stars does not seem to work well for SGRs, AXPs and X-ray dim isolated neutron stars (XDINs; specifically the Magnificent Seven or M7). Instead by appealing to a Color-Flavor Locked Quark Star (CFLQS) we find a more natural explanation. In this picture, the heating is provided by magnetic flux expulsion from a crust-less superconducting quark star. Combined with our previous studies concerning the possibility of SGRs, AXPs, and XDINs as CFLQSs, this provides another piece of evidence that these objects are all related. Specifically, we propose that XDINs are the descendants of SGRs and AXPs.

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arXiv2007

Nucleosynthesis in neutron-rich ejecta from quark-novae

We explore heavy-element nucleosynthesis by rapid neutron capture (r-process) in the decompressing ejecta from the surface of a neutron star. The decompression is triggered by a violent phase transition to strange quark matter (quark-nova scenario). The presence of neutron-rich large Z nuclei (40,95) < (Z,A) < (70,177), the large neutron-to-seed ratio, and the low electron fraction Ye ~ 0.03 in the decompressing ejecta present favorable conditions for the r-process. We perform network calculations that are adapted to the quark-nova conditions, and which mimic usual (n-gamma) equilibrium r-process calculations during the initially cold decompression phase. They match to dynamical r-process calculations at densities below neutron drip (4.10^11 g cm-3). We present results for the final element abundance distribution with and without heating from nuclear reactions, and compare to the solar abundance pattern of r-process elements. We highlight the distinguishing features of quark-novae by contrasting it with conventional nucleosynthetic sites such as type II supernovae and neutron star mergers, especially in the context of heavy-element compositions of extremely metal-deficient stars.

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arXiv2007

Ultra High Energy Cosmic ray and gamma-ray signatures of inductive acceleration in AGN jets

The highest energy cosmic rays could be produced by drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (i.e. FR II radiogalaxies; radio loud quasars and high power BL Lacs). We show that in such scenarios proton synchrotron radiation can give rise to detectable photon fluxes at energies ranging from hundreds of keV to tens of MeV.

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arXiv2007

A three stage model for the inner engine of Gamma Ray Burst: Prompt emission and early afterglow

We propose a new model within the ``Quark-nova'' scenario to interpret the recent observations of early afterglows of long Gamma-Ray Bursts (GRB) with the Swift satellite. This is a three-stage model within the context of a core-collapse supernova. Stage 1 is an accreting (proto-) neutron star leading to a possible delay between the core collapse and the GRB. Stage 2 is an accreting quark-star, generating the prompt GRB. Stage 3, which occurs only if the quark-star collapses to form a black-hole, consists of an accreting black-hole. The jet launched in this accretion process interacts with the ejecta from stage 2, and could generate the flaring activity frequently seen in X-ray afterglows. This model may be able to account for both the energies and the timescales of GRBs, in addition to the newly discovered early X-ray afterglow features.

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arXiv2007

Inductive acceleration of UHECRs in sheared relativistic jets

(Abridged)Relativistic outflows carrying large scale magnetic fields have large inductive potential and may accelerate protons to ultra high energies. We discuss a novel scheme of Ultra-High Energy Cosmic Ray (UHECR) acceleration due to drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (with jet luminosity $\geq 10^{46}$ erg s$^{-1}$). We point out that a positively charged particle carried by such a plasma is in an unstable equilibrium if ${\bf B} \cdot \nabla \times {\bf v}< 0$, so that kinetic drift along the velocity shear would lead to fast, {\it regular} energy gain. We show that if a seed of pre-accelerated particles with energy below the ankle $\leq 10^{18}$ eV is present, these particles can be boosted to energies above $ 10^{19}$ eV. A key feature of the mechanism is that the highest rigidity particles are accelerated most efficiently implying the dominance of light nuclei for energies above the ankle in our model: from a mixed population of pre-accelerated particle the drift mechanism picks up and boosts protons preferably. In addition, after a particle traversed large fraction of the available potential, its Larmor radius becomes of the order of the jet thickness. In this case, {\it the maximum possible acceleration rate of inverse relativistic gyro-frequency is achieved} and a particle finally become unconfined and leave the jet.

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