arXiv subjects

F. Casse

Publications and source records attributed to F. Casse.

17 recordsLinked to original sources

arXiv2025

A simple toy model for the electromagnetic variability of lump-dominated circumbinary disks around binary black holes

The electromagnetic detection of circumbinary disks around pre-merger binary black holes (BBHs) relies on theoretical predictions. These are generally obtained through expensive numerical simulations, but simple or fast toy models are lacking to unleash the potential of these theoretical advances for observational purposes. We aim to present a simple toy model to compute the electromagnetic variability of circumbinary disks around circular-orbit BBHs at relativistic separations, focusing on the impact of disk non-axisymmetries. We assume that the disk is threaded by spiral arms and hosts a hotspot linked to an overdense structure (the {\lq}lump{\rq}) preferably reported in binaries close to equal mass. We build a simple temperature distribution, and estimate its thermal emission, perceived by a distant observer, via a ray-tracing code in a BBH approximate metric. We propose a toy model reproducing the main lightcurve features and show it is consistent with 2D general-hydrodynamical simulations under the assumption of compressional heating and expansional cooling except for purely dynamical effects such as the binary-lump beat. The lightcurve exhibits a main modulation at the lump's period (i.e. a few times the orbital period), due to relativistic Doppler effect, and a shorter one at the orbital-like period, due to spiral arms or the beat. These are more prominent in the optical/UV band for a total binary mass $M\, {=} \, 10^{4-10}\mathrm{M_\odot}$, where the disk energy spectrum peaks. For $M=10^{9}\mathrm{M_\odot}$, a $4\%$-amplitude lump modulation is detectable with the Vera Rubin Observatory after six months of observation, up to $z\, {=}\, 0.5$. We proposed a new, simple toy model that can be used, for instance, to test the compatibility of the periodicity of BBH candidate sources with a circumbinary disk origin.

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arXiv2018

On the Rossby Wave Instability in accretion discs surrounding spinning black holes

We have performed general relativistic hydrodynamics (GRHD) simulations of 2D discs orbiting around spinning black holes and prone to the Rossby Wave Instability (RWI). We show that the RWI can develop at any location in the disc and for any spin parameter. After recovering the overall patterns of the instability in this general relativistic context, we have analysed its development and identified some modifications induced by the combined effects of the relativistic rotational profile of the disc and local time dilatation that affects the propagation of waves in the disc. We have found in particular that the saturation level of the instability increases significantly when RWI occurs in the very close vicinity of fast-rotating black holes where general relativistic effects are strong. Such finding suggests that even more strongly than in the case of Schwarzschild black-hole, it is necessary to complement such GRHD simulations with a full GR ray-tracing processing in order to provide synthetic observations of the disc in the distant observer frame.

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arXiv2016

Impact of Schwarzschild black hole's gravity upon the Rossby wave instability

In an early work the Rossby Wave Instability (RWI) has been proposed to explain variability thought to originate in the close vicinity of black-holes but this was done in the pseudo-Newtonian approach. Here we present the first general relativistic hydrodynamics simulations of this instability not only proving its theorized existence in a full general relativistic (GR) environment but also studying the effect of the strong gravity on the instability. To that end we performed a set of simulations increasingly closer to the black hole with our new GR version of the MPI-AMRVAC code.This allows us to study the minute changes in the behaviour of the instability.We found that the pseudo-Newtonian approach gives adequate results provided that time shifting induced by the black hole gravity is taken into account. Hence, to view the disc as a distant observer would a full GR ray-tracing post treatment of the simulations is a must.

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arXiv2016

A numerical investigation of wind accretion in persistent Supergiant X-ray Binaries I - Structure of the flow at the orbital scale

Classical Supergiant X-ray Binaries host a neutron star orbiting a supergiant OB star and display persistent X-ray luminosities of 10$^{35}$ to 10$^{37}$ erg/s. The stellar wind from the massive companion is believed to be the main source of matter accreted by the compact object. With this first paper, we introduce a ballistic model to characterize the structure of the wind at the orbital scale as it accelerates, from the stellar surface to the vicinity of the accretor. Thanks to the parametrization we retained and the numerical pipeline we designed, we can investigate the supersonic flow and the subsequent observables as a function of a reduced set of characteristic numbers and scales. We show that the shape of the permanent flow is entirely determined by the mass ratio, the filling factor, the Eddington factor and the $\alpha$-force multiplier which drives the stellar wind acceleration. Provided scales such as the orbital period are known, we can trace back the observables to evaluate the mass accretion rates, the accretion mechanism, the shearing of the inflow and the stellar parameters. We confront our model to three persistent Supergiant X-ray Binaries (Vela X-1, IGR J18027-2016, XTE J1855-026) and discuss the likelihood of wind-formed accretion discs around the accretors in each case, further investigated in a following paper.

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arXiv2014

Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY

The black hole at the Galactic centre exhibits regularly flares of radiation, the origin of which is still not understood. In this article, we study the ability of the near-future GRAVITY infrared instrument to constrain the nature of these events. We develop realistic simulations of GRAVITY astrometric data sets for various flare models. We show that the instrument will be able to distinguish an ejected blob from alternative flare models, provided the blob inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and the flare duration is >= 1h30.

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arXiv2013

Non-resonant magnetohydrodynamics streaming instability near magnetized relativistic shocks

We present in this paper both a linear study and numerical relativistic MHD simulations of the non-resonant streaming instability occurring in the precursor of relativistic shocks. In the shock front restframe, we perform a linear analysis of this instability in a likely configuration for ultra-relativistic shock precursors. This considers magneto-acoustic waves having a wave vector perpendicular to the shock front and the large scale magnetic field. Our linear analysis is achieved without any assumption on the shock velocity and is thus valid for all velocity regimes. In order to check our calculation, we also perform relativistic MHD simulations describing the propagation of the aforementioned magneto-acoustic waves through the shock precursor. The numerical calculations confirm our linear analysis, which predicts that the growth rate of the instability is maximal for ultra-relativistic shocks and exhibits a wavenumber dependence $\propto k_x^{1/2}$. Our numerical simulations also depict the saturation regime of the instability where we show that the magnetic amplification is moderate but nevertheless significant ($\delta B/B\leq 10$). This latter fact may explain the presence of strong turbulence in the vicinity of relativistic magnetized shocks. Our numerical approach also introduces a convenient means to handle isothermal (ultra-)relativistic MHD conditions.

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arXiv2012

Formation and long-term evolution of 3D vortices in protoplanetary discs

In the context of planet formation, anticyclonic vortices have recently received lots of attention for the role they can play in planetesimals formation. Radial migration of intermediate size solids toward the central star may prevent their growth to larger solid grains. On the other hand, vortices can trap the dust and accelerate this growth, counteracting fast radial transport. Multiple effects have been shown to affect this scenario, such as vortex migration or decay. The aim of this paper is to study the formation of vortices by the Rossby wave instability and their long term evolution in a full three dimensional protoplanetary disc. We use a robust numerical scheme combined with adaptive mesh refinement in cylindrical coordinates, allowing to affordably compute long term 3D evolutions. We consider a full disc stratified both radially and vertically that is prone to formation of vortices by the Rossby wave instability. We show that the 3D Rossby vortices grow and survive over hundreds of years without migration. The localized overdensity which initiated the instability and vortex formation survives the growth of the Rossby wave instability for very long times. When the vortices are no longer sustained by the Rossby wave instability, their shape changes toward more elliptical vortices. This allows them to survive shear-driven destruction, but they may be prone to elliptical instability and slow decay. When the conditions for growing Rossby wave-related instabilities are maintained in the disc, large-scale vortices can survive over very long timescales and may be able to concentrate solids.

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arXiv2011

Molecule survival in magnetized protostellar disk winds. I. Chemical model and first results

Molecular counterparts to atomic jets have been detected within 1000 AU of young stars. Reproducing them is a challenge for proposed ejection models. We explore whether molecules may survive in an MHD disk wind invoked to reproduce the kinematics and tentative rotation signatures of atomic jets in T Tauri stars. The coupled ionization, chemical and thermal evolution along dusty flow streamlines is computed for a prescribed MHD disk wind solution, using a method developed for magnetized shocks in the interstellar medium. Irradiation by wind-attenuated coronal X-rays and FUV photons from accretion hot spots is included, with self-shielding of H2 and CO. Disk accretion rates of 5e-6, 1e-6 and 1e-7 solar masses per year are considered, representative of low-mass young protostars (Class 0), evolved protostars (Class I) and very active T Tauri stars (Class II). The disk wind has an onion-like thermo-chemical structure, with streamlines launched from larger radii having lower temperature and ionisation, and higher H2 abundance. The coupling between charged and neutral fluids is sufficient to eject molecules from the disk out to 9 AU. The launch radius beyond which most H2 survives moves outward with evolutionary stage. CO survives in the Class 0 but is significantly photodissociated in the Class I/II. Balance between ambipolar heating and molecular cooling establishes an asymptotic temperature 700-3000 K, with cooler jets at earlier protostellar stages. Endothermic formation of H2O is efficient with abundances up to 1e-4, while CH+ and SH+ can exceed 1e-6 in the Class I/II winds. A centrifugal MHD disk wind launched from beyond 0.2-1 AU can produce molecular jets/winds up to speeds 100 km/s in young low-mass stars. The model predicts a high ratio H2/CO and an increase of molecular launch radius, temperature, and flow width as the source evolves, in agreement with current observed trends.

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arXiv2010

Rossby Wave Instability and three-dimensional vortices in accretion disks

Context. The formation of vortices in accretion disks is of high interest in various astrophysical contexts, in particular for planet formation or in the disks of compact objects. But despite numerous attempts it has thus far not been possible to produce strong vortices in fully three-dimensional simulations of disks. Aims. The aim of this paper is to present the first 3D simulation of a strong vortex, established across the vertically stratified structure of a disk by the Rossby Wave Instability. Methods. Using the Versatile Advection Code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone to the Rossby Wave Instability. Results. The simulation confirms the basic expectations obtained from previous 2D analytic and numerical works. The simulation exhibits a strong vortex that grows rapidly and saturates at a finite amplitude. On the other hand the third dimension shows unexpected additional behaviours that could be of strong importance in the astrophysical roles that such vortices can play.

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arXiv2010

Postshock turbulence and diffusive shock acceleration in young supernova remnants

The present article investigates magnetic amplification in the upstream medium of SNR blast wave through both resonant and non-resonant regimes of the streaming instability. It aims at a better understanding of the diffusive shock acceleration (DSA) efficiency considering various relaxation processes of the magnetic fluctuations in the downstream medium. Multi-wavelength radiative signatures coming from the SNR shock wave are used in order to put to the test the different downstream turbulence relaxation models. We confirm the result of Parizot et al (2006) that the maximum CR energies should not go well beyond PeV energies in young SNRs where X-ray filaments are observed. In order to match observational data, we derive an upper limit on the magnetic field amplitude insuring that stochastic particle reacceleration remain inefficient. Considering then, various magnetic relaxation processes, we present two necessary conditions to achieve efficient acceleration and X-ray filaments in SNRs: 1/the turbulence must fulfil the inequality $2-\beta-\delta_{\rm d} \ge 0$ where $\beta$ is the turbulence spectral index while $\delta_d$ is the relaxation length energy power-law index; 2/the typical relaxation length has to be of the order the X-ray rim size. We identify that Alv\'enic/fast magnetosonic mode damping does fulfil all conditions while non-linear Kolmogorov damping does not. Confronting previous relaxation processes to observational data, we deduct that among our SNR sample, the older ones (SN1006 & G347.3-0.5) fail to verify all conditions which means that their X-ray filaments are likely controlled by radiative losses. The younger SNRs, Cas A, Tycho and Kepler, do pass all tests and we infer that the downstream magnetic field amplitude is lying in the range of 200-300 $\mu$ Gauss.

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arXiv2008

Extragalactic jets with helical magnetic fields: relativistic MHD simulations

Extragalactic jets are inferred to harbor dynamically important, organized magnetic fields which presumably aid in the collimation of the relativistic jet flows. We here explore by means of grid-adaptive, high resolution numerical simulations the morphology of AGN jets pervaded by helical field and flow topologies. We concentrate on morphological features of the bow shock and the jet beam behind the Mach disk, for various jet Lorentz factors and magnetic field helicities. We investigate the influence of helical magnetic fields on jet beam propagation in overdense external medium. We use the AMRVAC code, employing a novel hybrid block-based AMR strategy, to compute ideal plasma dynamics in special relativity. The helicity of the beam magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. In comparison with equivalent low-relativistic jets which get surrounded by cocoons with vortical backflows filled by mainly toroidal field, the high speed jets demonstrate only localized, strong toroidal field zones within the backflow vortical structures. We find evidence for a more poloidal, straight field layer, compressed between jet beam and backflows. This layer decreases the destabilizing influence of the backflow on the jet beam. In all cases, the jet beam contains rich cross-shock patterns, across which part of the kinetic energy gets transferred. For the high speed reference jet considered here, significant jet deceleration only occurs beyond distances exceeding ${\cal O}(100 R_j)$, as the axial flow can reaccelerate downstream to the internal cross-shocks. This reacceleration is magnetically aided, due to field compression across the internal shocks which pinch the flow.

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arXiv2007

AMRVAC and Relativistic Hydrodynamic simulations for GRB afterglow phases

We apply a novel adaptive mesh refinement code, AMRVAC, to numerically investigate the various evolutionary phases in the interaction of a relativistic shell with its surrounding cold Interstellar Medium (ISM). We do this for both 1D isotropic as well as full 2D jetlike fireball models. This is relevant for Gamma Ray Bursts, and we demonstrate that, thanks to the AMR strategy, we resolve the internal structure of the shocked shell-ISM matter, which will leave its imprint on the GRB afterglow. We determine the deceleration from an initial Lorentz factor $γ=100$ up to the almost Newtonian $γ\sim{\cal O}(2)$ phase of the flow. We present axisymmetric 2D shell evolutions, with the 2D extent characterized by their initial opening angle. In such jetlike GRB models, we discuss the differences with the 1D isotropic GRB equivalents. These are mainly due to thermally induced sideways expansions of both the shocked shell and shocked ISM regions. We found that the propagating 2D ultrarelativistic shell does not accrete all the surrounding medium located within its initial opening angle. Part of this ISM matter gets pushed away laterally and forms a wide bow-shock configuration with swirling flow patterns trailing the thin shell. The resulting shell deceleration is quite different from that found in isotropic GRB models. As long as the lateral shell expansion is merely due to ballistic spreading of the shell, isotropic and 2D models agree perfectly. As thermally induced expansions eventually lead to significantly higher lateral speeds, the 2D shell interacts with comparably more ISM matter and decelerates earlier than its isotropic counterpart.

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arXiv2006

Two-component magnetohydrodynamical outflows around young stellar objects Interplay between stellar magnetospheric winds and disc-driven jets

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force.}{The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.

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arXiv2006

The ECLAIRs micro-satellite mission for gamma-ray burst multi-wavelength observations

Gamma-ray bursts (GRB), at least those with a duration longer than a few seconds are the most energetic events in the Universe and occur at cosmological distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide multi-wavelength observations of GRB, to study their astrophysics and to use them as cosmological probes. Furthermore in 2009 ECLAIRs is expected to be the only space borne instrument capable of providing a GRB trigger in near real-time with sufficient localization accuracy for GRB follow-up observations with the powerful ground based spectroscopic telescopes available by then. A "Phase A study" of the ECLAIRs project has recently been launched by the French Space Agency CNES, aiming at a detailed mission design and selection for flight in 2006. The ECLAIRs mission is based on a CNES micro-satellite of the "Myriade" family and dedicated ground-based optical telescopes. The satellite payload combines a 2 sr field-of-view coded aperture mask gamma-camera using 6400 CdTe pixels for GRB detection and localization with 10 arcmin precision in the 4 to 50 keV energy band, together with a soft X-ray camera for onboard position refinement to 1 arcmin. The ground-based optical robotic telescopes will detect the GRB prompt/early afterglow emission and localize the event to arcsec accuracy, for spectroscopic follow-up observations.

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arXiv2004

Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations

Motivated by the first detections of rotation signatures in the DG Tau jet (Bacciotti et al. 2002), we examine possible biases affecting the relation between detected rotation signatures and true azimuthal velocity for self-similar MHD disc winds, taking into account projection, convolution as well as excitation gradients effects. We find that computed velocity shifts are systematically smaller than the true underlying rotation curve. When outer slower streamlines dominate the emission, we predict observed shifts increasing with transverse distance to the jet axis, opposite to the true rotation profile. Determination of the full transverse rotation profile thus requires high angular resolution observations (< 5 AU) on an object with dominant inner faster streamlines. Comparison of our predictions with HST/STIS observations of DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda = 13 and an outer radius of the disc ~ 3 AU are able to reproduce detected velocity shifts, while cold disc wind models (lambda > 50) are ruled out for the medium-velocity component in the DG Tau jet.

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arXiv2003

Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory

Multidimensional magneto-hydrodynamical (MHD) simulations coupled with stochastic differential equations (SDEs) adapted to test particle acceleration and transport in complex astrophysical flows are presented. The numerical scheme allows the investigation of shock acceleration, adiabatic and radiative losses as well as diffusive spatial transport in various diffusion regimes. The applicability of SDEs to astrophysics is first discussed in regards to the different regimes and the MHD code spatial resolution. The procedure is then applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets. The ability of SDE to reproduce analytical solutions of the diffusion-convection equation for electrons is tested through the incorporation of an increasing number of effects: shock acceleration, spatially dependent diffusion coefficients and synchrotron losses. The SDEs prove to be efficient in various shock configuration occurring in the inner jet during the development of the Kelvin-Helmholtz instability. The particle acceleration in snapshots of strong single and multiple shock acceleration including realistic spatial transport is treated. In chaotic magnetic diffusion regime, turbulence levels $η_T=<δB^2>/(B^2+<δB^2>)$ around $0.2-0.3$ are found to be the most efficient to enable particles to reach the highest energies. The spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows), does not exhibit a power-law shape due to transverse momentum dependent escapes. Out of this range, the confinement is not so efficient and the spectrum cut-off above few hundreds of GeV, questioning the Chandra observations of X-ray knots as being synchrotron radiation. The extension to full time dependent simulations to X-ray extragalactic jets is discussed.

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arXiv2002

Waves and Instabilities in Accretion Disks: MHD Spectroscopic Analysis

A complete analytical and numerical treatment of all magnetohydrodynamic waves and instabilities for radially stratified, magnetized accretion disks is presented. The instabilities are a possible source of anomalous transport. While recovering results on known hydrodynamicand both weak- and strong-field magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a realistic accretion disk model demonstrates a much richer variety of instabilities accessible to the plasma than previously realized. We show that both weakly and strongly magnetized accretion disks are prone to strong non-axisymmetric instabilities.The ability to characterize all waves arising in accretion disks holds great promise for magnetohydrodynamic spectroscopic analysis.

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