arXiv subjects

Fabien Casse

Publications and source records attributed to Fabien Casse.

At least 19 recordsLinked to original sources

arXiv2026

Looking for observational signatures of early binary black hole systems

Context. A lot of recent studies have focused on the observables associated with near merger binary black-holes (BBHs) embedded in a circumbinary disk (CBD) but we still we lack knowledge of observables of BBHs in their early stage. In that stage the separation between the two black holes is so large that both black holes could potentially retain their individual accretion disk existing before the creation of the BBH. For such early BBH systems, it is interesting to look for observables originating in those individual disks whose structure is likely to differ from mini-disks often observed in simulations of later stages of BBHs. Aims. In a companion paper we presented a set of hydrodynamical simulations of an individual disk surrounding a primary black hole while being impacted by the presence of a secondary black-hole in an early BBH system, leading to the creation of three well-known characteristic features in the disk's structure. Here we explore the imprints of these three features on the observables associated with the thermal emission of the pre-existing black hole disk. The aim is two-fold, first to see which observables are best suited for detecting those early systems and, secondly, what could be extrapolated about these systems from observations. Methods. We used general relativistic ray-tracing in order to produce synthetic observations of the thermal emission emitted by early BBHs with different mass ratio and separations in order to search for distinctive observational features of early systems. Results. We found that in the case of early BBH with pre-existing disk(s) a necessary, although not unique, observational feature is the truncation of their disk(s). Conclusions. Such observable could be used for automated search of potential BBHs and discriminate some existing candidates.

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arXiv2025

Electromagnetic variability from circumbinary discs around binary black holes during their post-decoupling epoch

We present general-relativistic hydrodynamical simulations of inviscid circumbinary discs (CBDs) around near equal-mass binary black holes (BBH) in the binary-disc post-decoupling epoch. We use an approximate BBH spacetime with a post-Newtonian inspiral motion trajectory from ${\sim}80 (M/10^7 \mathrm{M_\odot}) \, \mbox{days}$ (separation of ${\sim}\,30$ gravitational radii) to ${\sim}100 (M/10^7 \mathrm{M_\odot}) \, \mbox{minutes}$ before merger. Initial data for the inspiral runs are produced from circular-orbits runs covering the formation timescale of the overdense {\lq}lump{\rq}, orbiting the CBD inner edge. The CBD non-axisymmetries (spiral waves and lump) lead to non-negligible angular momentum transport with effective viscosity ${\alpha_\mathrm{eff} \, {\sim} \, 10^{{-3}}{- 2\times 10}^{-2}}$. We post-process these simulations with a general-relativistic ray-tracing code to obtain synthetic observations in thermal emission. We find the lump and its associated electromagnetic (EM) modulation, already reported in the pre-decoupling epoch, to survive post-decoupling up until the end of the simulation. For LISA sources, our findings point to an active EM signature in UV during optimal gravitational wave source localization. For PTA sources and current BBH candidates detected through their optical periodicity: the lump{ in a low-viscosity CBD is a possible, though not unique, origin} for the observed periodicity.

astro-ph.HE↗

arXiv2025

Sculpting the outer edge of accretion disks in pre-circumbinary binary black hole systems

Binary black hole systems (BBHs) have become a vivid reality in astrophysics as stellar-mass black hole mergers are now detected through their related gravitational wave emission during the merger stage. If many studies were recently dedicated to the last stages of BBH where black holes are surrounded by a circumbinary disk (CBD), the structure of these systems prior to the formation of the CBD remains mostly unexplored. The aim of the present article is to investigate the potential modifications induced by the presence of a secondary black hole onto the structure of the accretion disk surrounding the primary black hole. We performed 2D classical hydrodynamical simulations of an accretion disk surrounding the primary black hole while taking into account all gravitational effects induced by both the primary black hole and the secondary black hole orbiting on circular orbits around the center of mass of the system.We report three main effects of the presence of a secondary black hole orbiting a circular orbit beyond the outer edge of the accretion disk: 1/ the outer radius of the accretion disk is significantly reduced and its ratio to the black hole separation is directly linked to only the mass ratio of the black holes; 2/ two spiral arms are visible in the gas density structure of the disk and 3/ the outer edge of the accretion disk exhibits an elliptical shape that mainly depends on the mass ratio of the black holes. Our results show that an accretion disk orbiting a primary black hole in a pre-CBD BBH exhibits specific features induced by the gravitational force generated by the presence of a secondary black hole beyond its outer edge. Such features, directly linked to the binary separation and mass ratio, has therefore the potential to help in the search and identification of BBH in the pre-CBD stage.

astro-ph.HE↗

arXiv2025

Can we disentangle between the emission of an accretion disc around a single black hole and a circumbinary disc ?

The detection of gravitational waves from binary black holes (BBHs) started the hunt for their pre-merger electromagnetic emission. In that respect, numerical simulations have been looking for the "smoking gun" signal that could help identify pre-merger systems. Here we study if any of the expected features of circumbinary discs, such as the periodic modulation from the orbiting "lump", could be used to identify pre-merger BBHs or if they could be easily confused with other systems. Indeed, while the timing feature associated with the "lump" seems to be present for a large part of the parameter space defined by the binary separation and mass ratio in circular binaries, it was recently proposed to form thanks to an instability occurring naturally at the edge of accretion discs around single black holes (SBH). In order to check if features of a circumbinary disc could be reproduced by a SBH system, we search for at least one SBH fit able to replicate the given synthetic observations of a circumbinary disc. We found that many of the features from a circumbinary disc can be reproduced by a SBH system with different masses, distances or inner disc positions. Interestingly, while we can always find a SBH model providing a good enough fit to the data, the presence of two variabilities, associated with the lump and the binary, or binary-lump beat, period, is a necessary condition for a wide range of BBH system parameters and should be used as a test to disqualify some BBH candidates.

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arXiv2024

Diffusive shock acceleration in relativistic, oblique shocks

Cosmic rays are charged particles that are accelerated to relativistic speeds by astrophysical shocks. Numerical models have been successful in confirming the acceleration process for (quasi-)parallel shocks, which have the magnetic field aligned with the direction of the shock motion. However, the process is less clear when it comes to (quasi-)perpendicular shocks, where the field makes a large angle with the shock-normal. For such shocks, the angle between the magnetic field and flow ensures that only highly energetic particles can travel upstream at all, reducing the upstream current. This process is further inhibited for relativistic shocks, since the shock can become superluminal when the required particle velocity exceeds the speed of light, effectively inhibiting any upstream particle flow. In order to determine whether such shocks can accelerate particles, we use the particle-in-cell (PIC) method to determine what fraction of particles gets reflected initially at the shock. We then use this as input for a new simulation that combines the PIC method with grid-based magnetohydrodynamics to follow the acceleration (if any) of the particles over a larger time-period in a two-dimensional grid. We find that quasi-perpendicular, relativistic shocks are capable of accelerating particles through the DSA process, provided that the shock has a sufficiently high Alfvenic Mach number.

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arXiv2023

Using PIC and PIC-MHD to investigate cosmic ray acceleration in mildly relativistic shocks

Astrophysical shocks create cosmic rays by accelerating charged particles to relativistic speeds. However, the relative contribution of various types of shocks to the cosmic ray spectrum is still the subject of ongoing debate. Numerical studies have shown that in the non-relativistic regime, oblique shocks are capable of accelerating cosmic rays, depending on the Alfvénic Mach number of the shock. We now seek to extend this study into the mildly relativistic regime. In this case, dependence of the ion reflection rate on the shock obliquity is different compared to the nonrelativistic regime. Faster relativistic shocks are perpendicular for the majority of shock obliquity angles therefore their ability to initialize efficient DSA is limited. We define the ion injection rate using fully kinetic PIC simulation where we follow the formation of the shock and determine the fraction of ions that gets involved into formation of the shock precursor in the mildly relativistic regime covering a Lorentz factor range from 1 to 3. Then, with this result, we use a combined PIC-MHD method to model the large-scale evolution of the shock with the ion injection recipe dependent on the local shock obliquity. This methodology accounts for the influence of the self-generated or pre-existing upstream turbulence on the shock obliquity which allows study substantially larger and longer simulations compared to classical hybrid techniques.

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arXiv2023

Gravitational waves or X-ray counterpart? No need to choose

Binary black holes emit gravitational waves as they inspiral towards coalescence. Searches for electromagnetic counterparts to these gravitational waves rely on looking for common sources producing both signals. In this paper, we take a different approach: we investigate the impact of radiation zone effects, including retardation effects and gravitational wave propagation onto the circumbinary disk around stellar-mass, spinning black holes, using general relativistic hydrodynamical simulations. Then we used a general relativistic ray-tracing code to extract its X-ray spectrum and lightcurve. This allowed us to show that radiation zone effects leave an imprint onto the disk, leading to quasi-periodic patterns in the X-ray lightcurve. The amplitude of the modulation is weak (<1%) but increases with time and is strongly dependent on the inclination angle.

astro-ph.HE↗

arXiv2023

On the origin of the lump in circumbinary discs

Accreting binary black holes (BBHs) are multi-messenger sources, emitting copious electromagnetic (EM) and gravitational waves. One of their most promising EM signatures is the lightcurve modulation caused by a strong, unique and extended azimuthal overdensity structure orbiting at the inner edge of the circumbinary disc (CBD), dubbed "lump". In this paper, we investigate the origin of this structure using 2D general-relativistic (GR) hydrodynamical simulations of a CBD in an approximate BBH spacetime. First, we use the symmetric mass-ratio case to study the transition from the natural m = 2 mode to m = 1. The asymmetry with respect to m = 2 grows exponentially, pointing to an instability origin. We indeed find that the CBD edge is prone to a (magneto-)hydrodynamical instability owing to the disc edge density sharpness: the Rossby Wave Instability (RWI). The RWI criterion is naturally fullfilled at the CBD edge and we report the presence of vortices, which are typical structures of the RWI. The RWI is also at work in the asymmetric mass-ratio cases (from 0.1 to 0.5). However, the CBD edge sharpness decreases with a decreasing mass ratio, and so the lump. By proposing a scenario for this lump formation, our work further supports its existence in astrophysical CBDs and potential source for an EM signature of BBHs. Finally, because the RWI is not caused by GR effects, it is also a robust candidate for the lump origin in CBDs around non-compact objects, e.g. binary protostars.

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arXiv2022

Impact of a binary black hole on its outer circumbinary disc

Accreting supermassive binary black holes (SMBBHs) are potential targets for multi-messenger astronomy as they emit gravitational waves (GW) while their environment emits electromagnetic (EM) waves. In order to get the most out of a joint GW-EM detection we first need to obtain theoretically-predicted EM signals unambiguously linked to BBHs. In that respect, this is the first of a series of papers dedicated to accreting pre-merger BBHs and their associated EM observables. Here, we extend our Numerical Observatory of Violent Accreting systems, e-NOVAs, to any spacetime. Unlike previous studies, almost exclusively focused on the inner regions, we investigated the impact of the BBH on its outer circumbinary disc, located in the radiation (or wave) zone, after implementing an approximate analytical spacetime of spinning, inspiralling BBHs in e-NOVAs. We follow the formation of a weak spiral structure in disc density arising from the retardation effects in the radiation zone metric. Simulation data are then post-processed with a general-relativistic ray-tracing code incorporating the same BBH spacetime, assuming SMBBH sources. The density spiral creates a small (<1%) but unambiguous modulation of the lightcurve at the semi-orbital period. This signal, although weak, is fundamentally different from that of an axisymmetric disc around a single BH providing a lower limit on the impact of a BBH on its outer disc. This potential difference being found, we study how binary parameters impact this modulation in order to find the optimal case which is a high source inclination of any binary mass ratio (from 0.1 to 1).

astro-ph.HE↗

arXiv2020

Particle acceleration in relativistic turbulence: A theoretical appraisal

We discuss the physics of stochastic particle acceleration in relativistic magnetohydrodynamic (MHD) turbulence, combining numerical simulations of test-particle acceleration in synthetic wave turbulence spectra with detailed analytical estimates. In particular, we study particle acceleration in wavelike isotropic fast mode turbulence, in Alfvén and slow Goldreich-Sridhar type wave turbulence (properly accounting for anisotropy effects), including resonance broadening due to wave decay and pitch-angle randomization. At high particle rigidities, the contributions of those three modes to acceleration are comparable to within an order of magnitude, as a combination of several effects (partial disappearance of transit-time damping for fast modes, increased scattering rate for Alfvén and slow modes due to resonance broadening). Additionally, we provide analytical arguments regarding acceleration beyond the regime of MHD wave turbulence, addressing the issue of nonresonant acceleration in a turbulence comprised of structures rather than waves, as well as the issue of acceleration in small-scale parallel electric fields. Finally, we compare our results to the existing literature and provide ready-to-use formulas for applications to high-energy astrophysical phenomenology.

astro-ph.HE↗

arXiv2019

Three-dimensional simulations of non-resonant streaming instability and particle acceleration near non-relativistic astrophysical shocks

We use particle-in-magnetohydrodynamics-cells to model particle acceleration and magnetic field amplification in a high Mach, parallel shock in three dimensions and compare the result to 2-D models. This allows us to determine whether 2-D simulations can be relied upon to yield accurate results in terms of particle acceleration, magnetic field amplification and the growth rate of instabilities. Our simulations show that the behaviour of the gas and the evolution of the instabilities are qualitatively similar for both the 2-D and 3-D models, with only minor quantitative differences that relate primarily to the growth speed of the instabilities. The main difference between 2-D and 3-D models can be found in the spectral energy distributions (SEDs) of the non-thermal particles. The 2-D simulations prove to be more efficient, accelerating a larger fraction of the particles and achieving higher velocities. We conclude that, while 2-D models are sufficient to investigate the instabilities in the gas, their results have to be treated with some caution when predicting the expected SED of a given shock.

astro-ph.HE↗

arXiv2019

Rossby Wave Instability and High-Frequency Quasi-Periodic Oscillations in accretion discs orbiting around black holes

The rather elusive High-Frequency Quasi-Periodic Oscillations(HFQPO) observed in the X-ray lightcurve of black holes have been seen in a wide range of frequencies, even within one source. It is also notable to have been detected in "pairs" of HFQPOs with a close to integer ratio between the frequencies. The aim of this paper is to investigate some of the possible observable that we could obtain from having the Rossby Wave Instability (RWI) active in the accretion disc surrounding the compact object. Using the newly developed GR-AMRVAC code able to follow the evolution of the RWI in a full general relativistic framework, we explore how RWI can reproduce observed HFQPO frequencies ratios and if it is compatible with the observations. In order to model the emission coming from the disc we have linked our general relativistic simulations to the general relativistic ray-tracing GYOTO code and delivered synthetic observables that can be confronted to actual data from binary systems hosting HFQPOs.}{We have demonstrated in our study that some changes in the physical conditions prevailing in the part of the disc where RWI can be triggered leads to various dominant RWI modes whose ratio recovers frequency ratios observed in various X-ray binary systems. In addition, we have also highlighted that when RWI is triggered near the last stable orbit of a spinning black hole, the amplitude of the X-ray modulation increases with the spin of the black hole. Revisiting published data on X-ray binary systems, we show that this type of relationship actually exists in the five systems where an indirect measure of the spin of the black hole is available.

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arXiv2018

Relativistic magnetohydrodynamical simulations of the resonant corrugation of a fast shock front

The generation of turbulence at magnetized shocks and its subsequent interaction with the latter is a key question of plasma- and high-energy astrophysics. This paper presents two-dimensional magnetohydrodynamic simulations of a fast shock front interacting with incoming upstream perturbations, described as harmonic entropy or fast magnetosonic waves, both in the relativistic and the sub-relativistic regimes. We discuss how the disturbances are transmitted into downstream turbulence and we compare the observed response for small amplitude waves to a recent linear calculation. In particular, we demonstrate the existence of a resonant response of the corrugation amplitude when the group velocity of the outgoing downstream fast mode matches the velocity of the shock front. We also present simulations of large amplitude waves to probe the non-linear regime.

astro-ph.HE↗

arXiv2017

On magnetic field amplification and particle acceleration near non-relativistic astrophysical shocks: Particles in MHD Cells simulations

We present simulations of magnetized astrophysical shocks taking into account the interplay between the thermal plasma of the shock and supra-thermal particles. Such interaction is depicted by combining a grid-based magneto-hydrodynamics description of the thermal fluid with particle in cell techniques devoted to the dynamics of supra-thermal particles. This approach, which incorporates the use of adaptive mesh refinement features, is potentially a key to simulate astrophysical systems on spatial scales that are beyond the reach of pure particle-in-cell simulations. We consider in this study non-relativistic shocks with various Alfvenic Mach numbers and magnetic field obliquity. We recover all the features of both magnetic field amplification and particle acceleration from previous studies when the magnetic field is parallel to the normal to the shock. In contrast with previous particle-in-cell-hybrid simulations, we find that particle acceleration and magnetic field amplification also occur when the magnetic field is oblique to the normal to the shock but on larger timescales than in the parallel case. We show that in our simulations, the supra-thermal particles are experiencing acceleration thanks to a pre-heating process of the particle similar to a shock drift acceleration leading to the corrugation of the shock front. Such oscillations of the shock front and the magnetic field locally help the particles to enter the upstream region and to initiate a non-resonant streaming instability and finally to induce diffuse particle acceleration.

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arXiv2015

Numerical simulations of axisymmetric hydrodynamical Bondi-Hoyle accretion onto a compact object

Bondi-Hoyle accretion configurations occur as soon as a gravitating body is immersed in an ambient medium with a supersonic relative velocity. From wind-accreting X-ray binaries to runaway neutron stars, such a regime has been witnessed many times and is believed to account for shock formation, the properties of which can be only marginally derived analytically. In this paper, we present the first results of the numerical characterization of the stationary flow structure of Bondi-Hoyle accretion onto a compact object, from the large scale accretion radius down to the vicinity of the compact body. For different Mach numbers, we study the associated bow shock. It turns out that those simulations confirm the analytical prediction by Foglizzo & Ruffert (1996) concerning the topology of the inner sonic surface with an adiabatic index of 5/3. They also enable us to derive the related mass accretion rates, the position and the temperature of the bow shock, as function of the flow parameters, along with the transverse density and temperature profiles in the wake.

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arXiv2012

On fan-shaped cold MHD winds from Keplerian accretion discs

We investigate under which conditions cold, fan-shaped winds can be steadily launched from thin (Keplerian) accretion discs. Such winds are magneto-centrifugal winds launched from a thin annulus in the disc, along open magnetic field lines that fan out above the disc. In principle, such winds could be found in two situations: (1) at the interface between an inner Jet Emitting Disc, which is itself powering magneto-centrifugally driven winds, and an outer standard accretion disc; (2) at the interface between an inner closed stellar magnetosphere and the outer standard accretion disc. We refer to Terminal or T-winds to the former kind and to Magnetospheric or M-winds to the latter. The full set of resistive and viscous steady state MHD equations are analyzed for the disc (the annulus), which allow us to derive general expressions valid for both configurations. We find that, under the framework of our analysis, the only source of energy able to power any kind of fan-shaped winds is the viscous transport of rotational energy coming below the inner radii. Using standard local $α$ prescriptions for the anomalous (turbulent) transport of angular momentum and magnetic fields in the disc, we derive the strength of the transport coefficients that are needed to steadily sustain the global configuration. It turns out that, in order for these winds to be dynamically relevant and explain observed jets, the disc coefficients must be far much larger than values expected from current knowledge of turbulence occurring inside proto-stellar discs. Either the current view on MHD turbulence must be deeply reconsidered or steady-state fan-shaped winds are never realized in Nature. The latter hypothesis seems to be consistent with current numerical simulations.

astro-ph.SR↗

arXiv2010

Dead zones in protostellar discs: the case of Jet Emitting Discs

Planet formation and migration in accretion discs is a very active topic. Among the many aspects related to that question, dead zones are of particular importance as they can influence both the formation and the migration of planetary embryos. The ionisation level in the disc is the key element in determining the existence and the location of the dead zone. This has been studied either within the Standard Accretion Disc (SAD) framework or using parametric discs. In this paper, we extend this study to the case of Jet Emitting Discs (JED), the structure of which strongly differ from SADs because of the new energy balance and angular momentum extraction imposed by the jets. We make use of the (r,z) density distributions provided by self-similar accretion-ejection models, along with the JED thermal structure derived in a previous paper, to create maps of the ionisation structure of JEDs. We compare the ionisation rates we obtain to the critical value required to trigger the magneto-rotational instability. It is found that JEDs have a much higher ionisation degree than SADs which renders very unlikely the presence of a dead zone in these discs. As JEDs are believed to occupy the inner regions of accretion discs, the extension of the dead zones published in the literature should be re-considered for systems in which a jet is present. Moreover, since JEDs require large scale magnetic fields close to equipartition, our findings raise again the question of magnetic field advection in circumstellar accretion discs.

astro-ph.EP↗

arXiv2007

Two-flow magnetohydrodynamical jets around young stellar objects

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. Our simulations show that the inner outflow is accelerated from the central object 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. We also show that when a dense inner stellar wind occurs, the resulting disc-driven jet have 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 which is in better agreement with the observations besides being more consistent.

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