arXiv subjects

Peggy Varniere

Publications and source records attributed to Peggy Varniere.

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 ${α_\mathrm{eff} \, {\sim} \, 10^{-3}{- 2\times 10}^{-2}}$. 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.

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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.

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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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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.

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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).

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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

Looking for the elusive 3:2 ratio of high-frequency quasi-periodic oscillations in the microquasar XTE J1550-564

Using the two main XTE J1550-564 outbursts (1998-99 and 2000) we gathered about 30 observations with confirmed detections of HFQPOs. While this is a small sample it is enough to start looking at the generic properties of these oscillations, especially focusing on their frequencies and their potential harmonic relationship. This then will provide us with a list of constraints, necessary for any attempt of modelling their origin. We defined five groups based on their similarities in the Fourier domain, namely the continuum of their power density spectra (PDS) and the HFQPO frequencies. We then combined the individual PDSs of each family to obtain a PDS with higher statistics to search for other potential, previously undetected, weaker peaks. While we have two $3σ$ potential detections of a pair of HFQPOs in our combined PDSs, none of them shows HFQPOs with frequencies in a previously claimed 3:2 ratio. Using the results presented here we propose an updated list of requirements for any model trying to explain the HFQPOs in microquasars.

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arXiv2016

X-ray Spectral Analysis of the Steady States of GRS 1915+105

We report on the X-ray spectral behavior within the steady states of GRS 1915+105. Our work is based on the full data set on the source obtained using the Proportional Counter Array on the Rossi X-ray Timing Explorer and 15 GHz radio data obtained using the Ryle Telescope. The steady observations within the X-ray data set naturally separated into two regions in the color-color diagram and we refer to them as steady-soft and steady-hard. GRS 1915+105 displays significant curvature in the coronal component in both the soft and hard data within the {\it RXTE}/PCA bandpass. A majority of the steady-soft observations displays a roughly constant inner disk radius (R_in), while the steady-hard observations display an evolving disk truncation which is correlated to the mass accretion rate through the disk. The disk flux and coronal flux are strongly correlated in steady-hard observations and very weakly correlated in the steady-soft observations. Within the steady-hard observations we observe two particular circumstances when there are correlations between the coronal X-ray flux and the radio flux with log slopes η~0.68 +/- 0.35 and η~ 1.12 +/- 0.13. They are consistent with the upper and lower tracks of Gallo et al. (2012), respectively. A comparison of model parameters to the state definitions show that almost all steady-soft observations match the criteria of either thermal or steep power law state, while a large portion of the steady-hard observations match the hard state criteria when the disk fraction constraint is neglected.

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arXiv2009

Simulating Magnetohydrodynamical Flow with Constrained Transport and Adaptive Mesh Refinement; Algorithms & Tests of the AstroBEAR Code

A description is given of the algorithms implemented in the AstroBEAR adaptive mesh refinement code for ideal magnetohydrodynamics. The code provides several high resolution, shock capturing schemes which are constructed to maintain conserved quantities of the flow in a finite volume sense. Divergence free magnetic field topologies are maintained to machine precision by collating the components of the magnetic field on a cell-interface staggered grid and utilizing the constrained transport approach for integrating the induction equations. The maintenance of magnetic field topologies on adaptive grids is achieved using prolongation and restriction operators which preserve the divergence and curl of the magnetic field across co-located grids of different resolution. The robustness and correctness of the code is demonstrated by comparing the numerical solution of various tests with analytical solutions or previously published numerical solutions obtained by other codes.

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arXiv2009

QPO Frequency - Color Radius Connection in GRS 1915+105: a Possible Turnover supporting AEI predictions

It is widely believed that the low frequency quasi-periodic X-ray oscillations observed in microquasars are correlated to, but do not originate at, the physical radius of the inner edge of the accretion disk. Models relating the QPO frequency and color radius are hindered by observations showing contradicting trend correlations between the microquasars GRO 1655-40, XTE J1550-564 and GRS 1915+105. The first shows a negative correlation and the latter two a positive one. By taking into account relativistic rotation in the accretion disk, the Accretion-Ejection Instability (AEI) model predicts a turnover in the frequency-radius relationship, and has been successfully compared with observations of GRO J1655-40 and GRS 1915+105. We present further evidence supporting the AEI model prediction by using observations of the microquasar GRS 1915+105. By combining a data set including $θ$-, $β$- and $α$-class X-ray light curves, we observe positive, negative and null correlations in the frequency-radius relationship. This is the first time a single source has shown a possible inversion in the QPO frequency-color radius curve predicted by the AEI model.

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arXiv2007

Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations

By combining data from different published 3-D simulations of Keplerian shearing boxes unstable to the magnetorotational instability (MRI), we highlight tight anti-correlations between the total effective inferred angular momentum transport parameter, $α_{tot}$, its separate Maxwell and Reynolds contributions $α_{mag}$ and $α_{kin}$, and the kinetic to magnetic pressure ratio $β$, defined with the initial or saturated (when available) thermal pressure. Plots of $Log (α_{kin}), Log (α_{mag})$, and $Log (α_{tot})$ vs $Log (β)$ are well fit by straight lines even as $α_{kin}$, $α_{mag}$,and $α_{tot}$ vary by four orders of magnitude over the simulations included. The ratio $α_{kin}/α_{mag}$ and the product $α_{tot}β$ are quite constant and largely independent of the presence or absence of weak mean fields, the choice of initial and boundary conditions, and the resolution. In short, simulations have more strongly constrained the product $α_{tot}β$ than $α_{tot}$ itself.

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arXiv2007

Evidence for Growth of Eccentricity and Mass Clearing in a Disc Interior to a Planet

We present computational results showing eccentricity growth in the inner portions of a protoplanetary disc. We attribute this to the evolving surface density of the disc. The planet creates a gap, which adjusts the balance between the 3:1 (eccentricity exciting) and 2:1 (eccentricity damping) resonances. The eccentricity of the inner disc can rise as high as 0.3, which is sufficient to cause it to be accreted onto the star. This offers an alternative mechanism for producing the large holes observed in the discs of CoKu Tau/4, GM Aur and DM Tau.

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arXiv2006

Accretion-Ejection Instability, MHD Rossby Wave Instability, diskoseismology, and the high-frequency QPO of microquasars

We present a possible explanation for the high-frequency Quasi-Periodic Oscillations of microquasars by an MHD instability that combines the physics developed, in different contexts, for the Accretion-Ejection Instability, the Rossby-Wave Instability, and the normal modes of diskoseismic models (which rely on the properties of the relativistic rotation curve in the vicinity of the Marginally Stable Orbit). This instability can appear as modes of azimuthal wavenumbers m=2, 3,... that have very similar pattern speeds ω/m, while the m=1 mode, which would appear as the fundamental of this discrete spectrum, is less unstable. This would readily explain the 2:3 (and sometimes higher) frequency ratio observed between these QPO. These instabilites form eigenmodes, i.e. standing wave patterns at a constant frequency in the disk; they are strongly unstable, and thus do not need an external excitation mechanism to reach high amplitudes. Furthermore, they have the property that a fraction of the accretion energy can be emitted toward the corona: this would explain that these QPO are seen in a spectral state where Comptonized emission from the corona is always present. Their existence depends critically on the existence of a magnetic structure, formed by poloidal flux advected in the accretion process, in the central region between the disk and the black hole.

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arXiv2005

Observational Properties of Proto-planetary Disk Gaps

We study the effects of an annular gap induced by an embedded proto-planet on disk scattered light images and the infrared spectral energy distribution. We find that the outer edge of a gap is brighter in the scattered light images than a similar location in a gap-free disk. The stellar radiation that would have been scattered by material within in the gap is instead scattered by the disk wall at the outer edge of the gap, producing a bright ring surrounding the dark gap in the images. Given sufficient resolution, such gaps can be detected by the presence of this bright ring in scattered light images. A gap in a disk also changes the shape of the SED. Radiation that would have been absorbed by material in the gap is instead reprocessed by the outer gap wall. This leads to a decrease in the SED at wavelengths corresponding to the temperature at the radius of the missing gap material and a corresponding flux increase at longer wavelengths corresponding to the temperature of the outer wall. We note that, unlike an inner hole in the disk, the presence of an annular gap does not change the bolometric IR flux; it simply redistributes the radiation, previously produced by material within the gap, to longer wavelengths. This implies that the changes in the SED generally will be smaller for gaps than holes. Although it will be difficult on the basis of the SED alone to distinguish between the presence of a gap and other physical effects, the level of changes can be sufficiently large to be measurable with current instruments (e.g., Spitzer).

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arXiv2005

Driving spiral arms in the circumstellar disks of HD 100546 and HD 141569A

With 2D hydrodynamical simulations of disks perturbed externally by stars, brown dwarfs or planets we investigate possible scenarios that can account for the spiral structure in circumstellar disks. We consider two scenarios, spiral structure driven by an external bound planet or low mass star and that excited by a previous stellar close encounter or flyby. We find that both scenarios produce morphology similar to that observed in the outer disks of HD 141569A and HD 100546; moderately open 2-armed outer spiral structure. The outer two-armed spiral structure observed in the disk of HD 141569A is qualitatively reproduced with tidal perturbations from its companion binary HD 141569B,C on a prograde orbit near periapse. Our simulation accounts for the outer spiral arms, but is less successful than the secular model of Augereau and Papaloizou at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk has been previously truncated by the tidal force from the binary. A bound object (stellar or planetary) is unlikely to explain the spiral structure in HD 100546. A co-eval planet or brown dwarf in the disk of sufficient mass to account for the amplitude of the spiral structure would be detectable in NICMOS and STIS images, however existing images reveal no such object. A previous encounter could explain the observed structure, provided that the encounter occurred less than a few thousand year ago. The object responsible for causing the spiral structure must then be within a few arcminutes of the star. However, the USNO-B proper motion survey reveals no candidate object. Moreover, the probability that a field star encountered HD 100546 in the past few thousand years is very low.

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arXiv2005

The Evolution of Protoplanetary Disk Edges

We investigate gap formation in gaseous protostellar disks by a planet in a circular orbit in the limit of low disk viscosity. This regime may be appropriate to an aging disk after the epoch of planet formation. We find that the distance of planet to the gap outer boundary can be between the location of the $m=2$ and $m=1$ outer Lindblad resonances. This distance is weakly dependent upon both the planet's mass and disk viscosity. We find that the evolution of the disk edge takes place on two timescales. The first timescale is set by the spiral density waves driven by the planet. The second timescale depends on the viscosity of the disk. The disk approaches a state where the outward angular momentum flux caused by the disk viscosity is balanced by the dissipation of spiral density waves which are driven at the Lindblad resonances. This occurs inefficiently however because of the extremely low gas density near the planet. We find that the distance between the planet and the peak density at the disk outer edge is only weakly dependent on the viscosity and planet mass, however the ratio of the gas density near the planet to that in the disk (or the slope of density along the disk edge) is strongly dependent upon both quantities. We find that the disk density profile along the edge scales approximately with disk viscosity divided by the square of the planet mass. We account for this behavior with a simple scenario in which the dissipation of angular momentum from the spiral density waves is balanced against diffusion in the steep edge of the disk.

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