arXiv subjects

Guillaume Dubus

Publications and source records attributed to Guillaume Dubus.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic winds in resistive compact binary discs

Dwarf Novae and low-mass X-ray binaries are eruptive binary systems comprised of a Roche-lobe overflowing solar-type star and an accreting compact object. Their recurrence time can be explained by a low-accreting phase, the quiescence, during which the angular momentum transport parameter is inferred to be $α\approx 0.01$ by the Disc Instability Model. Non-magnetics mechanisms, such as spiral wave transport, only achieve angular momentum transport an order of magnitude too low, at best, because these discs are so thin in quiescence. During this phase, the Magneto-rotational Instability is known to be suppressed by the increased resistivity of the weakly ionised plasma. Studying these thin magnetised discs is a numerical challenge because of the wide range of scales to be resolved. Thanks to the new GPU-accelerated code Idefix, we produce global 3D MHD simulations of a very thin disc $(H/R = 0.01)$ for the first time. We explore the possibility that an MHD wind arises and increases accretion in low magnetic Reynolds number $(\mathrm{Rm}\approx100)$ and realistic plasma parameter ($β\approx 1000$) regimes. We observe that the MRI is only quenched in the resistive disc bulk but survives in the disc atmosphere. This drives strong accretion and wind launching. We quantify the efficiency of the resulting wind and measure its global effect on the disc. We explore the effect of the initial disc magnetisation and compare the accretion/ejection regime with and without resistivity.

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arXiv2025

Orbital Phase-resolved Analysis of X-ray and Gamma-ray Observations of the High-Mass Gamma-ray Binary 4FGL J1405.1-6119

We present the results of multi-wavelength observations of the High-Mass Gamma-Ray Binary 4FGL J1405.1-6119. A pair of joint XMM-Newton and NuSTAR observations taken in 2019 (sampling the gamma-ray maximum and X-ray maximum) characterize the emission of soft and hard X-rays. We find variability of the hydrogen column density along our line of sight, $N_{\rm H}$, and photon index, $Γ$, and find no evidence of pulsations in X-rays. We also refine a new best-fit orbital period to $P=13.7157\pm0.0014$ days, the first orbital phase-resolved analysis based on nearly 16 years of Fermi--LAT observations of 4FGL J1405.1-6119 and the evolution of the spectral shape as a function of orbital phase. Finally, the X-ray and $γ$-ray spectra for the phases sampled in the new X-ray observations can be interpreted in the framework of the intrabinary shock model, previously applied to High-Mass Gamma-Ray binaries such as LS 5039.

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arXiv2025

Synthetic pulsar lightcurves from global kinetic simulations and comparison with the Fermi-LAT catalog

Rotation-powered pulsars represent the main class of identified gamma-ray sources in the Galaxy. The wealth of observational data collected by the AGILE and Fermi gamma-ray space telescopes in the GeV range, and by ground-based Cherenkov telescopes in the TeV band provide invaluable insights into how relativistic plasmas dissipate and accelerate particles. Decoding the information contained in the gamma-ray pulses profile is an important step to understand how pulsars work. In this study, we aim at putting an ab initio plasma model of pulsar magnetospheres to the test, in light of the most recent gamma-ray observations in the GeV and TeV bands. To this end, we present of a new series of global particle-in-cell simulations of an inclined pulsar magnetosphere. High-quality synthetic pulse profiles in the synchrotron and inverse Compton channels are reconstructed to study in greater details their morphology and their energy dependence. We also perform a fit of observed lightcurves with the model, using the third Fermi-LAT gamma-ray pulsar catalog. Reconnection in the wind current sheet powers synchrotron and inverse Compton emission. The modeled pulse profiles reproduce some of the salient features of observed gamma-ray pulsars, including the mysterious Vela-like lightcurves, such as: the generic double-peaked structure, the presence of a bridge or third peak in between the main pulses, the pulse narrowing with increasing energy. The bolometric synchrotron radiative efficiency is strictly limited by the reconnection rate. Our global kinetic simulations are able to match observed pulse profiles. Such direct comparisons will help drive and focus future simulation developments.

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arXiv2024

Cataclysmic Variables and the disc instability model in the Gaia DR3 colour-magnitude diagram

Cataclysmic Variables (CVs) are semi-detached binaries composed of a white dwarf orbiting a lower-mass K or M star. We investigate whether CVs are responsible for a new intriguing feature (the `hook') that appears in the Gaia DR3 colour-magnitude Hertzsprung-Russell diagram (HRD) when selecting sources with low extinction. We also aim to understand the location of CVs in the HRD based on the predictions of the disc instability model (DIM). The DIM is the foundation on which rests our basic understanding of stable (novae-like) and outbursting CVs (dwarf novae). We calculate the expected behaviour of CVs in the Gaia HRD taking into account the variable light contributed by the accretion disc, the companion, the white dwarf, and from the bright spot where the Roche lobe overflow stream from the companion intersects the disc. We find that the `hook' feature is most likely to be composed of CVs. The `hook' corresponds to the limited region where stable CVs (novae-likes) must be located in the HRD according to the DIM, with the bluest systems having the shortest orbital period. Unstable systems, giving rise to dwarf novae outbursts, trace counterclockwise loops in the HRD. The overall behaviour is consistent with the location of the various CV subtypes in the HRD. These results can be used as a basis to pinpoint interesting outliers in the HRD, either due to their location or their tracks. These outliers may signal new subtypes such as cold, stable CVs with truncated discs, or may challenge the disc instability model.

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arXiv2023

High-mass X-ray Binaries

Binary systems in which a neutron star or black hole accretes material from a high-mass star are known as high-mass X-ray binaries (HMXBs). This chapter provides a brief introduction to the physics of wind accretion and an observational view of HMXBs, including their classification, X-ray spectra, X-ray variability, orbital and compact object properties, as well as studies of Galactic and Magellanic HMXB populations. Two classes of X-ray sources whose possible connections to HMXBs have been debated, ultraluminous X-ray sources and gamma-ray binaries, are also discussed. Approximately 300 HMXBs residing either in the Milky Way or the Magellanic Clouds have been discovered. The majority of these HMXBs host wind-accreting neutron stars. Their X-ray properties depend both on the interaction of the accreting material with the neutron star's strong magnetic field and the properties of the donor star's wind. Most HMXBs are classified as either supergiant XBs or Be XBs based on the spectral type of the donor star; these classes exhibit different patterns of X-ray variability and occupy different phase space in diagrams of neutron star spin versus orbital period. While studies of HMXBs in the Milky Way and Magellanic Clouds find that their luminosity functions have similar shapes, an overabundance of Be XBs in the Small Magellanic Cloud points to important variations of the HMXB population with metallicity and age.

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arXiv2022

Images of magnetospheric reconnection-powered radiation around supermassive black holes

Accreting supermassive black holes can now be observed at the event-horizon scale at mm wavelengths. Current predictions for the image rely on hypotheses (fluid modeling, thermal electrons) which might not always hold in the vicinity of the black hole, so that a full kinetic treatment is in order. In this letter, we describe the first 3D global general-relativistic particle-in-cell simulation of a black-hole magnetosphere. The system displays a persistent equatorial current sheet. Synthetic images are computed by ray-tracing synchrotron emission from nonthermal particles accelerated in this current sheet by magnetic reconnection. We identify several time-dependent features of the image at moderate viewing angles: a variable radius of the ring, and hot spots moving along it. In this regime, our model predicts that most of the flux of the image lies inside the critical curve. These results could help understand future observations of black-hole magnetospheres at improved temporal and spatial resolution.

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arXiv2022

Intra-pulse variability induced by plasmoid formation in pulsar magnetospheres

Pulsars show irregularities in their pulsed radio emission that originate from propagation effects and the intrinsic activity of the source. In this work, we investigate the role played by magnetic reconnection and the formation of plasmoids in the pulsar wind current sheet as a possible source of intrinsic pulse-to-pulse variability in the incoherent, high-energy emission pattern. We used a two-dimensional particle-in-cell simulation of an orthogonal pulsar magnetosphere restricted to the plane perpendicular to the star spin axis. We evolved the solution for several tens of pulsar periods to gather a statistically significant sample of synthetic pulse profiles. The formation of plasmoids leads to strong pulse-to-pulse variability in the form of multiple short, bright subpulses, which appear only on the leading edge of each main pulse. These secondary peaks of emission are dominated by the dozen plasmoids that can grow up to macroscopic scales. They emerge from the high end of the hierarchical merging process occurring along the wind current layer. The flux of the subpulses is correlated with their width in phase. Although the full-scale separation is not realistic, we argue that the simulation correctly captures the demographics and the properties of the largest plasmoids, and therefore of the brightest subpulses. The prediction of subpulses at specific pulse phases provides a new observational test of the magnetic reconnection scenario as the origin of the pulsed incoherent emission. High-time-resolution observations of the Crab pulsar in the optical range may be the most promising source to target for this purpose.

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arXiv2021

The Galactic bulge millisecond pulsars shining in X rays: A gamma-ray perspective

If the mysterious Fermi-LAT GeV gamma-ray excess is due to an unresolved population of millisecond pulsars (MSP) in the Galactic bulge, one expects this very same population to shine in X rays. For the first time, we address the question of what is the sensitivity of current X-ray telescopes to an MSP population in the Galactic bulge. To this end, we create a synthetic population of Galactic MSPs, building on an empirical connection between gamma- and X-ray MSP emission based on observed source properties. We compare our model with compact sources in the latest Chandra source catalog, applying selections based on spectral observables and optical astrometry with Gaia. We find a significant number of Chandra sources in the region of interest to be consistent with being bulge MSPs that are as yet unidentified. This motivates dedicated multi-wavelength searches for bulge MSPs: Some promising directions are briefly discussed.

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arXiv2021

Synthetic gamma-ray lightcurves of Kerr black-hole magnetospheric activity from particle-in-cell simulations

Context: The origin of ultra-rapid flares of very high-energy radiation from active galactic nuclei remains elusive. Magnetospheric processes, occurring in the close vicinity of the central black hole, could account for these flares. Aims: We aim to bridge the gap between simulations and observations by synthesizing gamma-ray lightcurves in order to characterize the activity of a black-hole magnetosphere, using kinetic simulations. Methods: We perform global axisymmetric two-dimensional general-relativistic particle-in-cell simulations of a Kerr black-hole magnetosphere. We include a self-consistent treatment of radiative processes and plasma supply, as well as a realistic magnetic configuration, with a large-scale equatorial current sheet. We couple our particle-in-cell code with a ray-tracing algorithm, in order to produce synthetic lightcurves. Results: These simulations show a highly dynamic magnetosphere, as well as very efficient dissipation of the magnetic energy. An external supply of magnetic flux is found to maintain the magnetosphere in a dynamic state, otherwise the magnetosphere settles in a quasi-steady Wald-like configuration. The dissipated energy is mostly converted to gamma-ray photons. The lightcurves at low viewing angle (face-on) mainly trace the spark gap activity and exhibit high variability. On the other hand, no significant variability is found at high viewing angle (edge-on), where the main contribution comes from the reconnecting current sheet. Conclusions: We observe that black-hole magnetospheres with a current sheet are characterized by a very high radiative efficiency. The typical amplitude of the flares in our simulations is lower than what is detected in active galactic nuclei. Such flares could result from the variation of parameters external to the black hole

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arXiv2020

Dissipation of the striped pulsar wind and non-thermal particle acceleration: 3D PIC simulations

The formation of a large-scale current sheet is a generic feature of pulsar magnetospheres. If the magnetic axis is misaligned with the star rotation axis, the current sheet is an oscillatory structure filling an equatorial wedge determined by the inclination angle, known as the striped wind. Relativistic reconnection could lead to significant dissipation of magnetic energy and particle acceleration although the efficiency of this process is debated in this context. In this study, we aim at reconciling global models of pulsar wind dynamics and reconnection in the stripes within the same numerical framework, in order to shed new light on dissipation and particle acceleration in pulsar winds. To this end, we perform large three-dimensional particle-in-cell simulations of a split-monopole magnetosphere, from the stellar surface up to fifty light-cylinder radii away from the pulsar. Plasmoid-dominated reconnection efficiently fragments the current sheet into a dynamical network of interacting flux ropes separated by secondary current sheets which consume the field efficiently at all radii, even past the fast magnetosonic point. Our results suggest there is a universal dissipation radius solely determined by the reconnection rate in the sheet, lying well upstream the termination shock radius in isolated pair producing pulsars. The wind bulk Lorentz factor is much less relativistic than previously thought. In the comoving frame, the wind is composed of hot pairs trapped within flux ropes with a hard broad power-law spectrum, whose maximum energy is limited by the magnetization of the wind at launch. We conclude that the striped wind is most likely fully dissipated when it enters the pulsar wind nebula. The predicted wind particle spectrum after dissipation is reminiscent of the Crab Nebula radio-emitting electrons.

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arXiv2020

Magnetic field transport in compact binaries

Dwarf novae (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints on angular momentum transport mechanisms. We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations. We develop a version of the disk instability model that evolves the density, the temperature and the large scale vertical magnetic flux together. We take into account the accretion driven by turbulence or by a magnetized outflow. To evolve the magnetic flux, we use a toy model with physically motivated prescriptions depending mainly on the local magnetization. We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe lightcurves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk, where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as $\dot{M}^{-2/3}$. Models of DNe and LMXBs lightcurves typically require the outer, viscous disk to be truncated in order to match observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.

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arXiv2020

Multi-dimensional simulations of ergospheric pair discharges around black holes

Black holes are known to launch powerful relativistic jets and emit highly variable gamma radiation. How these jets are loaded with plasma remains poorly understood. Spark gaps are thought to drive particle acceleration and pair creation in the black-hole magnetosphere. In this paper, we perform 2D axisymmetric general-relativistic particle-in-cell simulations of a monopole black-hole magnetosphere with a realistic treatment of inverse Compton scattering and pair production. We find that the magnetosphere can self-consistently fill itself with plasma and activate the Blandford-Znajek mechanism. A highly time-dependent spark gap opens near the inner light surface which injects pair plasma into the magnetosphere. These results may account for the high-energy activity observed in active galactic nuclei and explain the origin of plasma at the base of the jet.

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arXiv2019

Magnetic wind-driven accretion in dwarf novae

Dwarf novae (DNe) and X-ray binaries exhibit outbursts thought to be due to a thermal-viscous instability in the accretion disk. The disk instability model (DIM) assumes that accretion is driven by turbulent transport, customarily attributed to the magneto-rotational instability (MRI). Recent results point out that MRI turbulence alone fails to reproduce the light curves of DNe. We aim to study the impact of wind-driven accretion on the light curves of DNe. Local and global simulations show that magneto-hydrodynamic winds are present when a magnetic field threads the disk, even for relatively high ratios of thermal pressure to magnetic pressure ($β\approx 10^{5}$). These winds are very efficient in removing angular momentum but do not heat the disk; they do not behave as MRI-driven turbulence. We add wind-driven transport in the angular momentum equation of the DIM, assuming a fixed magnetic configuration: dipolar or constant with radius. We use prescriptions for the wind torque and the turbulent torque derived from shearing box simulations. The wind torque enhances the accretion of matter, resulting in light curves that look like DNe outbursts when assuming a dipolar field with a moment $μ\approx10^{30}\,\mathrm{G\,cm^{3}}$. In the region where the wind dominates, the disk is cold, optically thin and the accretion speed is sonic. This acts as if the inner disk was truncated, leading to higher quiescent X-ray luminosities from the white dwarf boundary layer than expected with the standard DIM. The disk is stabilized if the wind-dominated region is large enough, potentially leading to `dark' disks emitting little radiation. Wind-driven accretion can play a key role in shaping the light curves of DNe and X-ray binaries. Future studies will need to include the time evolution of the magnetic field threading the disk to fully assess its impact on the dynamics of the accretion flow.

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arXiv2018

Kinetic modeling of the electromagnetic precursor from an axisymmetric binary pulsar coalescence

The recent detection of gravitational waves associated with a binary neutron star merger revives interest in interacting pulsar magnetospheres. Current models predict that a significant amount of magnetic energy should be released prior to the merger, leading to electromagnetic precursor emission. In this paper, we revisit this problem in the light of the recent progress in kinetic modeling of pulsar magnetospheres. We limit our work to the case of aligned magnetic moments and rotation axes, and thus neglect the orbital motion. We perform global two-dimensional axisymmetric particle-in-cell simulations of two pulsar magnetospheres merging at a rate consistent with the emission of gravitational waves. Both symmetric and asymmetric systems are investigated. Simulations show a significant enhancement of magnetic dissipation within the magnetospheres as both stars get closer. Even though the magnetospheric configuration depends on the relative orientations of the pulsar spins and magnetic axes, all configurations present nearly the same radiative signature, indicating that a common dissipation mechanism is at work. The relative motion of both pulsars drives magnetic reconnection at the boundary between the two magnetospheres, leading to efficient particle acceleration and high-energy synchrotron emission. Polar-cap discharge is also strongly enhanced in asymmetric configurations, resulting in vigorous pair production and potentially additional high-energy radiation. We observe an increase in the pulsar radiative efficiency by two orders of magnitude over the last orbit before the merger exceeding the spindown power of an isolated pulsar. The expected signal is too weak to be detected at high energies even in the nearby universe. However, if a small fraction of this energy is channeled into radio waves, it could be observed as a non-repeating fast radio burst.

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arXiv2018

Super-Keplerian Equatorial Outflows in SS 433. Centrifugal Ejection of the Circumbinary Disk

The microquasar SS 433 is the only known steady supercritical accretor in the Galaxy. It is well-known for its relativistic baryonic jets, but the system also drives equatorial outflows. These have been routinely detected in radio images, and components associated with a circumbinary disk have also been suggested in optical emission lines. We wish to spatially resolve the regions producing the stationary emission lines of SS 433 to shed light on its circumbinary structure and outflows. With an estimated binary orbit size $\lesssim 0.1 \text{ mas}$, this requires optical interferometry. We use the optical interferometer VLTI+GRAVITY to spatially resolve SS 433 in the near-infrared K band at high spectral resolution ($R\approx 4000$) on three nights in July 2017. The stationary Br$γ$ line is clearly dominated by an extended $\sim 1 \text{ mas} \sim 5 \text{ AU}$ circumbinary structure perpendicular to the jets and with a strong rotation component. The rotation direction is retrograde relative to the jet precession, in accordance with the slaved disk precession model. The structure has a very high specific angular momentum and is too extended to be a stable circumbinary disk in Keplerian rotation; interpreting it as such leads to a very high enclosed mass $M \gtrsim 400 M_{\odot}$. We instead interpret it as the centrifugal ejection of the circumbinary disk, with the implication that there must be an efficient transfer of specific angular momentum from the binary to the disk. We suggest that the equatorial outflows sometimes seen in radio images result from similar episodes of circumbinary disk centrifugal ejection. In addition to the equatorial structure, we find a very extended $\sim 6 \text{ mas} \sim 30 \text{ AU}$ spherical wind component to the Br$γ$ line: the entire binary is engulfed in an optically thin spherical line emission envelope.

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arXiv2018

The Collimated Radiation in SS 433. Constraints from Spatially Resolved Optical Jets and $\texttt{Cloudy}$ Modeling of the Optical Bullets

The microquasar SS 433 is well-known for its precessing, relativistic baryonic jets. Depending on their heating mechanism, the optical jet bullets may serve as a probe of the collimated radiation coming from the inner region close to the compact object. The optical interferometer VLTI/GRAVITY has allowed to spatially resolved the optical jets in SS 433 for the first time. We present here the second such observation taken over three nights in July 2017. In addition, we use the multi-wavelength XSHOOTER spectrograph at VLT to study the optical bullets in SS 433 in detail. GRAVITY reveals elongated exponential-like spatial profiles for the optical jets, suggestive of a heating mechanism acting throughout a long portion of the jet and naturally explained by photoionization by the collimated radiation. We also spatially resolve the movement of the optical bullets for the first time, detecting extended jet components corresponding to previous ejections. \texttt{Cloudy} photoionization models explain both the spatial intensity profiles measured with GRAVITY and the line ratios from XSHOOTER, and constrain the properties of the optical bullets and the ionizing radiation. We find that the latter must peak in the UV with an isotropic luminosity (as inferred by a face on observer) $\approx 10^{41}$ erg/s. Provided that the X-ray SED is sufficiently hard, the collimated X-ray luminosity could still be high enough so that the face on observer would see SS 433 as an ULX ($L_X \lesssim 10^{40}$ erg/s) and it would still be compatible with the H/He/He+ ionization balance of the optical bullets. The kinetic power in the optical jets is constrained to $3-20 \times 10^{38}$ erg/s, and the extinction in the optical jets to $A_V = 6.7 \pm 0.1$. We suggest there may be substantial $A_V \gtrsim 1$ and structured circumstellar extinction in SS 433, likely arising from dust formed in equatorial outflows.

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arXiv2018

Turbulent and wind-driven accretion in dwarf novae threaded by a large scale magnetic field

Dwarf novae (DNe) are accreting white dwarfs that show eruptions due to a thermal-viscous instability in the accretion disk. The outburst timescales constrain $α$, the ratio of the viscous stress to the thermal pressure, and so the mechanism of angular momentum transport. The eruptive state has $α\approx0.1$ while the quiescent state has $α\approx0.03$. Turbulent transport due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. Here, we perform 3D local magnetohydrodynamic (MHD) shearing box simulations including vertical stratification, radiative transfer and a net constant vertical magnetic flux to investigate how transport changes between the outburst and quiescent states of DNe. We find that a constant $B_z$ provides a higher $α$ in quiescence than in outburst, in opposition to what is expected. Including resistivity quenches MRI turbulence in quiescence, suppressing transport, unless the magnetic field is high enough, which again leads to $α\approx0.1$. A major difference between simulations with a net poloidal flux and simulations without is that angular momentum transport in the former is shared between turbulent and wind-driven transport. We find that wind-driven transport dominates in quiescence even for low magnetic fields $\sim 1$ G. This can have a major impact on observational signatures since wind-driven transport does not heat the disk. Furthermore, wind transport cannot be reduced to an $α$ prescription. We provide fits for $α$ and the wind torque with $β$, ratio of thermal to magnetic pressure. We conclude that the evolution of the thermal-viscous instability, and its consequences on the outburst cycles of CVs, needs to be seriously revised to take into account that most of the accretion energy may be carried away by a wind instead of being locally dissipated.

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arXiv2018

The geometric distance and binary orbit of PSR B1259-63

The pulsar/massive star binary system PSR B1259-63 / LS 2883 is one of the best-studied gamma-ray binaries, a class of systems whose bright gamma-ray flaring can provide important insights into high-energy physics. Using the Australian Long Baseline Array we have conducted very long baseline interferometric observations of PSR B1259-63 over 4.4 years, fully sampling the 3.4-year orbital period. From our measured parallax of $0.38\pm0.05$ mas we use a Bayesian approach to infer a distance of $2.6^{+0.4}_{-0.3}$ kpc. We find that the binary orbit is viewed at an angle of $154\pm3$ degrees to the line of sight, implying that the pulsar moves clockwise around its orbit as viewed on the sky. Taking our findings together with previous results from pulsar timing observations, all seven orbital elements for the system are now fully determined. We use our measurement of the inclination angle to constrain the mass of the stellar companion to lie in the range 15-31$M_{\odot}$. Our measured distance and proper motion are consistent with the system having originated in the Cen OB1 association and receiving a modest natal kick, causing it to have moved $\sim$8 pc from its birthplace over the past $\sim3\times10^5$ years. The orientation of the orbit on the plane of the sky matches the direction of motion of the X-ray synchrotron-emitting knot observed by the Chandra X-ray Observatory to be moving away from the system.

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