Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, $h_\mathrm{th}/r=0.3, 0.1$ and $0.03$. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with $h_\mathrm{th}/r=0.03$. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of $z/r = \pm 0.03$ and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGN). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass loss rates in good agreement with the properties of ultra-fast outflows observed in AGN.Comment: Accepted version. 19 pages, 18 Figures + 4 Appendix Figure

A Reemerging Bright Soft X-Ray State of the Changing-look Active Galactic Nucleus 1ES 1927+654:A Multiwavelength View

1ES1927+654 is a nearby active galactic nucleus (AGN) that has shown an enigmatic outburst in optical/UV followed by X-rays, exhibiting strange variability patterns at timescales of months to years. Here we report the unusual X-ray, UV, and radio variability of the source in its postflare state (2022 January–2023 May). First, we detect an increase in the soft X-ray (0.3–2 keV) flux from 2022 May to 2023 May by almost a factor of 5, which we call the bright soft state. The hard X-ray 2–10 keV flux increased by a factor of 2, while the UV flux density did not show any significant changes (≤30%) in the same period. The integrated energy pumped into the soft and hard X-rays during this period of 11 months is ∼3.57 × 10 ^50 erg and 5.9 × 10 ^49 erg, respectively. From the energetics, it is evident that whatever is producing the soft excess (SE) is pumping out more energy than either the UV or hard X-ray source. Since the energy source presumably is ultimately the accretion of matter onto the supermassive black hole, the SE-emitting region must be receiving the majority of this energy. In addition, the source does not follow the typical disk–corona relation found in AGNs, neither in the initial flare (from 2017 to 2019) nor in the current bright soft state (2022–2023). We found that the core (<1 pc) radio emission at 5 GHz gradually increased until 2022 March, but showed a dip in 2022 August. The Güdel–Benz relation ( L _radio / L _X-ray ∼ 10 ^−5 ), however, is still within the expected range for radio-quiet AGNs, and further follow-up radio observations are currently being undertaken

Accrétion dans les disques de novae naines

Dwarf novæ have been used for almost 50 years now as a test for the theory of accretion. These systems exhibit eruptions in optical light lasting approximately a week with a recurrence time scale of a month. Eruptions are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to put constraints on the mechanisms transporting angular momentum in the disk, which will be the subject of this thesis. Traditionally, transport is presumed to be turbulent where turbulence is due to the magneto-rotational instability (MRI). However, I show here, using local simulations of accretion disks with radiative transfer that there exists a discrepancy between observations and light curves obtained with MRI turbulence only. In quiescence, where the disk is poorly ionised, it is very unlikely that MRI can even survive. One of the key results of this thesis is that MRI do not participate to turbulent angular momentum transport only, but is also able to drive MHD outflows which extract angular momentum very efficiently, especially in quiescence. Wind-driven transport is, by nature, very different from turbulent transport, it induces a surface-torque on the disk and do not deposit thermal energy locally but extract energy from the disk instead. We included MHD wind-driven angular momentum transport in a disk instability model, model which is usually used to reproduce light curves of dwarf novæ. Using this new model, we were able to retrieve light curves looking alike observations, with a magnetic field consistent with what is expected from the dipolar magnetic field of a white dwarf. It is the first time that eruptions of dwarf novæ are modeled with success using prescriptions for angular momentum transport derived from first principles instead of ad hoc parameters.Les novæ naines permettent, depuis presque 50 ans maintenant, de tester les modèles d’accrétion. Ces systèmes montrent des éruptions en optique d’une durée de l’ordre de la semaine avec des temps de récurrence de l’ordre du mois. Ces éruptions sont communément attribuées à une instabilité thermo-visqueuse au sein du disque d’accrétion entourant la naine blanche. Les temps caractéristiques de ces éruptions posent de fortes contraintes sur les mécanismes de transport de moment cinétique pilotant l’accrétion dans le disque, mécanismes qui constituent l’objet de cette thèse. Il est souvent admis que l’instabilité magnéto-rotationnelle (MRI) est responsable du transport de moment cinétique via la turbulence qu’elle produit. Cependant, je montre ici, à l’aide de simulations locales de disque d’accrétion avec transfert radiatif, que le transport turbulent produit par la MRI ne permet pas de reproduire les courbes de lumière. En quiescence, où le disque est peu ionisé, il est même peu probable que de la turbulence MRI puisse survivre. Un des résultats majeurs de cette thèse est d’avoir mis en lumière que la MRI ne participe pas qu’au transport turbulent mais peut également lancer des vents magnéto-hydrodynamiques (MHD) qui transportent également du moment cinétique, voire dominent le transport dans l’état quiescent. Ces vents MHD induisent un couple magnétique de surface sur le disque et ne peuvent être réduits à une turbulence effective, en partie car ceux-ci ne déposent pas d’énergie thermique localement mais en emportent contrairement au transport turbulent. Nous avons inclus le transport de moment cinétique dû au couple du vent MHD dans un modèle d’instabilité de disque, modèle classiquement utilisé pour reproduire les éruptions de novæ naines. Avec ce nouveau modèle, nous avons montré qu’il est possible de reproduire les courbes de lumière des éruptions de novæ naines, en utilisant un champ magnétique à la surface de la naine blanche compatible avec ce qui est attendu. C’est la première fois que les éruptions de novæ naines sont modélisées avec succès en utilisant des prescriptions pour le transport de moment cinétique basées sur des simulations MHD et non sur les observations

Accretion in disks of dwarf novae

Les novæ naines permettent, depuis presque 50 ans maintenant, de tester les modèles d’accrétion. Ces systèmes montrent des éruptions en optique d’une durée de l’ordre de la semaine avec des temps de récurrence de l’ordre du mois. Ces éruptions sont communément attribuées à une instabilité thermo-visqueuse au sein du disque d’accrétion entourant la naine blanche. Les temps caractéristiques de ces éruptions posent de fortes contraintes sur les mécanismes de transport de moment cinétique pilotant l’accrétion dans le disque, mécanismes qui constituent l’objet de cette thèse. Il est souvent admis que l’instabilité magnéto-rotationnelle (MRI) est responsable du transport de moment cinétique via la turbulence qu’elle produit. Cependant, je montre ici, à l’aide de simulations locales de disque d’accrétion avec transfert radiatif, que le transport turbulent produit par la MRI ne permet pas de reproduire les courbes de lumière. En quiescence, où le disque est peu ionisé, il est même peu probable que de la turbulence MRI puisse survivre. Un des résultats majeurs de cette thèse est d’avoir mis en lumière que la MRI ne participe pas qu’au transport turbulent mais peut également lancer des vents magnéto-hydrodynamiques (MHD) qui transportent également du moment cinétique, voire dominent le transport dans l’état quiescent. Ces vents MHD induisent un couple magnétique de surface sur le disque et ne peuvent être réduits à une turbulence effective, en partie car ceux-ci ne déposent pas d’énergie thermique localement mais en emportent contrairement au transport turbulent. Nous avons inclus le transport de moment cinétique dû au couple du vent MHD dans un modèle d’instabilité de disque, modèle classiquement utilisé pour reproduire les éruptions de novæ naines. Avec ce nouveau modèle, nous avons montré qu’il est possible de reproduire les courbes de lumière des éruptions de novæ naines, en utilisant un champ magnétique à la surface de la naine blanche compatible avec ce qui est attendu. C’est la première fois que les éruptions de novæ naines sont modélisées avec succès en utilisant des prescriptions pour le transport de moment cinétique basées sur des simulations MHD et non sur les observations.Dwarf novæ have been used for almost 50 years now as a test for the theory of accretion. These systems exhibit eruptions in optical light lasting approximately a week with a recurrence time scale of a month. Eruptions are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to put constraints on the mechanisms transporting angular momentum in the disk, which will be the subject of this thesis. Traditionally, transport is presumed to be turbulent where turbulence is due to the magneto-rotational instability (MRI). However, I show here, using local simulations of accretion disks with radiative transfer that there exists a discrepancy between observations and light curves obtained with MRI turbulence only. In quiescence, where the disk is poorly ionised, it is very unlikely that MRI can even survive. One of the key results of this thesis is that MRI do not participate to turbulent angular momentum transport only, but is also able to drive MHD outflows which extract angular momentum very efficiently, especially in quiescence. Wind-driven transport is, by nature, very different from turbulent transport, it induces a surface-torque on the disk and do not deposit thermal energy locally but extract energy from the disk instead. We included MHD wind-driven angular momentum transport in a disk instability model, model which is usually used to reproduce light curves of dwarf novæ. Using this new model, we were able to retrieve light curves looking alike observations, with a magnetic field consistent with what is expected from the dipolar magnetic field of a white dwarf. It is the first time that eruptions of dwarf novæ are modeled with success using prescriptions for angular momentum transport derived from first principles instead of ad hoc parameters

Magnetic wind-driven accretion in dwarf novae

Context. 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). However, recent results point out that MRI turbulence alone fails to reproduce the light curves of DNe. Aims. Our aim is 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 ≈ 105). These winds are very efficient in removing angular momentum but do not heat the disk, thus they do not behave as MRI-driven turbulence. Methods. We add the effect of wind-driven magnetic braking in the angular momentum equation of the DIM but neglect the mass loss due to the wind. We assume 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. Results. 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 ≈ 1030 G cm3. In the region where the wind torque dominates the disk is cold and optically thin, and the accretion speed is super-sonic. The inner disk behaves as if 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 that emitting little radiation. Conclusion. 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

Nonthermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states

X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and a power-law tail above 10 keV. Empirical models fit the high-energy power-law tail to radiation from a nonthermal electron distribution, but the physical location of the nonthermal electrons and the reason for their power-law index and high-energy cut-off are still largely unknown. Here, we propose that the nonthermal electrons originate from within the black hole's innermost stable circular orbit (the ''plunging region''). Using an analytic model for the plunging region dynamics and electron distribution function properties from particle-in-cell simulations, we outline a steady-state model that can reproduce the observed spectral features. In particular, our model reproduces photon indices of $\Gamma\gtrsim2$ and power-law luminosities on the order of a few percent of the disk luminosity for strong magnetic fields, consistent with observations of the soft state. Because the emission originates so close to the black hole, we predict that the power-law luminosity should strongly depend on the system inclination angle and black hole spin. This model could be extended to the power-law tails observed above 400 keV in the hard state of X-ray binaries.Comment: 10 pages, 8 figures. Accepted in MNRA

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

International audienceDwarf novae (DNe) are accreting white dwarfs that show eruptions caused by a thermal-viscous instability in the accretion disk. The outburst timescales constrain α, the ratio of the viscous stress to the thermal pressure, which phenomenologically connects to the mechanism of angular momentum transport. The eruptive state has α  ≈  0.1 while the quiescent state has α  ≈  0.03. Turbulent transport that is due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. The presence of a large-scale poloidal field threading the disk is known to enhance MRI-driven transport. 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 net vertical constant magnetic field, as could be provided by the white dwarf or by its stellar companion, 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  ≈  0.1. A major difference between simulations with a net poloidal flux and simulations without a net flux is that angular momentum transport in the former is shared between turbulent radial transport and wind-driven vertical transport. We find that wind-driven transport dominates in quiescence even for moderately low magnetic fields ∼1 G. This can have a great 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 to the dependence of α with β, the ratio of thermal to magnetic pressure, and Teff, the effective temperature of the disk, as well as a prescription for the wind torque as a function of β that is in agreement with both local and global simulations. We conclude that the evolution of the thermal-viscous instability, and its consequences on the outburst cycles of CVs, needs to be thoroughly revised to take into account that most of the accretion energy may be carried away by a wind instead of being locally dissipated.Key words: accretion / accretion disks / magnetohydrodynamics (MHD) / turbulence / convection / stars: dwarf nova