Numerical simulations of hybrid accretion-ejection configurations in Black-Hole X-ray Binaries

Accretion disks are very common structures in astrophysics. They are found around forming stars (protoplanetary disks), interacting binary systems such as cataclysmic variables (where the primary is a white dwarf) and X-ray binaries (where the primary is either a neutron star or a stellar black hole), or around the supermassive black hole at the centre of active galaxies and quasars. In addition to plasma accretion signatures on the central object, all these systems also show signatures of jets (fast and very narrow bipolar flows) or winds (slow and massive, uncollimated flows), and sometimes even both, emitted perpendicular to the plane of the disk (Ray & Ferreira 2021, Petrucci et al. 2021).

It is now theoretically established that a large-scale vertical magnetic field passing through an ionised disk would generate both jets and winds, as well as their correlation with accretion (Ferreira 1997, Zimniak et al. 2024, 2026). Theoretical models show that it would be responsible not only for the mass loss of the disk but also for the accretion-ejection cycles in X-ray binaries, involving a hybrid disk structure. The inner part, very strongly magnetised and producing jets (called JED, Ferreira 1997), would give way beyond a transition radius to a weakly magnetised disk producing winds (called WED, Jacquemin-Ide et al. 2019), and then to a standard, non-ejecting disk (called SAD) (Ferreira et al. 2006, Marcel et al. 2022).

The objective of this thesis is to perform axisymmetric magnetohydrodynamic (MHD) simulations of JED-WED hybrid disks using MHD codes (PLUTO, https://plutocode.ph.unito.it/ and/or IDEFIX, https://github.com/idefix-code). The study will focus on the conditions necessary for the stationary formation of such hybrid disks, particularly by manipulating MHD turbulence profiles. Special attention will be paid to the dynamics of the transition zone between the JED and the WED, and to the interaction between the internal fast jet and the external slow wind.

Once the conditions for obtaining a stationary hybrid configuration have been established, numerous applications can then be explored in the context of X-ray binary systems. Among these, we will explore the impact of the evolution of the JED-WED transition radius on jet dynamics. Indeed, this radius varies during an activity burst (Marcel et al., 2022), with unexplored consequences on the collimation properties of the jets. Another potentially high-impact aspect will be the study of the coupling between accretion-ejection dynamics and disk thickness, itself dependent on the disk's thermal equilibrium (Marcel et al., 2018). Inaccessible with 3D simulations, this exploration can be easily conducted with 2D simulations.

Finally, an analytical and semi-analytical approach will also be considered. This will involve obtaining self-similar solutions for 'puffy' disks with low magnetisation (see Jacquemin-Ide et al., 2021). Such solutions do not yet exist, yet they are required to explain the winds emitted by WEDs in many astrophysical objects. Obtaining these new solutions will require leveraging the turbulent prescriptions used for stationary 2D numerical simulations.