Modeling the X-ray variability of microquasars within the framework of the Jet Emitting Disk model

Microquasars are binary systems in which one of the components is a black hole. They undergo outbursts from time to time, becoming extremely bright in X-rays. During these outbursts, their spectrum evolves between so-called “hard” states, highly variable, dominated by a power-law-type emission up to 100 keV and showing signatures of powerful radio jets, and so-called “soft” states, which are much less variable and dominated by soft X-ray emission (keV) originating from an accretion disk, where the jet has disappeared. The emission observed in the hard states requires the presence of a hot plasma (several billion degrees), called “hot corona”, whose physical origin is still debated. Recent constraints from X-ray polarimetry measurements by the IXPE satellite, launched in 2021, suggest that this corona is located in the inner regions of the accretion flow.
For several years, we have been developing at IPAG an accretion-ejection model, the Jet Emitting Disk (JED, Ferreira et al. 2006; Marcel et al. 2018a,b, 2019, 2020, 2022). A JED is a disk threaded by large-scale magnetic field lines that enable the formation of a jet, as observed in the hard state of microquasars. The impact of the jet on the disk is significant, extracting both mass and angular momentum. As a result, a JED has properties very different from those of a standard, non-ejecting disk. In particular, its accretion velocity is much higher and, for a given accretion rate, its density is therefore much lower. We have shown that the inner regions of a JED can reach very high temperatures and thus reproduce the hard spectra of microquasars. During his PhD (2015–2018), G. Marcel developed a Python code, DYPLO, which makes it possible to compute the spectrum produced by arbitrary JED configurations. This model has been successfully applied to various sources (Petrucci et al. 2010; Marcel et al. 2019; Barnier et al. 2021; Marino et al. 2021) and is now well recognized within the community. However, the model is currently stationary and cannot reproduce the observed X-ray variability.

The goal of this PhD project is therefore to make the JED model time-dependent. This will be achieved through two approaches. The first will focus on applying an analytical method for the propagation of fluctuations in a JED structure, following the formalism developed by Malzac et al. (in preparation). This method has been successfully applied to reproduce the timing properties of the microquasar MAXI J1820+070. The idea is to apply this formalism to additional observations of this object, as well as to other microquasars, using the many archival datasets available (notably from the RXTE satellite, but also from XMM, NICER, NuSTAR, and HXMT). This work is expected to occupy the first year of the PhD.
The second approach, which is at the heart of this PhD, will involve the use of the Monte Carlo code MONK. MONK is a Monte Carlo code developed by Dr. W. Zhang, with whom we collaborate. It properly accounts for the geometry of the accretion flow, includes all radiative processes (both local and non-local), naturally incorporates time-delay effects related to the illumination of the outer regions of the accretion flow by the inner regions, and also includes general relativistic effects. The objective will be to implement, within MONK, the propagation of fluctuations in a JED structure computed by DYPLO.

Such a code, based on physically grounded accretion-ejection solutions, would be the first of its kind able to reproduce at the same time the spectral and temporal characteristics of microquasars. It will be directly compared to the many timing signatures that characterize microquasars (power spectra, time lags between light curves at different energies, QPOs, etc.) in order to better constraints the origin of their X-ray emission.