I work on theoretical models and numerical simulations of black hole accretion process. My work is mainly focused on time-dependent magneto-hydrodynamical modeling of accretion flows, wind outflows and jet ejections. The aim is to explain emission properties of disks and jets observed in persistent, transient, and explosive sources, such as active galaxies, stellar mass black hole binaries, and gamma ray bursts.
We study the structure and time evolution of an accretion disk that powers relativistic jets in systems such as radio loud AGN and blazars, or Gamma Ray Bursts. In these sources ultra-fast, magnetized jets of plasma are launched on the cost of rotational energy of the black hole. The extraction of this energy is mediated by the magnetic fields, anchored to the black hole horizon. Toroidal magnetic fields in the jet are twisted because of the black hole rotation.
For these simulations, we are using my personal version of the code HARM. This General Relativistic MHD scheme has been extended form the 2D version into full 3D, and parallelized with a hybrid MPI-Open MP parallelisation scheme, originally developed in my work in the years 2013-18.
This code has been also supplemented with the nuclear equation of state module. This tabulated EOS is adequately describing hot and dense matter in the engine of a Gamma Ray Burst. The simulation results can be post-processed to study nucleosynthesis of heavy isotopes in the outflows from GRB engine.
The code is also developed to work in a dynamically updated Kerr metric, to mimic the evolution of the Kerr black hole which grows in mass and changes its angular momentum, during the collapse of a massive star. The code HARM_COOL has been used in multiple reserach papers, linked in this website.
The standard accretion disks accreting at high percentage of the Eddington rate exhibit thermal instabilities if their stress-energy tensor is supported by the radiation pressure. This instability may acts as a source of thermal runaway, and lead to the disk destruction, if only the system is not stabilized. The stabilizing mechanism was found to be the advective cooling, which removes some of the excess energy from the unstable disk regions and transports it radially towards the black hole. Hence, the instability does not cause the disk break-up, but manifests itself in a limit-cycle type of oscillations. This non linear hydrodynamical effect is known as the Radiation Pressure Instability.
I study the consequences of Prad instability in various types of accreting systems, from transient X-ray binaries, through intermediate mass black holes, up to the Active Galactic Nuclei. The timescales and amplitudes of the activity cycles, or flaring activities, are consistent with the quasi-periodic flare-like events observed in some microquasars. They are also plausible mechanism for Changing-Look AGN phenomenon.
My numerical calculations are utilizing the code GLADIS (for Global Accretion Disk Instability Simulation). The code has been designed and developed originally by myself, and is under development since my PhD in 2002. It enables for long-term time-dependent simuations of disk variability (viscous timescale) in its global scale (disk size of order of 1000 Schwarzschild radii).
The code was basis of multiple research articles, linked in this website.
