Numerical Simulations of Black Hole Accretion Flows

Abstract

We model the structure and evolution of black hole accretion disks using numerical simulations. The numerics is governed by the equations of general relativistic magneto-hydrodynamics (GRMHD). Accretion disks and outflows can be found at the base of very energetic ultra-relativistic jets produced by cosmic explosions, so called gamma-ray bursts (GRBs). Another type of phenomena are blazars, with jets emitted from the centers of galaxies.Long-lasting, detailed computations are essential to determine the physics of these explosions, and confront the theory with potential observables. From the point of view of numerical methods and techniques, three ingredients need to be considered. First, the numerical scheme must work in a conservative manner, which is achieved by solving a set of non-linear equations to advance the conserved quantities from one time step to the next. Second, the efficiency of computations depends on the code parallelization methods. Third, the analysis of results is possible via the post-processing of computed physical quantities, and visualization of the flow properties. This is done via implementing packages and libraries that are standardized in the field of computational astrophysics and supported by community developers.In this paper, we discuss the physics of the cosmic sources. We also describe our numerical framework and some technical issues, in the context of the GRMHD code which we develop. We also present a suite of performance tests, done on the High-Performance Computer cluster (HPC) in the Center for Mathematical Modeling of the Warsaw University.

GRMHD Evolution of Interacting Double Accretion Tori Orbiting a Central Black Hole

“…The evolution of the GR hydrodynamical RAD configuration considered in Pugliese & Stuchlík (2017) has been tested in the simplest case of double torus configuration around a central Schwarzschild BH using 2.5D GRMHD numerical simulation in the HARM code Gammie et al (2003), Janiuk et al (2018), Sapountzis & Janiuk (2019.…”

“…Recently, there have been different numerical GRMHD simulations based on the force-free (infinite conductivity) approximation (for the inverse zero conductivity approximation, see Stuchlík et al 2020) demonstrating the BH magnetosphere in a more sophisticated way Tchekhovskoy (2015), Nakamura et al (2018), but a simple, realistic, and well-accepted model for the BH magnetosphere is still missing. Due to the GRMHD simulations of the accretion processes, one can expect that the BH magnetosphere has timedependent complicated structure Tchekhovskoy (2015), Janiuk et al (2018). At small scales, the turbulent magnetic field inside the accretion disk is very important, since it enables the angular momentum transport inside the accretion disk due to the MRI Sapountzis & Janiuk (2019).…”

“…HARM is a conservative shock-capturing scheme for evolving the equations of GRMHD, developed by C. Gammie et al (Gammie et al 2003) and later improved, parallelized, and released as HARM COOL (Janiuk et al 2018;Palit et al 2019;Sapountzis & Janiuk 2019). The HARM code is GRMHD code with ideal conductivity and without including electromagnetic radiation, but some dissipative processes can still be observed due to numerical resistivity that is present in the code.…”

“…On the other hand, the exact shape of the EM field around the BH, i.e., the BH magnetosphere, is still not yet properly resolved, although a strong connection to the accretion processes is evident (Komissarov 2004;Punsly 2009;Meier 2012). A simple and elegant solution of a uniform magnetic field (Wald 1974) could be used as a basic simple approximation to the real BH magnetosphere model, but from GRMHD simulations of accretion processes, one can expect that the BH magnetosphere has a more complicated structure, eventually changing in time (Tchekhovskoy 2015;Janiuk et al 2018). At small scales, the turbulent magnetic field inside the accretion disk is very important, since it enables the angular momentum transport inside the disk due to the magnetorotational instability (MRI; Balbus & Hawley 1991;Sapountzis & Janiuk 2019).…”