We study the role of adiabatic index in determining the critical points in the transonic low angular momentum accretion flow onto a black hole. We present the general relativistic 2D hydrodynamic simulations of axisymmetric, inviscid accretion flows in a fixed Kerr black hole gravitational field. A relativistic fluid where its bulk velocity is comparable to the speed of light, flowing in the accretion disk very close to the horizon can be described by an adiabatic index of 4/3 < γ < 5/3. The time dependent evolution of the shock position and respective effects on mass accretion rate and oscillation frequency with varying adiabatic index is discussed in the context of the observed microquasars.

We compute the evolution of a quasi-spherical, slowly rotating accretion flow around a black hole, whose mass and spin evolve adequately to the mass-energy transfer through the horizon. Our model is relevant for the central engine driving a long gamma ray burst, that originates from the collapse of a massive star. The computations of a GRB engine in a dynamically evolving spacetime metric are important specifically due to the transient nature of the event, in which a huge amount of mass is accreted and changes the fundamental black hole parameters, its mass and spin, during the process. We discuss the results in the context of angular momentum magnitude of the collapsing star. We also study the possible formation and evolution of shocks in the envelope, which may temporarily affect accretion. Our results are important for the limitations on the mass and spin range of black holes detected independently by electromagnetic observations of GRBs and gravitational waves. We speculate on the possible constraints for the final masses and spins of these astrophysical black holes. It is shown that the most massive BHs were rahter not formed in a powerful GRB explosion if the cores of their progenitors were only weakly rotating.

Structured jets have recently been invoked to explain the complex emission of gamma-ray bursts (GRBs), such as GW170817. Based on accretion simulations, the jets are expected to have a structure that is more complex than a simple top-hat structure. Also, the structure of the launch regions of blazar jets should influence their large-scale evolution. This was recently revealed by the interactions of jet components in TXS 0506+056, where the jet was observed at a viewing angle close to zero. Observational studies have also shown an anticorrelation between the jet variability, measured, e.g., by its minimum variability timescale, and the Lorentz factor, which spans several orders of magnitude and covers both blazars and GRBs samples. Motivated by those observational properties of black hole sources, we investigate the accretion inflow and outflow properties by means of numerical gamma-ray MHD simulations. We perform axisymmetric calculations of the structure and evolution of a central engine, composed of a magnetized torus around a Kerr black hole that is launching a nonuniform jet. We probe the jet energetics at different points along the line of sight, and we measure the jet-time variability as localized in these specific regions. We quantify our results by computing the minimum variability timescales and power density spectra. We reproduce the MTS–Γ correlation and we attribute it to the black hole’s spin as the main driving parameter of the engine. We also find that the power density spectral slope is not strongly affected by the black hole’s spin, while it differs for various viewing angles.

Cygnus X-1 is one of the brightest X-ray sources observed and shows the X-ray intensity variations on timescales from milliseconds to months in both the soft and hard X-rays. The accretion onto the black hole is believed to be wind fed due to focused stellar wind from the binary companion HDE-226868. We aim to understand the physical mechanism responsible for the short timescale X-ray variability (<100 s) of the source in its hard/low state. We compute the 2D relativistic hydrodynamic simulation of the low angular momentum accretion flow with a time-dependent outer boundary condition that reflects the focused, clumpy wind from the supergiant in this X-ray binary system. We follow the dynamical evolution of our model for about 100 s and present the results showing an oscillatory shock, being a potential explanation of variability observed in hard X-rays. The simulated model with shock solutions is in good agreement with the observed power density spectra of the source.