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.

We present the results of simulations done with the code HARM-COOL developed in the CTP PAS Warsaw research group over the years 2017-2019. It is based in the original GR MHD scheme proposed by Gammie et al. (2003) for the simulation of Active Galactic Nucleus, but now it has been suited for the engine of a short Gamma Ray Burst event. We compute time-dependent evolution of a black hole accretion disk, in two-dimensional, axisymmetric scheme. The code includes neutrino cooling and accounts for nuclear structure of dense, degenerate matter. Free protons, neutrons, and electron-positron pairs form a neutron-rich, magnetically driven outflow that provides site for subsequent r-process nucleosynthesis. Here the heavy elements up to the Uranium and Gold are synthesized and may contribute to the chemical enrichment of the circumburst medium. Their radioactive decay will give signal in lower energies in a timescale of weeksmonths after the GRB prompt phase. In addition, the magnetic fields are responsible for the launching of ultra-relativistic jets along the rotation axis of the central black hole, according to the well-known Blandford-Znajek mechanism. These jets are sites of variable high energy emission in gamma rays. We find that the magnetic field and the black hole spin account for the observed variability timescales and jet energetics.

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We investigate the dependence of the gamma-ray burst (GRB) jet structure and its evolution on the properties of the accreting torus in the central engine. Our models numerically evolve the accretion disk around a Kerr black hole using three-dimensional general relativistic magnetohydrodynamic simulations. We use two different analytical hydrodynamical models of the accretion disk, based on the Fishbone–Moncrief and Chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star (BNS) merger, respectively. We impose poloidal magnetic fields of two different geometries upon the initial stable solutions. We study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. The jets produced in our models are structured and have a relatively hollow core and reach higher Lorentz factors at an angle ≳9° from the axis. The jet in our short GRB model has an opening angle of up to ∼25° while our long GRB engine produces a narrower jet, of up to ∼11°. We also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. The application of our models to the GRB jets in the BNS postmerger system and to the ultrarelativistic jets launched from collapsing stars are briefly discussed.