Variability of magnetically-dominated jets from accreting black holes

Abstract: Structured jets are recently invoked to explain the complex emission of gamma ray bursts, such as GW 170817. Based on the accretion simulations, the jets are expected to have a structure that is more complex than a simple top-hat. Also, the structure of launching regions of blazar jets should influence their large scale evolution. This is recently revealed by the interactions of jet components in TXS 0506+056, where the jet is observed at a viewing angle close to zero. Observational studies have also shown an anti-correlation between the jet variability, measured e.g. by its minimum variability time scale, and the Lorentz factor, that 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 GR MHD simulations. We perform axisymmetric calculations of the structure and evolution of central engine, composed of magnetized torus around Kerr black hole that is launching a non-uniform 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-$\Gamma$ correlation and we attribute it to the black hole spin as the main driving parameter of the engine. We also find that the PDS slope is not strongly affected by the black hole spin, while it differs for various viewing angles.