Numerical study of MHD instabilities and their impact on the multi-wavelength emission signatures of relativistic jets

Abstract

Highly collimated outflows from supermassive black holes, or more generally, from a “central engine" at the cores of active galaxies, are referred to as “relativistic jets". In 1918, M87, an elliptical galaxy in the Virgo cluster, made the first reported discovery of a jet. Our present notion is that jets originate from a SMBH present in the heart of the Active Galactic Nuclei (AGN). These black holes and possibly their associated accretion discs fuel the jets, which in turn carry energy, momentum, and angular momentum across enormously long distances despite being susceptible to a variety of instabilities. Blazars are a class of AGN, with the relativistic jet pointing along our line of sight. These are characterised by multi-timescale flux variability, high linear polarization, strong 훾-ray emission and spectra that cover the whole gamut of the electromagnetic spectrum. Despite progress on both observational and theoretical fronts, some fundamental issues still persist as a result of degeneracy in existing models or explanations for observed events. In this thesis, we aim to address the question of how the dynamics of jet instabilities manifest in terms of non-thermal emission. In particular, this thesis focuses on understanding the role played by kink and Kelvin Helmholtz instability in governing the observed features in light curves and the spectral energy distribution of relativistic and magnetized jets. We have performed high resolution three dimensional simulations of a representative section of a jet using relativistic magneto-hydrodynamics. The plasma column under consideration is prone to both kink and Kelvin-Helmholtz instability. A comprehensive parameter survey is performed to understand the impact of several key properties on the growth of instabilities. With the aim of bridging dynamical features to observables, we have adopted two strategies in terms of particle spectral evolution: (i) static particle spectra assumes a power-law non-thermal electron population that remains fixed in time, and (ii) evolving particle spectra uses a hybrid framework that couples Lagrangian particles (collection of leptons) with an Eulerian grid (underlying fluid). Here, the particle spectral evolution is governed by losses due to processes like synchrotron and External Compton and also particle acceleration due to shocks is accounted for. The work presented in this thesis has demonstrated the role of kink instability in the structural deformations of highly magnetized jets. These deformations translate into variable emission signatures when coupled with static particle spectral modelling. Further, the growth of instabilities and the variablity amplitude estimated from the synthetic lightcurve show a linear co-relation. This finding is consistent with the helical jet model for long term variability in blazars, whereby the emitting region of the jet lies along the line of sight, resulting in boosted emission. For cases with lower magnetization, such variablity is absent due to supression of kink instability and associated structural deformations. The salient feature from our study of coupling the evolving particle spectral model with the MHD unstable plasma column was the impact of turbulent shocks on emission signatures. The simulations presented in this thesis have clearly demonstrated the effects of shocks in re-accelerating particles to produce a secondary population of electrons near the localized shock sites. This localized secondary population of electrons exhibits itself in the form of flaring in the X-ray lightcurve and associated spectral hardening due to synchrotron emission. The subsequent radiative cooling of these electrons presents a sequential lag between the various multi-wavebands. Additionally, the spectral flattening due to shocks shows characteristics of slope variation in the broad band SED, particularly in the UV and X-ray range. All these synethetically observed features show a consistent match when qualitatively compared with typical multi-wavband emission from blazars. In summary, this thesis has resulted in the development of a state-of-the-art multi-zone and time-dependent modelling framework. This framework couples the micro-physics of shock acceleration and radiation losses to bridge the dynamical effects of magneto-hydrodynamic instabilities with the multi-waveband variability and spectral energy distribution of relativistic jets.