Magneto-hydrodynamic instabilities in jets : dynamical and emission properties

Abstract

The e↵ect of MHD instabilities on the non-thermal continuum emission spectra and dynamics of kilo-parsec scale jets is being investigated. The dynamical evolution of a cylindrical jet configuration with a helical magnetic field and a radially sheared axial flow has been studied using non-relativistic three-dimensional numerical simulations. The helical magnetic field makes the jet prone to kink instabilities whereas the case with a uniform magnetic field exhibits Kelvin Helmholtz instability on account of MHD turbulence. The kink instabilities suppress vortex growth at the jet surface by enhancing the azimuthal magnetic field which has a stabilizing e↵ect on the jet. We have developed a simplistic prescription of emission modeling to study the e↵ects of instabilities on the jet emission - the multi-zone model. The results in a limiting case of this model- a “ Onezone model” have been validated using an open-source package, the NAIMA code. The radiative models include synchrotron and inverse-Compton emission. Multi-zone SED modeling has been carried out for the synchrotron component alone for accurate modeling of the continuum spectra assuming a power-law distribution of relativistic cosmic-ray electrons and a line of sight inclined at two di↵erent viewing angles with the jet axis. The total integrated flux levels for the jet with a helical magnetic field are higher as compared to the one with a uniform axial field due to the additional azimuthal magnetic field. The di↵erence between the two drops with increasing values of the line of sight inclination ✓. As the multi-zone model fails to capture localized physical e↵ects such as shocks and particle acceleration, a hybrid macro-particle based model has been used for more accurate emission modeling. The e↵ect of jet velocity on the emission has been studied for the helical and uniform magnetic field cases by comparing the results from both the multi-zone and the hybrid macro-particle based models for three di↵erent magnitudes of the jet velocity on-axis. If the Kelvin-Helmholtz instability dominates, it disrupts the flow causing shock formation which results in a flatter emission spectrum whereas the interaction between the kink and Kelvin-Helmholtz instabilities has a stablilizing e↵ect on the jet making the emission spectra steep. Having studied the e↵ects of MHD instabilities on the jet emission, our future investigations will focus on the polarization aspect of the jet emission to probe into the magnetic field structure.