An investigation of variability and its associated synchrotron emission in relativistic AGN jets using numerical hydrodynamic simulations

[摘要] English: Active regions at the centres of certain galaxies known as Active Galactic Nuclei (AGN) aresome of the most energetic and violent sources of emission in the universe. Certain types ofAGN can produce jet-like emission structures that extend hundreds of kiloparsec in length.The jet-like sources show intricate time dependent structure and are believed to consistof collimated flows of relativistic plasma. Many studies have focused on investigating thestructure and emission of these sources. The evolution time scale of the jets are much longerthan their recorded history which makes observational studies of their evolution challengingand, due to the relativistic nature of these jets, they have not been accurately reproducedin laboratory experiments. Instead many studies have employed fluid dynamic numericalsimulations of these sources to study their properties. To accurately compare a fluid dynamicsimulation to that of observational data the emission emitted by such an environment mustbe modelled. In this study a fluid dynamic simulations of a relativistic jet is constructedand a synchrotron emission model is applied to the simulations to reproduce intensity mapsat radio frequencies which is comparable to observational data of AGN jet sources. Thenumerical fluid dynamic simulation was created and evolved using the PLUTO software andconsisted of a three dimensional environment containing ambient medium, into which a jetis injected through a nozzle on the lower z boundary. The injected material consisted of aless dense medium with a super-sonic bulk motion of Lorentz factor T = 10. The simulationreproduced a jet structure containing a relativistic beam of material propagating throughthe ambient medium. The beam of material was surrounded by a turbulent cocoon regionwith asymmetric structure. The entire structure was encased in a bow shock. Intensity mapsof the three dimensional fluid simulation were created by applying a post-processing code tothe simulation data. The emission model estimated the synchrotron emission by assumingthat the entire population of electrons in the jet had a power-law energy distribution. Theintensity maps were able to reproduce emission structures that resemble those of FR IItype radio galaxies with a dominant cocoon region containing time dependent hot spots and laments. To investigate the effects of Doppler boosting, intensity maps were calculatedat different polar angles and the results were consistent with the current unified model ofAGN and showed a significant increase in the intensity of the relativistic beam at small polarangels. The intensity maps were able to reproduce time dependent emission structures due tofluid dynamic instabilities that formed during the simulation. The time dependent structureled to the production of variability with an amplitude of �? 10% in the total intensity. Itwas therefore shown that some variability observed within these sources occurs due to fluiddynamic instabilities rather than a change in the injection parameters. However, large flareswhich have been observed from these sources require additional perturbations in the flow.This study serves as a good basis for future in depth investigation of AGN emission.

[发布机构] University of the Free State