Spectral Evolution in Blazars

Abstract

Active Galactic Nuclei (AGN) are among the most powerful objects in the universe. In their centre they host a supermassive black hole (BH) with up to 10¹⁰ solar masses and an accretion disk is formed around them feeding the system. A fraction of the in-falling mater is ejected perpendicularly to the accretion disk forming the so-called jets. These relativistic flows are highly collimated and propagate up to kiloparsec distances from their central engine. The observed emission of AGN jets shows strong variability throughout the electro-magnetic spectrum which reflects variations in source intrinsic parameters such as the magnetic field and the rest-mass density. The variation in the emission of AGN jets can be best studied in their most powerful representatives, the blazars (AGN jets seen under a small viewing angle).<br /> The blazar CTA 102 underwent a historic radio outburst in April 2006 which provides a perfect laboratory for studying the spectral evolution. CTA 102 has been a target of single-dish and VLBI observations for several years. In this work we use both kind of observations to study and model the spectral evolution during the flare. We use the dense sampling of the single-dish observations to trace the evolution of the flare in the turnover-frequency and turnover flux density plane and modelled the results with a modified shock-in-jet model, assuming a travelling shock recollimation shock interaction. <br /> To test this hypothesis, we combine archival VLBI observations from the MOJAVE program (15 GHz) and Boston University Blazar Monitoring program (43 GHz) with our multi-frequency VLBI observations during the 2006 flare. The VLBI kinematic provides a unique view on the parsec-scale structure of CTA 102 over the last 15 years and reveals several stationary features. Our hypothesis of a shock-shock interaction as possible mechanism behind the 2006 is confirmed by a detailed spectral analysis of the multi-frequency VLBI observations. <br /> We use 2D relativistic hydrodynamic simulations (RHD) to bridge the sparse time sampling of the observations and to further investigate the non-linear process of travelling shock recollimation shock interaction. From the simulations we compute the non-thermal emission taking adiabatic and radiative losses into account. The synthetic single dish spectra and radio maps can reproduce the observed structure in the VLBI maps and variation in the single dish spectra during the flare. In addition, we present observable predictions for the interaction between a travelling shock and a recollimation shock.