The impact of thermal gas in AGN jets on the low-frequency emission^*

¹ Institut für Theoretische Physik, Lehrstuhl IV, Ruhr-Universität Bochum, 44780 Bochum, Germany e-mail: ms@tp4.ruhr-uni-bochum.de
² Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA

Received: 29 March 2004
Accepted: 25 May 2004

Abstract

We study the effect of non-relativistic, thermal matter in the jets of active galaxies (AGN) on the low-frequency non-thermal emission and the variability thereof. In matter-dominated jets, sizable quantities of gas should exist, in particular in the compression zones near the collision fronts that are an implicit ingredient of Fermi-type particle acceleration scenarios. Non-relativistic thermal gas in AGN jets noticably contributes to the optical depth at radio to infrared frequencies, and much less to the emission, with an efficiency that is strongly temperature-dependent. The observable flux of low-frequency emission is thus modulated by the temperature evolution of the thermal gas, and it can therefore display very complicated variability. For a particular particle energisation scenario we calculate the temperature evolution of the thermal plasma as well as the radiation transport of low-frequency emission, and thus derive simulated light curves at different frequencies and their typical correlation properties.