Extragalactic jets and their interaction with their environment

Abstract

We describe three optical observational projects, each carried out in conjunction with studies at other wavelengths. These emphasize the usefulness of multifrequency studies in elucidating the nature of extragalactic jets. The first part concerns regions of optical line and continuum emission found at the sites of X-ray-radio knots in the inner 2 kpc of the Centaurus A jet. We show that these regions are physically associated with the jet and cite evidence for shock excitation within the emitting clumps. The full-spectrum continuum emission is attributed to the synchrotron process. Next we report the preliminary results of a search for optical emission in the fields of five radio jets. In the field of Abell 194 we find evidence for a jet-galaxy interaction. The optical-emission region known as Minkowski's Object is found to lie in the path of a radio jet. Spectroscopic investigation supports the view that it results from a burst of star formation, triggered by the impact of the jet on a cloud of material in its path. Last, we describe simultaneous multifrequency observations of the highly variable BL Lac object Mkn421. Dramatic changes in X-ray flux on time scales of weeks are not reproduced at other wavelengths. The X-ray variations are such that the lowest energy photons undergo the least change. These results allow some discrimination between the various theoretical jet models that have been proposed to describe the properties of BL Lacs. We also discuss possible interpretations for shorter time-scale (~10 000 s) variations with 10% amplitude, which were observed in X-ray light curves when the source was in an X-ray bright state.