Very high energy emission and multi-wavelength campaigns of the BL Lac object PKS 2155-304
This thesis is concerned with multi-wavelength observations of the prominent southern hemisphere high energy BL Lac object, (HBL), PKS 2155-304. After a general introduction covering the particulars of Active Galactic Nuclei, the pysical processes and models that are believed to play an important role in the observations and ground based γ-ray astronomy, the results and interpretation of four individual multi-wavelength campaigns on PKS 2155-304 are presented. Lead by observations of the High Energy Stereoscopic System (H.E.S.S.), these campaigns include observations with RXTE, NTT at ESO, ROTSE, Spitzer, JCMT and the NRT. During all these campaigns, the source appeared to be in a quiescent state, allowing us to probe the jet paramaters and emission models for a quiescent HBL for the first time. The models for VHE emission from HBLs have evolved over the last 14 years since Mkn 421 was detected as a VHE emitter. However, the sensitivity of the past generations of VHE telescopes has only been sufficient to observe VHE emission from HBLs during apparent active states. These evolved models are therefore essentially high active state emission models. Since the sensitivity of H.E.S.S. now allows us to detect these objects in a quiescent state , the obvious question is, do the 'active state' models still accurately predict the SED and in particular the VHE emission?