The AGN phenomenon: open issues
  The aim of this short paper is to motivate and encourage research in the
field of Active Galactic Nuclei (AGN). Here we summarize the main open
questions concerning the central engine. Is the central black hole rapidly
spinning and can we prove this? What is the dominant accretion mechanism in
AGN? Why do some AGN form jets while others don't and how do the jets
originate? What keeps jets collimated out to distances of 100 kpc? Is the
emission of blazars dominated rather by synchrotron self-Compton or by external
Compton processes? Which parameters are important in the unified model? We
outline the status of related research, formulate the questions and try to hint
at research projects able to tackle these fundamental topics. Deep surveys,
polarization measurements, improved models, faster and more accurate
simulations as well as bridging the gap in the MeV range can be part of the
tools to bring us closer to an understanding of AGN.
