Accretion, obscuration and the X-ray background

Xavier Barcons, Instituto de Fisica de Cantabria (CSIC-UC)

It is now widely accepted that the cosmic X-ray background (XRB) is due to the summed emission from growing massive black holes. Much of this accretion occurs in obscured objects. However the quantitative description of all this is still "work in progress", mostly limited by observations. In this presentation I will review our current understanding (and where appliable on-going arguments) of key aspects like X-ray vs optical obscuration, accretion efficiency, evolution of absorption among others.