Left: the SED of BL Lacertae showing the expected emission from the host galaxy (just below the observed non-thermal radiation, giant elliptical, orange line) and the blue bump emission (blue line, see also Raiteri et al. 2009). Right: the SED of 3C273 showing the thermal emission from the blue bump and the expected X-ray emission from accretion including 1, 2, and 3σ bands (purple and blue lines) derived from Eq. (2). The vertical parallel lines represent the optical window (4000–10000Å). The green lines are the best-fit to the simultaneous non-thermal data using a third degree polynomial function.