2005MNRAS.363...57L

I show that the summed spectral energy distribution (SED) L(ν) of any extended blackbody radiator will scale in a predictable way if all parts of the body change in temperature by the same factor X, such that L'(ν) =X³L(ν/X). This should, for example, apply to accretion discs around black holes, where X is relative accretion rate, or external heating rate, but will not apply to changes in black hole mass. I summarize evidence that active galactic nucleus (AGN) optical-ultraviolet (UV) SEDs become progressively redder with decreasing luminosity, and show that the trend in colour versus luminosity shown by Mushotzky & Wandel is matched extremely well by taking a template high-luminosity SED and scaling it in the manner described above. This agreement is striking because it involves no adjustable parameters. The agreement breaks down at low luminosities because of stellar contamination and reddening. I then consider the colour changes of an individual AGN (NGC 5548) during luminosity changes, which according to the popular X-ray reprocessing model should follow the scaling law well. However the observed changes are clearly not consistent with the simple scaling prediction. Instead, these colour changes are quite well explained by the mixing of a constant red component and variable blue component. Overall, there is then strong support for the ideas (i) that AGN optical-UV SEDs arise from accretion discs, (ii) that accretion rate plays a significant role in the very large range of luminosity seen in AGN, and (ii) that the inner regions of AGN vary independently of the outer accretion disc.