Blue, Green, and Red Bumps in AGN

Abstract

I show that the summed spectral energy distribution (SED) $L(\nu )$ 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 $L^{\prime }(\nu )=X^{3}L(\nu /X)$. This should for example apply to accretion disks around black holes, where $X$ is relative accretion rate, or external heating rate, but will not apply to changes in black hole mass. I summarise evidence that AGN optical-UV SEDs become progressively redder with decreasing luminosity, and show that the trend in colour versus luminosity shown by Mushotzky and Wandel (1989) 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 {\em 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 {\em 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.

Accepted for publication in MNRAS, 8 pages, 5 figures

References in corpus (3)

• Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
• A Composite Extreme Ultraviolet QSO Spectrum from the Far Ultraviolet Spectroscopic Explorer
• The buried Balmer-edge signatures from quasars

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• Accretion Disc-Jet Decomposition from the Optical-Near Infrared Monitoring of Fermi Blazars