Abstract / Description of output
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.
Original language | English |
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Pages (from-to) | 57-63 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 363 |
Publication status | Published - 1 Oct 2005 |
Keywords / Materials (for Non-textual outputs)
- quasars: general
- accretion
- galaxies: active
- accretion discs