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Unformatted text preview: Structure and Central Masses in Active Nuclei AGNs have been observed to vary everywhere from the X-ray to radio regimes. This variability is associated with the continuum (core) sources and the BLR, with NLR variability mild (as would be inferred from its large size). Early work used broad-band photographic data (from archival plates, for example, giving spotty coverage over spans as large as 90 years) or photoelectric observations (accurate but time-consuming). These are sensitive mostly to the continuum (at low redshifts contamination by lines is usually of order 15%). The variations may be episodic (turning on and off, frustrating planning of major campaigns). They are of different kinds - flares, dips, with multiple timescales present. Timescales range from days to decades in the optical. Most QSOs are only slightly variable - 5% or so over a couple of years. Many of these results are summarized in Variability of Active Galactic Nuclei , ed. Miller and Wiita (Cambridge, 1991). In the continuum, one might look for the spectral shape of the variable component - if nothing else, there should be a nonvariable componant from the surrounding galaxy, and possible from any other large-size continuum components. In Seyferts and QSOs, the bluer wavelengths are generally most variable, and there is little or no variability in the IR. A few violently variable (OVV) QSOs and the Bl Lac objects often show "grey" variations, with no wavelength...
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