Broad-band properties of the CfA Seyfert galaxies. III - Ultraviolet variability

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Abstract

Six hundred fifty-seven archived IUE spectra are used to study the ultraviolet variability properties of the six members of the CfA Seyfert 1 galaxy sample with many repeated observations. All Show strong evidence for continuum and line variations. There is a tendency for less luminous objects to be more strongly variable. Most objects show a clear correlation at zero lag between ultraviolet spectral index and luminosity, in the sense that the spectrum becomes harder as the source becomes brighter. This is evidence that the variable component is an accretion disk around a black hole which is systematically smaller in less luminous sources. No correlation is seen between the continuum luminosity and equivalent width of the C IV, Mg II, and C III] emission lines when the entire sample of Seyfert 1 galaxies is examined. However, a clear anticorrelation is present when only repeated observations of individual objects are considered. This is explained by a combination of light-travel time effects in the broad- line region and the nonlinear response of lines to continuum fluctuations. Three objects (NGC 4151, NGC 5548, and 3C 273) have a sufficient number of observations to attempt to construct continuum autocorrelation and line- continuum cross-correlation functions. In the case of NGC 4151, the ultraviolet line emission appears to emanate from an unstratified region with a limit to the characteristic size scale <~ 40 lt-day. The data for the other two objects are not adequate to permit unambiguous detailed interpretation.

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