Ultraviolet and optical variations in active galactic nuclei - The Seyfert 1 galaxy NGC 5548
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Abstract
We describe the results of a project to study the combined ultraviolet- optical spectrum of the Seyfert I galaxy NGC 5548 over the period 1978- 1986. Of the many spectrophotometric observations made with a typical resolution of 5-8 A, 11 epochs were simultaneous and nine cover the range from 1150 to 7000 A. The combined IUE and ground-based observations, frequently obtained only a few days apart, enable us to study, for the first time, the time variability of many UV and optical lines, the overall shape of the nonstellar continuum, and the relations between the two. The variable continuum was found to be well represented by a power law, Fnu_ is proportional to ν^-α^, with α varying between 0.1 and 0.5 over 1.5 mag changes in the UV continuum. We have measured the strength of the Balmer continuum and the Fe II blends with the aid of theoretical models and found small variations in the strengths of the Fe II multiplets in the UV only, and large variations in the Balmer continuum. The detailed continuum analysis allowed us to study the line profiles and show that they are well represented by five components. The narrow (< 1400 km s^-1^) component in most lines does not vary with the continuum except for Lyα where the variations suggest a 3 year delay with respect to the continuum variations. A very broad component (10,000 km s^-1^) is seen in all high-excitation lines, most notably in He II, while a broad (5000 km s^-1^) component produces most of the flux in the hydrogen Balmer lines, and the low-excitation lines such as Mg II λ2798. In some lines there are also "blue" and "red" broad components. The behavior of the continuum and individual line components is studied in detail. We find among other results, that: a) The very broad component has a blueshift of 2000 km s^-1^, possibly indicating obscuration in the inner-most parts of an accretion disk; b) Unlike Lyα, the very broad C IV λ1549 does not continue to increase with the UV continuum at large flux levels. This may indicate a transition to C^+4^, a matter bounded situation, or it could be a consequence of hysteresis type behavior in undersampled data. c) For the broad component, the ratio C III] λ1909/C IV λ1549 is independent of the continuum luminosity, suggesting a large ionization parameter. d) The Balmer decrement in the broad component is smaller when the continuum is brighter. Lyα and Hγ increase more than Hα and Hβ with the increasing UV flux. e) The He II lines are very broad and change by a factor of up to 4. They seem to react quickly to UV continuum changes, and there is no need to invoke "accretion" events to explain their variability.
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