Variations of the ultraviolet Fe II and Balmer continuum emission in the Seyfert galaxy NGC 5548

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Abstract

We present measurements of the Balmer continuum/Fe II emission blend (the "small blue bump") between 2160 and 4130 A in the Seyfert galaxy NGC 5548. The measurements are from spectra obtained as part of the combined space-based and ground-based monitoring program of this object in 1988- 1989. An iterative scheme is used to determine and subtract the continuum emission underlying the emission blend so as to obtain a light curve sampled once every 4 days. The small blue bump is an important component of the emission-line cooling, constituting about one third of the line flux in this object. Its flux varies with an amplitude of approximately +/- 20% about the mean, similar to the amplitude of the Balmer line variations during the same period. Its light curve resembles that of Lyα, with a lag of ~10 days behind the continuum variations. The bump variation amplitude is independent of the wavelength interval where it is measured, which indicates that both the Balmer continuum and Fe II emission have comparable variation amplitudes. These results suggest that the Fe II UV multiplets and the Balmer continuum are emitted in the same parts of the broad-line region as most other broad emission lines in this object.

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