Variability of FeiiEmission Features in the Seyfert 1 Galaxy NGC 5548

We study the low-contrast Fe II emission blends in the ultraviolet(1250--2200A) and optical (4000--6000A) spectra of the Seyfert 1 galaxy NGC5548 and show that these features vary in flux and that these variations arecorrelated with those of the optical continuum. The amplitude of variability ofthe optical Fe II emission is 50% - 75% that of Hbeta and the ultraviolet Fe IIemission varies with an even larger amplitude than Hbeta. However, accuratemeasurement of the flux in these blends proves to be very difficult even usingexcellent Fe II templates to fit the spectra. We are able to constrain onlyweakly the optical Fe II emission-line response timescale to a value less thanseveral weeks; this upper limit exceeds all the reliably measured emission-linelags in this source so it is not particularly meaningful. Nevertheless, thefact that the optical Fe II and continuum flux variations are correlatedindicates that line fluorescence in a photoionized plasma, rather thancollisional excitation, is responsible for the Fe II emission. The ironemission templates are available upon request.Comment: 34 pages including 12 figures and 2 tables. Accepted for publication by ApJ (tentatively in vol. 626 June 10, 2005