The Seyfert 1 galaxy NGC5548 has been monitored between 1988 December and 1989 October in the optical by an international collaboration (Peterson et al., 1991). Parallel to the optical monitoring, this galaxy was observed every 4 days with the IUE-satellite from Dec. to Aug. (Clavel et al., 1991). The internal calibration of the spectra was done by scaling with respect to narrow forbidden lines.

The optical emission lines Hα, Hβ, HeI5876, and HeII4686 show the same variability pattern as the UV continuum, the Lyα, and CIV1548 lines, but with different delays for the various lines (Fig. 1a — d) (Dietrich, Kollatschny, et al., 1992). We estimated the extent of the broad line region for different lines by cross-correlating the UV-continuum and the emission lines. For the Hα and Hβ lines we got lags of 17 and 19 days respectively. The helium lines originate closer to the central continuum source, as indicated by delays of 7 days (HeII4686) and 11 days (HeI5876).

In Figure 2a,b we have plotted the temporal evolution of the difference spectra of Hα and Hβ for several epochs with respect to that epoch when the galaxy was in the minimum state. The full width at zero intensity (FWZI) remained constant. This indicates a turbulent velocity field in the broad-line region of NGC5548. Furthermore, a blue component (vblue ≈ −2000kms−1) is visible in addition to the central component. This blue component varies independently and with a different amplitude than the central component (Kollatschny and Dietrich, 1991).