Ultraviolet variability of NGC 5548 - Dynamics of the continuum production region and geometry of the broad-line region

Abstract

We used the data obtained in the 1989-1990 IUE monitoring of NGC 5548 to derive a mean shape for the ionizing spectrum; mean line profiles; the continuum fluctuation power spectrum in three UV bands; cross-correlation functions relating these bands; cross-correlation functions relating fluctuations in eight emission lines to fluctuations in the UV continuum; and the response functions through which the continuum fluctuations appear to generate the fluctuations in each of these eight lines. The mean shape of the UV continuum is well fit by an accretion disk model with a black hole mass of 1 x 10^8^h^-1^/sin i M_sun_; however, an additional component is required to reproduce the observed soft X-ray flux. Construction of mean line profiles gives both a separation of the broad and narrow components in the lines and also a measurement of the velocity widths for the broad components. The widths of the lines increase with increasing ionization stage. We find that the continuum fluctuation power spectrum is very steep, with most of the variance coming from ~1 yr time scales. The entire optical/UV continuum rises and falls almost simultaneously, so that the logarithmic slope of the power spectrum (between -2 and -3) is nearly the same for all bands, but the flux at higher photon frequencies varies with larger amplitude. Within the context of thermal accretion disk models for the production of the UV continuum, these results lead to two important inferences: that orbital dynamics at the place where the radiation is generated do not control the dominant fluctuations; and the radial group speed of accretion disk temperature fluctuations is greater than c/10. A possible explanation for the near-simultaneity of fluctuations in the UV and optical bands is that much of the optical continuum is due to reprocessing by outer, cooler material of UV photons created closer in. The emission-line response functions map the marginal emission-line emissivity onto a set of paraboloidal isodelay surfaces. Combining the response functions with photoionization models, we find that the emission- line material around this Seyfert nucleus may best be described by a highly ionized inner zone of high and nearly constant pressure (nT ~ 10^15^ K cm^- 3^) that stretches from ~4-14 lt-days from the center, and an outer, more weakly ionized zone of considerably lower ionization at least ~20-30 lt- days out. Both zones are suggested by photoionization models of the mean line ratios. while the inner region is probably roughly spherical, there are indications that the outer region is flattened. It is possible that the low-ionization region forms a ringlike structure of radius ~100 lt-day which is roughly edge-on to the line of sight. Most of the mass in the emission-line region is in the low-ionization portion, but the bulk of the luminosity (excluding Fe II) comes from the high-ionization part. The dispersion in line-of-sight velocity for each line is well correlated with the lag at which the cross-correlation function with respect to the UV continuum peaks: σ is proportional to τ_l_^-0.51^. Such a correlation is consistent with the view that gravity is responsible for the motions of the line-emitting material if the central mass is ~10^7^-10^8^ M_sun_; this mass range is (barely) consistent with the mass estimated on the basis of the accretion disk spectral fit.