The Geometry and Kinematics of the Broad-Line Region in NGC 5548 from HST and IUE Observations
I. Wanders, M.R. Goad, K.T. Korista, B.M. Peterson, K. Horne, G. Ferland, A.P. Koratkar, R.W. Pogge, and J.C. Shields: 1995, ApJ 453, L87

The spatial and radial-velocity distribution of broad-line emitting gas in the Seyfert~1 galaxy NGC~5548 is examined through the process of reverberation mapping, which is done by detailed comparison of continuum and emission-line variations. Recent spectroscopic monitoring of NGC~5548 with HST and IUE allows us to resolve the ``transfer function'' (TF) that relates the continuum and emission-line variability. We also examine the radial velocity-resolved TFs, and confirm that predominantly radial motions of the line-emitting clouds can be excluded. We find that a broad-line region comprised of clouds that are orbiting a central source of mass $\sim\!10^{8}\,\Msun$ along randomly inclined Keplerian orbits and irradiated by a beamed continuum source yields a TF and line profile that are qualitatively consistent with the observations. In this model, the clouds that produce the variable \civ\ emission lie within 12 light days of the central source, and the continuum radiation is confined to a wide biconical beam (semi-opening angle 35 -- 60$^\circ$) with the observer viewing into the cone.