Echo mapping of broad H-beta emission in NGC 5548

A one-dimensional echo map of the broad Hβ emission-line region in the Seyfert 1 galaxy NGC 5548 is reconstructed by using a maximum entropy technique to model the variable optical continuum and integrated Hβ emission-line fluxes observed during the 1989-1990 monitoring campaign as reported by Peterson et al. The echo map has a strong peak at a time delay of 20 lt-days which has an unresolved full width of 20 days at half-maximum. The Hβ response at time delay zero is less than one-third of that at 20 days, implying a deficit of Hβ-emitting gas near the line of sight to the continuum source. This rules out spherically symmetric and edge-on disk geometries for isotropically emitting Hβ clouds. Further monitoring will be necessary to determine the reliability of secondary structure in the echo map at 100-180 days.