An Intrinsic Baldwin Effect in the H-beta Broad Emission Line in the   Spectrum of NGC 5548
  We investigate the possibility of an intrinsic Baldwin Effect (i.e.,nonlinear emission-line response to continuum variations) in the broad H-beta emission line of the active galaxy NGC 5548 using cross-correlation techniques to remove light travel-time effects from the data. We find a nonlinear relationship between the H-beta emission line and continuum fluxes that is in good agreement with theoretical predictions. We suggest that similar analysis of multiple lines might provide a useful diagnostic of physical conditions in the broad-line region. 
Karoline M. Gilbert and Bradley M. Peterson