Is the slope of the intrinsic Baldwin effect constant ?

We investigate the relationship between emission-line strength and continuum luminosity in the best-studied nearby Seyfert 1 galaxy NGC 5548. Our analysis of 13 years of ground-based optical monitoring data reveals significant year-to-year variations in the observed Hβ emission-line response in this source. More specifically, we confirm the result of Gilbert and Peterson (2003) of a non-linear relationship between the continuum and Hβ emission-line fluxes. Furthermore, we show that the slope of this relation is not constant, but rather decreases as the continuum flux increases. Both effects are consistent with photoionisation model predictions of a luminosity-dependent response in this line.