Photoionized Hβ emission in NGC 5548:: it breathes!

Emission-line regions in active galactic nuclei (AGNs) and other photoionized nebulae should become larger in size when the ionizing luminosity increases. This 'breathing' effect is observed for the H beta emission in NGC 5548 by using H beta and optical continuum light curves from the 13-yr (1989-2001) AGN Watch monitoring campaign. To model the breathing, we use two methods to fit the observed light curves in detail: (i) parametrized models and, (ii) the memecho reverberation-mapping code. Our models assume that optical continuum variations track the ionizing radiation, and that the H beta variations respond with time-delays tau due to light travel-time. By fitting the data using a delay-map Psi(tau, F-c) that is allowed to change with continuum flux F-c, we find that the strength of the H beta response decreases and the time-delay increases with ionizing luminosity. The parametrized breathing models allow the time-delay and the H beta flux to depend on the continuum flux so that, tau proportional to F-c(beta) and F(H beta)proportional to F-c(alpha). Our fits give 0.1 < beta < 0.46 and 0.57 < alpha < 0.66. alpha is consistent with previous work by Gilbert and Peterson, and Goad, Korista and Knigge. Although we find beta to be flatter than previously determined by Peterson et al. using cross-correlation methods, it is closer to the predicted values from recent theoretical work by Korista and Goad.