Photoionized Hβ emission in NGC 5548: it breathes!

Abstract

Emission-line regions in active galactic nuclei and other photoionized nebulae should become larger in size when the ionizing luminosity increases. This “breathing” effect is observed for the Hβ emission in NGC 5548 by using Hβ and optical continuum lightcurves from the 13-year 1989-2001 AGN Watch monitoring campaign. To model the breathing, we use two methods to fit the observed lightcurves in detail: (i) parameterized models and, (ii) the MEMECHO reverberation mapping code. Our models assume that optical continuum variations track the ionizing radiation, and that the Hβ variations respond with time delays τ due to light travel time. By fitting the data using a delay map �(τ, Fc) that is allowed to change with continuum flux Fc, we find that the strength of the Hβ response decreases and the time delay increases with ionizing luminosity. The parameterized breathing models allow the time delay and the Hβ flux to depend on the continuum flux so that, τ / F � c and FH� / F � c . Our fits give 0.1 < β < 0.46 and 0.57 < α < 0.66. α is consistent with previous work by Gilbert & Peterson (2003) and Goad, Korista & Knigge (2004). Although we find β to be flatter than previously determined by Peterson et al. (2002) using crosscorrelation methods, it is closer to the predicted values from recent theoretical work by Korista & Goad (2004).