Anomalous Broad-Line Region Responses to Continuum Variability in Active Galactic Nuclei.
In the standard AGN reverberation-mapping model variations in broad-line region (BLR) fluxes are predicted from optical continuum variability (taken as a proxy for variations in the ionizing continuum) convolved with an impulse function that depends on the geometry of the emitting and reprocessing regions. However, it has long been known that BLR variability can deviate from these predictions. We analyze extensive long-term H$\beta$ and continuum monitoring of NGC 5548 and a large sample of high-quality H$\beta$ light curves of other AGNs to investigate the frequency and characteristics of anomalous responses of the BLR. Anomalies are very common and probably occur in every object. Onsets can be on a timescale only slightly longer than the light-crossing time and durations are of the order of the characteristic timescale of variability of the optical continuum to several times longer. Anomalies are larger when NGC 5548 is in a low state, but otherwise there is no correlation with continuum variability. There is abundant evidence for the optical continuum of AGNs varying independently of the higher-energy continua and this is sufficient to explain the anomalous responses. There are good reasons for believing that the frequent lack of correlation between different spectral regions is due to anisotropic and non-axisymmetric emission. Rapid changes in line profiles and velocity-dependent lags are consistent with this. Motion of compact absorbing clouds across the line of sight is another possible cause of anomalies. The prevalence of anomalies needs to be taken into account when planning reverberation-mapping campaigns.