Continuum reverberation mapping and a new lag-luminosity relationship for AGN

High cadence, high quality observations of active galactic nuclei (AGN) clearly show continuum variations with lags, relative to the shortest observed variable UV continuum that increase with wavelength ('lag spectra'). These have been attributed to the irradiation and heating of the central accretion disc by the central X-ray emitting corona. An alternative explanation, connecting the observed lag-spectra to line and continuum emission from gas in the broad line region (BLR), has also been proposed. In this paper I show the clear spectral signature of the time-dependent diffuse gas emission in the lag-spectrum of 6 AGN. I also show a new lag-luminosity relationship for 9 objects which is a scaled down version of the well known τ(Hβ)-L5100 relationship in AGN. The shape of the lag-spectrum, and its noalization, are entirely consistent with diffuse emission from radiation pressure supported clouds in a BLR with a covering factor of about 0.2. While some contributions to the continuum lag from the irradiated disc cannot be excluded, there is no need for this explanation.