Bloated stars as AGN broad-line clouds: the emission-line response to continuum variations

Abstract

The `Bloated Stars Scenario' proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Alexander and Netzer (1994, 1996) established that ~ 5e4 BSs with dense, decelerating winds can reproduce the observed emission line spectrum and line profiles while avoiding rapid collisional destruction. Here, I investigate a third prediction of the model, related to the size of the broad line region, by deriving the emission line response to variations in the ionizing continuum (`line reverberation') and comparing it to observations. The expected time lags, as well as the order of response of the various lines, strongly depend on the typical variability time scale of the ionizing continuum. The BS model studied here corresponds to a bright Syfert 1 galaxy (Sy1) or a low luminosity QSO. I find that the BS model is consistent with the observed correlation between the Balmer lines time lags and the AGN luminosity, which at present is the only line reverberation information available for this luminosity class. The model displays also some of the anti-correlation between the time lags of the metal emission lines and their degree of ionization that has been observed in a few low-luminosity Sy1s. However, the bright Sy1 model results differs from the low-luminosity Sy1 data in that the Hb time lag is relatively shorter and the CIV time lag relatively longer.