The ''ghost of L gamma alpha'' as evidence for radiative acceleration in quasars

In a previous study Arav and coworkers uncovered a promising signature for radiative acceleration in quasars-''the ghost of Ly alpha''-which is seen in the spectra of a few broad absorption line (BAL) QSOs. The ghost appears as an absorption hump in the C IV lambda 1549 BAL and is centered around 5900 km s(-1) to the blue of the corresponding broad emission line (BEL). This velocity separation is the same as the velocity separation between the N V lambda 1240 and Ly alpha lines. Thus, when the N V BAL and the C IV BAL are plotted as a function of velocity (where zero velocity corresponds to the peaks of these BELs), the hump in the C IV BAL appears at the same velocity as the Ly alpha BEL and qualitatively resembles it. The details of the dynamical model that can successfully fit the observed signatures are presented here, together with analysis of individual ghosts. The relation between the so-called double trough phenomenon and the ghost signature is also discussed, as well as the dynamical selection criteria that allow us to predict which objects should show a ghost signature in their spectrum. In addition, we present evidence that supports our dynamical prediction that the far-UV continuum in the ghost objects is strongly suppressed relative to other BALQSOs and quantify our assertion that a strong ghost signature is seen if and only if all the predicted dynamical criteria are satisfied. When combined together, the different aspects of this investigation give strong support to the idea that the ghost phenomenon is indicative of the main acceleration mechanism of BAL outflows: radiative acceleration via resonance line scattering.