MODELING THE DOUBLE-TROUGH STRUCTURE OBSERVED IN BROAD ABSORPTION-LINE QSOS USING RADIATIVE ACCELERATION

We present a model explaining the double trough, separated by DELTAups ilon approximately 5900 km s-1, observed in the C IV lambda1549 broad absorption line (BAL) in a number of BALQSOs. The model is based on ra diative acceleration of the BAL outflow, and the troughs result from m odulations in the radiative force. Specifically, where the strong flux from the Lyalpha lambda1215 broad emission line is redshifted to the frequency of the N v lambda1240 resonance line, in the rest frame of t he accelerating N v ions, the acceleration increases and the absorptio n is reduced. At higher velocities the Lyalpha emission is redshifted out of the resonance and the N v ions experience a. declining flux whi ch causes the second absorption trough. A strongly nonlinear relations hip between changes in the flux and the optical depth in the lines is shown to amplify the expected effect. This model produces double troug hs for which the shallowest absorption between the two troughs occurs at upsilon congruent-to 5900 km s-1. Indeed, we find that a substantia l number of the observed objects show this feature. A prediction of th e model is that all BALQSOs that show a double-trough signature will b e found to have an intrinsic sharp drop in their spectra shortward of approximately 1200 angstrom.