Optical spectroscopy of two broad absorption line QSOs and implications for spherical-symmetric absorbing wind models

Low resolution spectroscopic observations of two broad absorption line QSOs, spanning 3200 Å to 1 µm, are described and briefly discussed. For Q1011 + 091 the absorption present in its spectrum is very extensive and complex in character; on the basis of the Mg IIλ 2798 emission line alone, which appears not to be strongly absorbed, we obtain the approximate redshift 2.27. For Q1413 + 117 we find the emission redshift 2.546±0.003 from several lines; again, Mg IIλ 2798 is essentially unabsorbed. In this object, N III] λ 750 seems to be unusually strong. Applying a moments method first developed by Castor, Lutz & Seaton to Q1413 + 117, an estimate is derived from the optically-thin Si IVλ 1397 line profile of the quantity $$\textit f(\text{Si}^{3+})\dot M$$, which is the product of the QSO mass-loss rate and mean Si³⁺/Si fraction. Consistency is sought with a model for broad absorption line QSOs, in which the absorbing medium is a spherically-symmetric wind, by comparing this quantity with the results of ionization and thermal equilibrium calculations carried out for a variety of wind and QSO parameters. It is found that the spherically-symmetric wind model is only viable for QSOs emitting weakly at X-ray energies and that the mass-loss rate needs to be of the order of 40M_⊙yr⁻¹. The resultant wind is likely to be thermally unstable. The rarity of the broad absorption line phenomenon may be attributable to the rarity of weak X-ray continua in QSOs.