The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541

We have analyzed the broad absorption line system of the bright (V ∼ 16.5) high-redshift (z = 2.361) QSO SBS1542+541 using UV spectra from the HST FOS along with optical data from the MMT and the Steward Observatory 2.3m telescope. These spectra offer continuous wavelength coverage from 1200 to 8000 Å, corresponding to ∼ 340− 2480 Å in the QSO rest frame. The line of sight to the object contains only three identified intervening Lyman-limit absorption systems. Only one of these is optically thick at the Lyman edge, a low-redshift (z = 0.156) system with a strong Lyman edge observed at 1055 Å (314 Å in the rest frame) in a Hopkins Ultraviolet Telescope spectrum from the Astro-2 mission. The spectra therefore offer a rare opportunity to study broad absorption lines in the rest-frame extreme UV. We find that the broad absorption line system is lacking in species of relatively low ionization often seen in broad absorption systems, such as C III, O III, and Si IV. Instead, the system is dominated by very high-ionization species. The strongest features correspond to O VI, Ne VIII, and Si XII. In addition to other high-ionization lines, we identify apparently saturated broad Lyman-series lines of order Lyγ and higher. There is strong evidence for partial occultation of the QSO emission source, particularly from the higher-order Lyman lines which indicate a covered fraction less than 0.2. With the exception of C IV and N V, which are low-ionization species in the context of this system, all of the other lines depress the flux by more than 20%. Absorption from Lyα also depresses the flux more than 20%, indicating that there are at least two different regions contributing to H I absorption. Overall, the data suggest a correlation between a larger covered fraction and a higher state of ionization. These observations reveal inhomogeneity in the ionization structure of the broad absorption line gas. We have used photoionization models to constrain the total column density and ionization state of the system. A single-slab model consistent with our observational