The very highly ionized broad absorption line system of the QSO SBS 1542+541

We have analyzed the broad absorption line system of the bright (V similar to 16.5) high-redshift (z = 2.361) QSO SBS 1542+541 using UV spectra from t he Hubble Space Telescope Faint Object Spectrograph, along with optical dat a from the Multiple Mirror Telescope and the Steward Observatory 2.3 m tele scope. These spectra offer continuous wavelength coverage from 1200 to 8000 Angstrom which corresponds to similar to 340-2480 Angstrom in the QSO rest frame. The line of sight to the object contains only three identified inte rvening Lyman-limit absorption systems. Only one of these is optically thic k at the Lyman edge, a low-redshift (z = 0.156) system with a strong Lyman edge observed at 1055 Angstrom (314 Angstrom in the rest frame) in a Hopkin s Ultraviolet Telescope spectrum from the Astro-2 mission. The spectra ther efore 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 rela tively low ionization often seen in broad absorption systems, such as C m, O m, and Si IV. Instead, the system is dominated by very high ionization sp ecies. The strongest features correspond to O vr, Ne VIII, and Si XII. In a ddition to other high-ionization lines, we identify apparently saturated br oad Lyman-series lines of order Ly gamma and higher. There is strong evidence for partial occultation of the QSO emission source , particularly from the higher order Lyman lines that indicate a covered fr action less than 0.2. With the exception of C rv and N v, which are low-ion ization species in the context of this system, all of the other lines depre ss the flux by more than 20%. Absorption from Ly alpha also depresses the f lux more than 20%, indicating that there are at least two different regions contributing to II I absorption. Overall, the data suggest a correlation b etween a larger covered fraction and a higher state of ionization. These ob servations reveal inhomogeneity in the ionization structure of the broad ab sorption line gas. We have used photoionization models to constrain the total column density a nd ionization state of the system. A single-slab model consistent with our observational limits on the column densities requires N-H approximate to 5 x 10(22) cm(-2) and an incident ionization parameter U approximate to 2 . S ince the observed covered fractions suggest multiple zones, we also produce d a two-slab model and find 10(21) cm(-2) < N-H < 10(23) cm(-2) and 0.08 < U < 4 for the smaller zone, N-H greater than or similar to 3 x 10(21) cm(-2 ) and U greater than or similar to 2 for the larger zone. We suggest that t he different covered fractions can be explained either by a special line of sight through a disklike geometry or by the existence of density fluctuati ons of a factor of greater than or similar to 2 in the broad absorption lin e gas. The large column density and high state of ionization suggest that t he system is likely associated with an X-ray "warm absorber."