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=16.5)high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS alongwith optical data from the MMT and the Steward Observatory 2.3m telescope.These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,corresponding to 340 to 2480 Angstroms in the QSO rest frame. This objecttherefore offers a rare opportunity to study broad absorption lines in therest-frame extreme UV. We find that the absorption system is dominated by veryhigh-ionization species, including O VI, NeVIII, and SiXII. We also identifyapparently saturated broad 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 which indicate a coveredfraction less than 0.2. Overall, the data suggest a correlation between alarger covered fraction and a higher state of ionization. We suggest that thedifferent covered fractions can be explained by either a special line of sightthrough a disk-like geometry or by the existence of density fluctuations of afactor >2 in the BAL gas. Our photoionization models of the system indicate alarge column density and high ionization state similar to that found in X-ray``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap