Session 103 - Quasars & Blazars.
Display session, Saturday, January 10
Exhibit Hall,

## [103.06] Very High Ionization and Partial Covered Fractions in the BAL System of the QSO SBS1542+541

R. C. Telfer, G. A. Kriss, W. Zheng, A. F. Davidsen (Johns Hopkins U.)

In a previous poster (BAAS, 189, #98.09) we presented early results concerning the broad absorption line system of the z=2.361 QSO SBS1542+541. Based on analysis of UV spectra from the HST FOS and optical spectra from the MMT and Steward Observatory 2.3m telescope, we reported strong broad absorption lines from many high ionization species including the lithium-like ions OVI, NeVIII, MgX, and SiXII. We also identified absorption from HI and weak features from CIV and NV, typically strong in BAL objects.

Here we present more detailed analysis of the system, including photoionization models and implications for the geometry and physical structure of the BAL gas. We find that there is strong evidence for partial occultation of the continuum source from lines that appear to be saturated but are far from black. This is particularly clear from the resolved Lyman series lines Ly\gamma through Ly\epsilon, as well as from the MgX and SiXII doublets that are also resolved in velocity space. Overall, the covered fractions for this system suggest a general correlation between a higher ionization state and a higher covered fraction.

We have created both one-zone and two-zone photoionization models and used them along with estimated lower limits on the ionic column densities to constrain the basic parameters of the system. We find that a total column density of N_H > 10^21 \ cm^-2 and an ionization parameter U>1 are necessary to produce a consistent model. We suggest that the different covered fractions could be explained by either a special line of sight through a disk-like geometry or by the existence of density fluctuations in the BAL gas by a factor >2. The large column density and high state of ionization suggest that the system should be associated with an X-ray warm absorber''.