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Astron. Astrophys. 340, 371-380 (1998)

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1. Introduction

Broad absorption line quasi-stellar objects (BAL QSOs) are characterized by the presence in their spectra of broad, often deep, absorption troughs in the resonance lines of highly-ionized species like C iv, Si iv, or N v. These BALs appear blueshifted with respect to the corresponding emission lines. They are generally attributed to the ejection of matter at very high velocities ([FORMULA]0.1 c). About 12% of optically selected QSOs have BALs in their spectra, although this fraction could be underestimated if at least some BAL QSOs have their continuum more attenuated than non-BAL QSOs (Goodrich 1997). Apparently all BAL QSOs are radio-quiet 1 (Stocke et al. 1992). A recent account of BAL QSO properties may be found in Arav et al. (1997).

The fact that the broad emission line properties are essentially similar for BAL and non-BAL QSOs suggests that all radio-quiet QSOs could have a BAL region (BALR) of small covering factor, the BAL QSOs themselves being those objects with the BALR along the line of sight (e.g. Weymann et al. 1991, hereafter WMFH). Alternately, BAL and non-BAL QSOs may constitute two physically distinct populations of objects, BAL QSOs possibly representing an early stage in an evolutionary process towards normal QSOs (e.g. Boroson & Meyers 1992).

As first noticed by Stockman, Moore & Angel (1984), a number of BAL QSOs show high optical polarization ([FORMULA] 3%) in the continuum while other radio-quiet QSOs (i.e. non-BAL ones) have generally low polarization ([FORMULA] 1%). This important result has been recently confirmed by Hines & Schmidt (1997) on the basis of a larger sample. The fact that there is little or no variability of the polarization clearly distinguishes BAL QSOs from the so-called blazars. Since polarization is sensitive to the geometry of the objects (without spatially resolving them), a detailed understanding of BAL QSO polarization properties may provide important clues on the nature of the outflows and the status of these objects among AGN.

In this view, we have started a systematic polarimetric study of BAL QSOs. The present paper is devoted to the analysis of new broad-band polarization measurements obtained for a sample of 29 BAL QSOs, to which a number of normal radio-quiet QSOs have been added for comparison. Since an important issue is the study of possible correlations between polarization and other spectral characteristics, the objects have been essentially picked out from the WMFH sample which provides many useful quantitative spectral indices. Further, at least one BAL QSO is known to be gravitationally lensed. Its polarization could then be affected or induced by microlensing effects, i.e. by the selective magnification of some regions. We have therefore added to our sample several gravitationally lensed non-BAL QSOs with the aim of detecting any possible polarization difference.

The paper is organized as follows: the observing strategy and techniques are described in Sect. 2, as well as the methods for reducing the data and extracting accurate measurements. Instrumental polarization and de-biasing are also discussed in this section. In Sect. 3, the final sample of observed objects is detailed, sub-classes are defined, and several quantities characterizing the optical spectra are presented. Results are given in Sect. 4, including correlation searches between the various quantities. Conclusions and discussion form the last section.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998