Polarization properties of broad absorption line QSOs: New statistical clues

Abstract

Abstract.We report the results of several statistical tests performed on a large sample of 139 broad absorption line (BAL) QSOs with good quality optical spectra and/or optical polarization data. Correlations between ten optical indices and the polarization degree p 0 are systematically searched for. We find six significant non-trivial correlations. In order to identify the most important correlations, we perform a principal component analysis with a sample of 30 BAL QSOs and eight quantities (including p 0 ). Most of the variance (∼57%) in the data is contained in two principal components called PC1 and PC2. PC1 is mainly dominated by the correlation between the balnicity index BI and the strength of the Fe  emission; it may be driven by the accretion rate of matter onto the central compact object. The variance in PC2 is essentially due to the anti-correlation between p 0 and the detachment index DI, indicating that BAL QSOs with P Cygni profiles (DI ) are usually more polarized than those objects with C  absorption troughs well detached from the corresponding emission lines (DI ). We show that PC2 may be related to the orientation of the BAL QSOs with respect to the line of sight. We also present new spectropolarimetric observations of six BAL QSOs. By adding spectropolarimetric data from the literature, we build a sample of 21 BAL QSOs for which we define four spectropolarimetric indices describing the polarization properties of the absorption and emission lines. We find that the polarization of the C ] emission line is systematically higher than the polarization of the C  emission line, and that the highest polarization in the troughs is correlated to the balnicity index. Another important result emerging from the statistical tests performed on this spectropolarimetric sample is a possible anti-correlation between the detachment index and a quantity SI which measures the ratio of the depths of the C  absorption in the polarized flux and in the total flux. This correlation indicates that in BAL QSOs with P cygni profiles, the BAL troughs in the polarized flux are nearly as deep as in the total flux while, in BAL QSOs with detached absorptions, the BAL troughs in the polarized flux are much weaker than in the total flux. We show that our main results may be explained in the framework of a "two-component" wind model which is a natural extension of the classical wind-from-disk models. In this model, the broad absorption occurs in a dense equatorial wind emerging from the accretion disk, while scattering and polarization mainly take place in a polar region. The orientation relative to the observer drives the correlations p 0 -DI and DI -SI. While most of our observations can be explained within this framework, there are also several indications that other polarization mechanisms, and more particularly resonance scattering, may also be at work.

Microlensing in H1413+117: disentangling line profile emission and absorption in a broad absorption line quasar

“…Goodrich & Miller 1995;Lamy & Hutsemékers 2004) provide additional evidence favoring this kind of scenario. First, the polarization angle rotates within the absorption line profiles, suggesting the existence of at least two sources and/or mechanisms of polarization.…”

“…First, the polarization angle rotates within the absorption line profiles, suggesting the existence of at least two sources and/or mechanisms of polarization. The polar outflow and the disk, expected to produce perpendicular polarizations, can play this role, especially in the case of BAL QSOs with P Cygni-type profiles (Goodrich 1997;Hutsemékers et al 1998;Lamy & Hutsemékers 2004). Furthermore, the absorption in the polarized spectrum is clearly narrower than the absorption in the direct spectrum (this is best observed in Fig.…”

“…While pure spherically symmetric winds appeared too simple to account for the variety of observations (Hamann et al 1993;Ogle et al 1999), equatorial disks, rotating winds, polar flows, or combinations thereof have been proposed more or less successfully to interpret the observations of individual objects or small groups of them (e.g. Murray et al 1995;Schmidt & Hines 1999;Lamy & Hutsemékers 2004;Zhou et al 2006). Given the large parameter space characterizing non spherically symmetric winds, BAL profile modeling must then be combined with other techniques to determine the outflow properties in individual objects (e.g.…”

Multiwavelength campaign on Mrk 509

“…Finally, Wang et al (2010) summarised the observational evidence which implies that indeed the N v line is generally strongly contaminated by the resonance scattering of Lyα photons. Strong support is provided by the 10% polarisation fraction of the N v emission line (Lamy & Hutsemékers 2004), while the finding that other broad emission lines are unpolarised indicates that at least part of the emission is scattered light. Furthermore, Weymann et al (1991) found a difference between the strength of the N v line in BAL and non-BALQSOs, with excess N v emission extending to 10 000 km s −1 , much broader than the broad emission lines.…”

Resolving the Structure at the Heart of BAL Quasars Through Microlensing Induced Polarisation Variability

“…It was conceded, however, that this would require a very precise lensing configuration. Furthermore, the polarisation for the summed images has been seen to vary in the blue wing of the C IV λ1549 emission line, with fluctuations between ∼10% (Lamy & Hutsemékers 2004) and ∼20% (Schmidt & Hines 1999). Interestingly, these studies also found that continuum polarisation near the C IV feature was ∼2%, indicating some polarisation is still occurring away from the absorption troughs (Wang, Wang, & Wang 2005).…”