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2.3. BALQSOs and warm absorbers

More than half of the Seyfert 1s show K-shell edges of warm oxygen (O VII and O VIII at 0.739 and 0.870 keV respectively), characteristic of optically thin, photoionized material along the line of sight to the central engine, the so-called ``warm absorbers'' ([347]; [141]). The column densities of the ionized material are typically in the range NH ~ 1021-1023 cm-2 ([141]); in many objects, the column density is variable, but the O VII and O VIII features do not vary simultaneously leading to a two zone model ([142]).

Warm absorber objects are generally more likely to have high optical polarization than objects with no detected ionized absorption ([263]); objects displaying deep O VII edges often show significant optical reddening ([347]; [349]; [171]). These observations suggest that the warm absorber is associated with dust.

As much as 50% of all Seyfert 1s show narrow (100-300 km s-1), high ionization UV absorption lines (C IV, N V, O VI, Si IV), blueshifted from 0 to 1 500 km s-1 and variable on time scales of weeks to years ([99]). The X-ray warm absorbers seem to be associated with these absorption lines ([284]; [464]). The iron coronal lines observed in some Seyfert 1s could be emitted by the warm absorbers ([349]).

A low-energy X-ray cut-off is sometimes found in radio loud QSOs; it is due to photoelectric absorption with column densities of ~ 1021 cm-2; these objects usually show a narrow C IV absorption line ([108]).

In Seyfert 1s and QSOs, the broad Halpha and Hbeta luminosities are tightly correlated with (in fact proportional to) the X-ray luminosity ([230]; [489]; [202]). However, a distinct class of ``X-ray-weak'' QSOs has been identified, which form ~ 10% of the population, and where the X-ray emission is smaller by a factor 10-30 than expected from their broad Hbeta luminosity ([246]; [202]); there is a strong correlation between the soft X-ray weakness and the C IVlambda1550 absorption EW which suggests that absorption is the primary cause of the soft X-ray weakness ([62]).

BALQSOs (Broad Absorption Line QSOs) are a special class of QSOs that show mostly highly ionized gas flowing away from the central source at speeds up to 10 000 to 30 000 km s-1 or more. The BALs appear in the spectra of about 10% of all QSOs ([427]). The observed radial terminal velocity of the gas being ejected from BALQSOs is strongly anticorrelated with the radio power ([471]). The continuum and emission line properties of BALQSOs and non-BALQSOs are remarkably similar ([472]).

BALQSOs show systematically higher optical polarization than other QSOs, the degree of polarization increasing toward shorter wavelengths; it seems that broad absorption is observed in BALQSOs because they are inclined at intermediate inclinations where our line of sight passes through gas clouds located near the surface of the dusty torus, the polarization being due to scattering by this material ([374]; [320]).

BALQSOs are very weak X-ray sources, often being 30-100 times less luminous in X-rays than expected from their optical luminosities indicating that the X-ray flux is reduced by high column densities (NH > 5 1023 cm-2) of either cold or ionized material; the absence of selective reddening in BALs, combined with the high inferred X-ray column densities argues for very little dust ([135]).

In conclusion, it seems that there is a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, soft X-ray weak QSOs and BALQSOs ([62]).

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