Differences between the Optical/Ultraviolet Spectra of X‐Ray Bright and X‐Ray Faint QSOs

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Abstract
We contrast measurements of composite optical and ultraviolet (UV) spectra constructed from samples of QSOs defined by their soft X-ray brightness. X-ray bright (XB) composites show stronger emission lines in general, but particularly from the narrow-line region. The difference in the [O III]/Hβ ratio is particularly striking, and is even more so when blended Fe II emission is properly subtracted. The correlation of this ratio with X-ray brightness was a principal component of QSO spectral diversity found by Boroson & Green. We find here that other, much weaker narrow optical forbidden lines ([O II] and [Ne V]) are enhanced by factors of 2 to 3 in our XB composites, and that narrow line emission is also strongly enhanced in the XB UV composite. Broad permitted-line fluxes are slightly larger for all XB spectra, but the narrow-/broad-line ratio stays similar or increases strongly with X-ray brightness for all strong permitted lines except Hβ. Spectral differences between samples divided by their relative X-ray brightness (as measured by αox) exceed those seen between complementary samples divided by luminosity or radio loudness. We propose that the Baldwin effect may be a secondary correlation to the primary relationship between αox and emission line equivalent width. We conclude that either (1) Wλ depends primarily on the shape of the ionizing continuum, as crudely characterized here by αox, or (2) both Wλ and αox are related to some third parameter characterizing the QSO physics. One such possibility is intrinsic warm absorption; a soft X-ray absorber situated between the broad and narrow line regions can successfully account for many of the properties observed.
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