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 [OIII]/Hbeta
ratio is particularly striking, and even more so when blended FeII emission is
properly subtracted. The correlation of this ratio with X-ray brightness were
principal components of QSO spectral diversity found by Boroson & Green (1992).
We find here that other, much weaker narrow optical forbidden lines ([OII] and
NeV) 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 Hbeta.
Spectral differences between samples divided by their relative X-ray
brightness (as measured by alpha_{ox}) 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
alpha_{ox} and emission line equivalent width. We conclude that either 1)
equivalent width depends strongly on the SHAPE of the ionizing continuum, as
crudely characterized here by alpha_{ox} or 2) both equivalent width and
alpha_{ox} 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.Comment: 16 pages including 3 figures, AAS latex, plus 4 tables totaling 5
pages, to appear in ApJ Vol. 498, May 1, 199