Differences Between The Optical/Uv Spectra Of X-Ray Bright And X-Ray Faint QSOs

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 [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.