The PG X-ray QSO sample: Links between the UV-X-ray Continuum and Emission Lines

The UV to soft X-rays of luminous AGNs dominate their bolometric luminosity, driven by an accretion-powered dynamo at the center. These photons ionize the surrounding gas, thereby providing clues to fueling and exhaust. Two sets of important relationships – neither of them understood – link the continuum and gas properties. (i) Boroson & Green’s ‘eigenvector 1’ relationships: Steeper soft X-ray spectra are clearly related to narrower Hβ emission and stronger optical Fe II emission from the BLR, and weaker [O III]λ5007 from the NLR. We show that these relationships extend to UV spectra: narrower C III]λ1909, stronger low ionization lines, larger Si III]λ1892/C III]λ1909 (a density indicator), weaker C IVλ1549 but stronger higher-ionization NVλ1240. We speculate that high accretion rates are linked to high columns of dense (10 – 10 cm), nitrogen-enhanced, low-ionization gas from nuclear starbursts. Linewidth, inverse Fe II–[O III] and inverse Fe II–C IV relationships hint at the geometrical arrangement of this gas. (ii) The Baldwin effect (inverse equivalent width – luminosity relationships): Our correlation analyses suggest that these are independent of the above eigenvector 1 relationships. The eigenvector 1 relationships can therefore be used in future work, to reduce scatter in the Baldwin relationships, perhaps fulfilling the dream of using the Baldwin effect for cosmological studies.