Relation between O VI Emission and X-Ray Intensity in Active Galactic Nuclei
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Abstract
Based on HUT observations of two Seyfert 1 galaxies and lUE and HST observations of 30 QSOs, we find a correlation between the ratio of O VI λ1034 to Lyα λ1216 and the effective power-law index between 2500 Å and 2 keV: log (O VI/Lyα) = -0.74 αOX + 0.38. From X-ray-strong to weak objects the O VI/Lyα ratio decreases by a factor of ∼8. Since the ionization threshold for O VI emission is in the soft X-ray range, our finding supports photoionization models in which high-ionization emission lines are powered by extreme UV and soft X-ray photons. Adding in ground-based optical data for 29 high-redshift QSOs, we find that, over five decades of luminosity, the O VI equivalent width shows an anti-correlation with luminosity: EW(O VI) = 4.0 × 1010 LUV-0.30, the strongest Baldwin effect among all known emission lines in AGNs. Between Seyfert 1 galaxies and high-z QSOs the equivalent widths of Lyα, C IV, and O VI decrease approximately by factors of 2, 6, and 30, respectively. The significant change in high-ionization line intensity with luminosity is consistent with the assumption that the soft X-ray continuum between 0.1 and 1 keV in high-luminosity QSOs is weaker than that in low-luminosity AGNs.