BAT AGN spectroscopic survey–II. X-ray emission and high-ionization optical emission lines

Abstract

We investigate the relationship between X-ray and optical line emission in 340 nearby (z ≃ 0.04) AGN selected above 10 keV using Swift BAT. We find a weak correlation between the extinction corrected [O iii] and hard X-ray luminosity (⁠|$L_{\left[{\rm O}\,\small {III}\right] }^{\text{int}} \propto L_{14\text{--}195}$|⁠) with a large scatter (RPear = 0.64, σ = 0.62 dex) and a similarly large scatter with the intrinsic 2–10 keV to [O iii] luminosities (RPear = 0.63, σ = 0.63 dex). Correlations of the hard X-ray fluxes with the fluxes of high-ionization narrow lines ([O iii], He ii, [Ne iii] and [Ne v]) are not significantly better than with the low-ionization lines (H α, [S ii]). Factors like obscuration or physical slit size are not found to be a significant part of the large scatter. In contrast, the optical emission lines show much better correlations with each other (σ = 0.3 dex) than with the X-ray flux. The inherent large scatter questions the common usage of narrow emission lines as AGN bolometric luminosity indicators and suggests that other issues such as geometrical differences in the scattering of the ionized gas or long-term AGN variability are important.