BAT AGN Spectroscopic Survey II: X-ray Emission and High Ionization Optical Emission Lines

TL;DR

This study examines the correlation between X-ray and optical emission lines in nearby AGN, revealing large scatter and questioning the reliability of optical lines as indicators of AGN luminosity.

Contribution

It provides a comprehensive analysis of the relationship between X-ray and optical emission lines in a large AGN sample, highlighting the significant scatter and factors affecting these correlations.

Findings

Weak correlation between [O III] and X-ray luminosities.

Optical emission lines correlate better with each other than with X-ray flux.

Large scatter suggests optical lines are unreliable for AGN bolometric luminosity.

Abstract

We investigate the relationship between X-ray and optical line emission in 340 nearby AGN selected above 10 keV using Swift BAT. We find a weak correlation between the extinction corrected [O III] and hard X-ray luminosity (14-195 keV) with a [OIII] large scatter (R_Pear = 0.64, sigma = 0.62 dex) and a similarly large scatter with the intrinsic 2-10 keV to [O III] luminosities (RPear=0.63, sigma = 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 (Halpha, [SII]). 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 (sigma = 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.