BASS XLVIII: [Ne v] {\lambda}3427 Emission in Powerful Nearby Active Galactic Nuclei

TL;DR

This study examines the [Ne v] 3427 emission line in a large sample of X-ray selected AGN, finding it a reliable tracer of AGN activity even in obscured systems, with consistent emission ratios and no strong dependence on AGN properties.

Contribution

It provides the first large-scale analysis of [Ne v] emission in a diverse AGN sample, confirming its robustness as an AGN indicator across different obscuration levels.

Findings

[Ne v] detection in 43% of AGN, unaffected by obscuration

Median line-to-X-ray luminosity ratios are stable and comparable to other tracers

No significant correlation between [Ne v] strength and AGN/SMBH properties

Abstract

We investigate the high-ionization, narrow [Ne v] $\lambda$3427 emission line in a sample of over 340 ultrahard X-ray (14-195 keV) selected Active Galactic Nuclei (AGN) drawn from the BASS project. The analysis includes measurements in individual and stacked spectra, and considers several key AGN properties such as X-ray luminosity, supermassive black hole (SMBH) mass, Eddington ratios, and line-of-sight column density. The [Ne v] $\lambda$3427 line is robustly detected in ~43% (146/341) of the AGN in our sample, with no significant trends between the detection rate and key AGN/SMBH properties. In particular, the detection rate remains high even at the highest levels of obscuration (>70% for log[N_H/cm^-2] > 23). On the other hand, even some of our highest signal-to-noise spectra (S/N > 50) lack a robust [Ne v] detection. The typical (median) scaling ratios between [Ne v] line emission and (ultra-)hard X-ray emission in our sample are log L[Ne v]/L(14-150 keV) = -3.75 and log L[Ne v]/L(2-10 keV) = -3.36. The scatter on these scaling ratios, of ~0.5 dex, is comparable to, and indeed smaller than, what is found for other commonly used tracers of AGN radiative outputs (e.g., [O III] $\lambda$5007). Otherwise, we find no significant relations between the (relative) strength of [Ne v] and the basic AGN/SMBH properties under study, in contrast with simple expectations from models of SMBH accretion flows. Our results reaffirm the usability of [Ne v] as an AGN tracer even in highly obscured systems, including dual AGN and high redshift sources.