BASS. XLIX. Characterization of highly luminous and obscured AGNs: local X-ray and [NeV]$λ$3426 emission in comparison with the high-redshift Universe

Alessandro Peca, Michael J. Koss, Kyuseok Oh, Claudio Ricci, Benny Trakhtenbrot, Richard Mushotzky, Ezequiel Treister, C. Megan Urry, Andrealuna Pizzetti, Kohei Ichikawa, Alessia Tortosa, Federica Ricci, Matilde Signorini, Darshan Kakkad, Chin-Shin Chang, Giovanni Mazzolari, Turgay Caglar, Macon Magno, Ignacio del Moral-Castro, Peter G. Boorman, Tonima T. Ananna, Fiona Harrison, Daniel Stern, David Sanders

First listed 2025-07-14 | Last updated 2025-07-14

Abstract

We present a detailed analysis of the most luminous and obscured Active Galactic Nuclei (AGNs) detected in the ultra-hard X-ray band (14-195 keV) by Swift/BAT. Our sample comprises 21 X-ray luminous (log $L_X/{\rm erg\,s^{-1}}>44.6$, 2-10 keV) AGNs at $z<0.6$, optically classified as Seyfert 1.9-2. Using NuSTAR, XMM-Newton, Suzaku, and Chandra, we constrain AGN properties such as absorption column density $N_H$, photon index $Γ$, intrinsic $L_X$, covering factor, and iron K$α$ equivalent width. For sources with black hole mass estimates (12/20), we find a weak correlation between $Γ$ and Eddington ratio ($λ_{Edd}$). Of these, six ($50\pm13\%$) lie in the $N_H$-$λ_{Edd}$ "forbidden region'' and exhibit a combined higher prevalence of $N_H$ variability and outflow signatures, suggesting a transitional phase where AGN feedback may be clearing the obscuring material. For the 13/21 sources with multi-epoch X-ray spectra, $82^{+6}_{-16}\%$ exhibit variability in either 2-10 keV flux ($73^{+9}_{-16}\%$) or line-of-sight $N_H$ ($33^{+15}_{-10}\%$). For the 20/21 sources with available near-UV/optical spectroscopy, we detect [NeV]$λ$3426 in 17 ($85^{+5}_{-11}\%$), confirming its reliability to probe AGN emission even in heavily obscured systems. When normalized to the same [OIII]$λ$5007 peak flux as $z = 2$-$9$ narrow-line AGNs identified with JWST, our sample exhibits significantly stronger [NeV]$λ$3426 emission, suggesting that high-redshift obscured AGNs may be intrinsically weaker in [NeV]$λ$3426 or that [NeV]$λ$3426 is more challenging to detect in those environments. The sources presented here serve as a benchmark for high-redshift analogs, showing the potential of [NeV]$λ$3426 to reveal obscured AGNs and the need for future missions to expand X-ray studies into the high-redshift Universe.

Short digest

A BAT-selected local analog set of 21 highly luminous, obscured Seyfert 1.9–2 AGN at z<0.6 (log L_X,2–10keV>44.6) is modeled with NuSTAR, XMM-Newton, Suzaku, and Chandra to recover N_H, Γ, intrinsic L_X, covering factor, Fe Kα, and [NeV] λ3426. The sample is highly variable: 82% vary overall (73% in 2–10 keV flux; 33% in line-of-sight N_H), and 6/12 with M_BH estimates fall in the N_H–λ_Edd “forbidden region,” where they also show enhanced N_H variability/outflow signatures—consistent with a clearing phase. [NeV] λ3426 is detected in 85% and, when renormalized to the same [OIII] peak, is significantly stronger than in JWST-selected z=2–9 narrow-line AGN. These local benchmarks calibrate [NeV]-based obscured AGN searches and frame expectations for future high‑z X-ray follow-up.

Key figures to inspect

• Figure 1 — Verify the luminosity cut (log L_X,2–10keV>44.6) and how the 21 selected Seyfert 1.9–2s sit within the broader BASS DR3 distributions; this shows the extreme-luminosity, obscured regime the paper targets.
• Figure 2 — Inspect the fitted Γ–λ_Edd relation and its scatter versus literature trends (Risaliti+09; Trakhtenbrot+17; Brightman+16) to gauge whether these luminous, obscured systems follow or deviate from known accretion–slope scalings.
• Figure 3 — Examine the N_H–λ_Edd plane to identify the six sources in the ‘forbidden region’; note their L_X color-coding and connect this placement to reported N_H variability/outflow indicators suggesting a transitional clearing phase.
• Figure 4 — Check λ_Edd versus torus covering factor for the six objects with both measurements; compare the median point to the Ricci+22 relation to assess whether covering factor declines with accretion in this extreme-luminosity subset.