NuSTAR Observations of AGN with Low Observed X-ray to [OIII] Luminosity Ratios: Heavily Obscured AGN or Turned-Off AGN?

TL;DR

This study uses NuSTAR and other X-ray observations to investigate whether low X-ray to [OIII] luminosity ratios in certain AGN are due to heavy obscuration or the black hole being inactive, revealing most are heavily obscured or turned off.

Contribution

The paper provides new high-energy X-ray data and analysis of a sample of AGN, clarifying their obscuration status and identifying a changing-look AGN, thus improving understanding of AGN activity states.

Findings

Most AGN are heavily obscured, including three confirmed Compton-thick.

One galaxy appears to be recently deactivated.

Discovered a new X-ray changing-look AGN in NGC 6890.

Abstract

Type 2 active galactic nuclei (AGN) show signatures of accretion onto a supermassive black hole through strong, high-ionization, narrow emission lines extended on scales of 100s to 1000s of parsecs, but they lack the broad emission lines from close in to the black hole that characterize type 1 AGN. The lack of broad emission could indicate obscuration of the innermost nuclear regions, or could indicate that the black hole is no longer strongly accreting. Since high-energy X-rays can penetrate thick obscuring columns, they have the power to distinguish these two scenarios. We present high-energy NuSTAR observations of 9 Seyfert 2 AGN from the IRAS 12 micron survey, supplemented with low-energy X-ray observations from Chandra, XMM-Newton, and Swift. The galaxies were selected to have anomalously low observed 2-10 keV luminosities compared to their [O III] optical luminosities, a traditional diagnostic of heavily obscured AGN, reaching into the Compton-thick regime for the highest hydrogen column densities ($N_{\rm H} > 1.5 \times 10^{24}\, {\rm cm}^{-2}$). Based on updated [O III] luminosities and intrinsic X-ray luminosities based on physical modeling of the hard X-ray spectra, we find that one galaxy was misclassified as type 2 (NGC 5005) and most of the remaining AGN are obscured, including three confirmed as Compton-thick (IC 3639, NGC 1386, and NGC 3982). One galaxy, NGC 3627, appears to be recently deactivated. Compared to the original sample the 9 AGN were selected from, this is a rate of approximately 1%. We also find a new X-ray changing-look AGN in NGC 6890.