Call of the Duty Cycle: Searching for Fading AGN in the 12 μm Galaxies

The primary observational signatures of an active galactic nucleus (AGN) are an X-ray continuum from the corona, ultraviolet and optical emission lines from the broad-line region (BLR), and near-infrared continuum from hot dust beyond the edges of the BLR. However, in heavily obscured AGN where the hydrogen column density to the nucleus exceeds 10²³ cm^-2, these signatures are blocked. Obscured AGN can still be identified through mid-infrared (MIR) emission from the obscuring torus, and the high ionization forbidden lines of the narrow-line region (NLR). However, because the torus and NLR are further away from the supermassive black hole (SMBH), it is possible for accretion onto the SMBH to be currently shut off but still preserve the MIR and NLR emission, making an inactive galaxy look like an obscured AGN. X-rays with energies greater than 10 keV can penetrate heavy obscuration and determine whether the AGN is currently active or not. NuSTAR can detect these high-energy X-rays, making it capable of determining the nature of obscured AGN. I will present the results from an X-ray survey of nine candidate heavily obscured AGN from the IRAS 12 μm sample of galaxies, each with NuSTAR observations. I have found one candidate recently faded AGN (NGC 3627) and one candidate recently rising AGN (NGC 6890).