We present the multi-epoch monitoring with NuSTAR and XMM-Newton of NGC 1358, a nearby Seyfert 2 galaxy whose properties made it a promising candidate X-ray changing-look active galactic nucleus (AGN), i.e., a source whose column density could transition from its 2017 Compton-thick (having LOS hydrogen column density N H,LOS > 1024 cm−2) state to a Compton-thin (N H,LOS < 1024 cm−2) one. The multi-epoch X-ray monitoring confirmed the presence of significant N H,LOS variability over timescales of weeks to years, and allowed us to confirm the changing-look nature of NGC 1358, which has most recently been observed in a Compton-thin status. Multi-epoch monitoring with NuSTAR and XMM-Newton is demonstrated to be highly effective in simultaneously constraining three otherwise highly degenerate parameters: the torus average column density and covering factor, and the inclination angle between the torus axis and the observer. We find a tentative anticorrelation between column density and luminosity, which can be understood under the framework of chaotic cold accretion clouds driving recursive AGN feedback. The monitoring campaign of NGC 1358 has proven the efficiency of our newly developed method to select candidate N H,LOS-variable, heavily obscured AGN, which we plan to soon extend to a larger sample to better characterize the properties of the obscuring material surrounding accreting supermassive black holes, as well as to constrain AGN feeding models.

Abstract The EUV provides most of the ionization that creates the high equivalent width (EW) broad and narrow emission lines (BELs, NELs) of quasars. Spectra of Hypermassive Schwarzschild black holes (HMBHs, MBH ≥ 1010M⊙) with α-discs, decline rapidly in the EUV suggesting much lower EWs. Model spectra for black holes of mass 106 − 1012M⊙ and accretion rates 0.03 ≤ Lbol/Ledd ≤ 1.0 were input to the CLOUDY photoionization code. BELs become ∼100 times weaker in EW from MBH ∼ 108M⊙ to MBH ∼ 1010M⊙. The high ionization BELs (O VI 1034 Å, C IV 1549 Å, He II 1640 Å) decline in EW from (MBH ≥ 106M⊙, reproducing the Baldwin effect, but regain EW for MBH ≥ 1010M⊙). The low ionization lines (MgII 2798 Å, Hβ 4861 Å and Hα 6563 Å) remain weak. Lines for maximally spinning HMBHs behave similarly. Line ratio diagrams for the BELs show that high OVI/Hβ and low CIV/Hα may pick out HMBH, although OVI is often hard to observe. In NEL BPT diagrams HMBHs lie among star forming regions, except for highly spinning, high accretion rate HMBHs. In summary, the BELs expected from HMBHs would be hard to detect using the current optical facilities. From 100 to 1012M⊙, the emission lines used to detect AGN only have high EW in the 106 − 109M⊙ window, where most AGN are found. This selection effect may be distorting reported distributions of MBH.

Context. We present the broad-band X-ray spectral analysis (0.6–50 keV) of seven Compton-thick active galactic nuclei (CT-AGN; line-of-sight (LOS) column density > 1024 cm−2) candidates selected from the Swift-BAT 100 month catalogue using archival NuSTAR data. Aims. We aim to obtain a complete census of the heavily obscured AGN in the local Universe (z < 0.05). Methods. This work is in continuation of the ongoing research of the Clemson-INAF group to classify CT-AGN candidates at redshift z < 0.05 using physically motivated torus models. Results. Our results confirm that three out of seven targets are bona fide CT-AGN. Adding our results to the previously analysed sources using NuSTAR data, we increase the population of bona fide CT-AGN by ∼9%, bringing the total number to 35 out of 414 AGN. We also performed a comparative study using MYTorus and borus02 on the spectra in our sample, finding that both physical models are strongly consistent in the parameter space of LOS column density and photon index. Furthermore, we also investigate the clumpiness of the torus clouds by separately computing the LOS and average torus column densities in each of the seven sources. Adding our results to all the previous 48 CT-AGN candidates analysed by the Clemson-INAF research team for which NuSTAR observations are available, we find that 78% of the sources are likely to have a clumpy distribution of the obscuring material surrounding the accreting supermassive black hole.