Compton-Thick AGN in the NuSTAR era VIII: A joint NuSTAR-XMM-Newton monitoring of the changing-look Compton-thick AGN NGC 1358

TL;DR

This study uses multi-epoch NuSTAR and XMM-Newton observations to confirm the changing-look nature of NGC 1358, revealing rapid variability in its obscuring material and demonstrating a new method to identify variable, heavily obscured AGN.

Contribution

The paper introduces a novel multi-epoch monitoring approach that effectively constrains key parameters of the AGN torus and confirms rapid NH,los variability in NGC 1358, supporting the changing-look AGN scenario.

Findings

Confirmed NH,los variability over weeks in NGC 1358

Demonstrated effectiveness of combined NuSTAR and XMM-Newton monitoring

Identified a tentative anti-correlation between column density and luminosity

Abstract

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 AGN, i.e., a source whose column density could transition from its 2017 Compton-thick (CT-, having line-of-sight Hydrogen column density NH,los>10^24 cm^-2) state to a Compton-thin (NH,los<10^24 cm^-2) one. The multi-epoch X-ray monitoring confirmed the presence of significant NH,los variability over time-scales as short as weeks, 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 anti-correlation between column density and luminosity, which can be understood in 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 NH,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 constrain AGN feeding models.