The NuSTAR view of the changing look AGN ESO 323-G77

TL;DR

This study presents NuSTAR observations of the Changing Look AGN ESO 323-G77, revealing complex absorption features, a Compton-thick reflector, ionized absorbers at various distances, and constraints on coronal properties, enhancing understanding of AGN variability.

Contribution

First NuSTAR monitoring of ESO 323-G77 providing detailed insights into its variable obscuration and ionized absorbers, linking small-scale absorbers to the larger torus structure.

Findings

Presence of a Compton-thick reflector ($N_{H,refl}=5×10^{24}$ cm$^{-2}$).

Detection of two ionized absorbers within 1.5 pc and 0.01 pc.

Possible detection of a fast ($0.2c$) ionized outflow.

Abstract

The presence of an obscuring torus at pc-scale distances from the central black hole is the main ingredient for the Unified Model of Active Galactic Nuclei (AGN), as obscured sources are thought to be seen through this structure. However, the Unified Model fails to describe a class of sources that undergo dramatic spectral changes, transitioning from obscured to unobscured and vice-versa through time. The variability in such sources, so-called Changing Look AGN (CLAGN), is thought to be produced by a clumpy medium at much smaller distances than the conventional obscuring torus. ESO 323-G77 is a CLAGN that was observed in various states through the years with Chandra, Suzaku, Swift-XRT and XMM-Newton, from unobscured ($N_{\rm H}<3\times10^{22}$ cm$^{-2}$) to Compton-thin ($N_{\rm H}\sim1-6\times10^{23}$ cm$^{-2}$) and even Compton-thick ($N_{\rm H}>1\times10^{24}$ cm$^{-2}$), with timescales as short as one month. We present the analysis of the first NuSTAR monitoring of ESO 323-G77, consisting of 5 observations taken at different timescales (1, 2, 4 and 8 weeks from the first one) in 2016-2017, in which the AGN was caught in a persistent Compton-thin obscured state ($N_{\rm H}\sim2-4\times10^{23}$ cm$^{-2}$). We find that a Compton-thick reflector is present ($N_{\rm H,refl}=5\times10^{24}$ cm$^{-2}$), most likely associated with the presence of the putative torus. Two ionized absorbers are unequivocally present, located within maximum radii of $r_{\rm max,1}=1.5$ pc and $r_{\rm max,2}=0.01$ pc. In one of the observations, the inner ionized absorber is blueshifted, indicating the presence of a possible faster ($v_{\rm out}=0.2c$) ionized absorber, marginally detected at $3\sigma$. Finally, we are able to constrain the coronal temperature and the optical depth of ESO 323-G77, obtaining $kT_e=38$ keV or $kT_e=36$ keV, and $\tau=1.4$ or $\tau=2.8$, depending on the coronal geometry assumed.