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

The presence of an obscuring torus at parsec-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, appearing as obscured or unobscured through time. The variability in such sources, so-called Changing Look AGN, is thought to be produced by a clumpy medium at much smaller distances than the conventional obscuring torus. ESO 323-G77 is a Changing Look AGN that was observed in various states through the years with Chandra, Suzaku, Swift-XRT and XMM-Newton, from unobscured (𝑁𝐻 ∼ 10^22 cm^−2 ) to Compton-thin obscured (𝑁𝐻 ∼ 1 − 6 × 10^23 cm^−2 ) and even
Compton-thick obscured (𝑁𝐻 > 10^24 cm^−2 ) with timescales as low as a few months. 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) in 2016-2017, in which the AGN was caught in a persistent Compton-thin obscured state (𝑁𝐻 ∼ 2 − 4 × 10^23 cm^−2 ) .