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

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: roberto.serafinelli@inaf.it
² INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121, Milano, Italy & Via Bianchi 46, Merate, (LC), Italy
³ Department of Physics, Institute for Astrophysics and Computational Sciences, The Catholic University of America, Washington, DC 20064, USA
⁴ Eureka Scientific, Inc, 2452 Delmer St. Suite 100, Oakland, CA 94602, USA

Received: 16 October 2022
Accepted: 28 January 2023

Abstract

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, transitioning from obscured to unobscured and vice versa through time. The variability in these sources, which are known as 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_H < 3 × 10²² cm⁻²) to Compton-thin (N_H ∼ 1 − 6 × 10²³ cm⁻²) and even Compton-thick (N_H > 1 × 10²⁴ cm⁻²), on timescales as short as one month. We present an analysis of the first NuSTAR monitoring of ESO 323-G77, consisting of five 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_H ∼ 2 − 4 × 10²³ cm⁻²). We find that a Compton-thick reflector is present (N_H, refl = 5 × 10²⁴ cm⁻²), most likely associated with the presence of the putative torus. Two ionized absorbers are unequivocally present, located within maximum radii of r_max, 1 = 1.5 pc and r_max, 2 = 0.01 pc. In one of the observations, the inner ionized absorber is blueshifted, indicating the presence of a possible faster (v_out = 0.2c) ionized absorber, marginally detected at 3σ. 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 τ = 1.4 or τ = 2.8, depending on the coronal geometry assumed.