Discovery of changing-look behavior in AGN NGC3822: A long-term multiwavelength study

Abstract

We present a comprehensive long-term multiwavelength study of the active galactic nucleus NGC 3822, based on 17 yr (2008─2025) of X-ray, ultraviolet (UV), and optical observations. The data set includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope. Our multiwavelength light-curve analysis reveals flux variations across X-ray to optical/UV bands, with an increased variability amplitude at shorter wavelengths. X-ray spectral analysis indicates the presence of intrinsic absorption during the 2016 and 2022 observations; however, this absorption disappeared before and after these epochs. The presence and absence of the absorber are attributed to clouds moving in and out of the line of sight. During the long-term monitoring period, the bolometric luminosity of the source varies between (1.32 and 17) × 1043 erg s−1. Optical spectroscopic monitoring reveals changing-look (CL) behavior in NGC 3822, characterized by the appearance and disappearance of broad emission lines (BELs). These CL transitions are associated with changes in the Eddington ratio rather than changes in the obscuration. The BELs appear only when the Eddington ratio is relatively high (∼3.8 × 10−3) and disappear when it drops to a lower value (∼0.9 × 10−3).