Multi-phase gas nature in the sub-pc region of the active galactic nuclei II: Possible origins of the changing-state AGNs

Abstract

Multi-wavelength observations of active galactic nuclei (AGNs) often reveal various time scales of variability. Among these phenomena, "changing-look AGNs" are extreme cases where broad emission lines become faint/bright or even disappear/emerge between multi-epoch observations, providing crucial information about AGN internal structures. We here focus on "changing-state" AGNs, specifically investigating the transition of optical spectra over years to tens of years. Based on the axisymmetric radiation-hydrodynamical simulations (Paper I) for the gas dynamics within the dust-sublimation radius, we investigate the spectral properties of ionized gas exposed to the radiation from an AGN with a 10^7 Msun supermassive black hole. We find significant time-dependent variations in the Balmer emission lines by utilizing post-process pseudo-three-dimensional calculations and the spectral synthesis code CLOUDY. The equivalent width of Halpha and Hbeta changes by a factor of 3, or the emission lines even disappear during 30 years for the same viewing angle. The time-dependent behaviour arises primarily from gas dynamics, particularly the formation of non-steady, radiation-driven outflows within the innermost region of the disc (r <10^-3 pc). The intricate interplay between non-spherical radiation sources at the core of AGNs and the dynamic behavior of gas within the dust sublimation radius gives rise to radiation-driven outflows. This non-steady outflow potentially contributes to the observed variability in Balmer line emissions over multi-year timescales in certain AGNs.