A Decade-scale Ionization Echo of Black Hole Accretion in Galactic Nuclei

Abstract

Abstract Circumnuclear structures in active galactic nuclei (AGN) respond to accretion variability over different spatial and temporal scales, but whether compact narrow-line gas responds on decade timescales has not been systematically established. We use repeat Sloan Digital Sky Survey and Dark Energy Spectroscopic Instrument spectroscopy of 473 variability-selected galactic nuclei with evidence for recent nuclear brightening. We identify 19 sources whose narrow [O III ] λ 5007 flux increased by more than a factor of 2 over rest-frame baselines of 12.0–20.4 yr, which we term narrow-line ionization echoes (NIEs). In equal-weight stacked spectra, [O III ] increased by 229% ± 22%, while lower-ionization lines remained nearly unchanged or declined, indicating an ionization-dependent response rather than uniform spectral rescaling. The NIE hosts also moved coherently toward higher excitation in optical diagnostic diagrams, with mean increases of 0.46 ± 0.18 dex in log ( [ O III ] / H β ) and 0.58 ± 0.13 dex in log ( [ O III ] / [ O II ] ) . Photoionization models reproduce this evolution with a higher ionization parameter and a harder AGN-like ionizing spectrum at fixed gas abundance. For 13 sources with well-defined mid-infrared (MIR) outbursts, the integrated W1+W2 excess energy correlates strongly with the absolute [O III ] luminosity change ( r = 0.87), linking the dust and narrow-line responses to the same nuclear event. The MIR timescale and spectroscopic baseline constrain the variable [O III ]-emitting gas to characteristic scales of 1.5–5.2 pc. Compact narrow-line gas therefore appears to form an intermediate, decade-scale response layer between the infrared dust echo and the long-lived extended narrow-line region.