{
  "abstract": "Circumnuclear structures in active galactic nuclei 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] $\\lambda5007$ flux increased by more than a factor of two 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 $229\\pm22\\%$, 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 $0.46\\pm0.18$ dex in $\\log([\\mathrm{O III}]/\\mathrm{H}β)$ and $0.58\\pm0.13$ dex in $\\log([\\mathrm{O III}]/[\\mathrm{O II}])$. 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 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 mid-infrared 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.10373",
  "author_total": 19,
  "authors": [
    {
      "name": "Wei-Jian Guo"
    },
    {
      "name": "Shuo Zhai"
    },
    {
      "name": "Chen Hu"
    },
    {
      "name": "David M. Alexander"
    },
    {
      "name": "Bin Luo"
    },
    {
      "name": "Pu Du"
    },
    {
      "name": "Matthew J. Temple"
    },
    {
      "name": "Leah K. Morabito"
    },
    {
      "name": "Feige Wang"
    },
    {
      "name": "Jinyi Yang"
    },
    {
      "name": "Victoria A. Fawcett"
    },
    {
      "name": "Swayamtrupta Panda"
    },
    {
      "name": "Claire L. Greenwell"
    },
    {
      "name": "Yan-Rong Li"
    },
    {
      "name": "Cheng Cheng"
    },
    {
      "name": "Shengxiu Sun"
    },
    {
      "name": "Zhi-Qiang Chen"
    },
    {
      "name": "Alessandro Marconi"
    },
    {
      "name": "Jian-Min Wang"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/4sqh3k02pt4wk.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.10373",
  "fulltext_url": null,
  "id": "4sqh3k02pt4wk",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.10373",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7221012207",
  "pdf_url": "https://arxiv.org/pdf/2610.10373",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-07",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/4sqh3k02pt4wk.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:27:37Z",
      "native_id": "2610.10373",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.10373"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "A Decade-Scale Ionization Echo of Black-Hole Accretion in Galactic Nuclei",
  "title_text": "A Decade-Scale Ionization Echo of Black-Hole Accretion in Galactic Nuclei",
  "type": "preprint",
  "updated_at": "2026-10-30T12:27:37Z",
  "url": "https://research.draconitic.com/paper/4sqh3k02pt4wk",
  "venue": "arXiv",
  "versions": []
}