{
  "abstract": "Doppler boosting allows blazars to be detected out to high z , making them promising probes of the intergalactic medium through the 21 cm forest. We report 0.144−11 GHz observations of the most distant known blazar, VLASS J041009.05−013919.88 at z ∼ 7, obtained with the upgraded Giant Metrewave Radio Telescope (uGMRT), the LOw Frequency ARray (LOFAR), and the Very Large Array (VLA). The first uGMRT epoch (300−820 MHz, 2023 April) revealed an inverted radio spectrum that, combined with earlier (2021–2022) VLA data (1.5–11 GHz), unveiled a double-peaked spectrum potentially indicative of multiepoch jet activity. A second uGMRT campaign (2023 August), simultaneous with new VLA observations (1.5–11 GHz), instead revealed a flat low-frequency and a peaked high-frequency spectrum, ruling out this interpretation. While limited by the two uGMRT epochs, variability analysis favors intrinsic jet processes, indicating a highly relativistic, closely aligned jet ( θ < 3°, δ > 19.3, Γ > 9.7). The equipartition magnetic field (>1 mG) exceeds the equivalent cosmic microwave background field at z ∼ 7 (0.2 mG), indicating that synchrotron losses dominate. The inferred Doppler boosting ( δ > 19.3) implies that J0410−0139 is intrinsically radio-weak. As a blazar, it traces a much larger parent population of radio quasars at z ∼ 7, detectable only with deep (≤ μ Jy) next-generation radio observations. LOFAR 144 MHz observations (2024 April and July) yielded ∼2 mJy, well below the ∼8 mJy predicted from uGMRT epochs, confirming strong variability at rest-frame ∼1 GHz. Low-frequency monitoring will be crucial for identifying high radio intensity states suitable for future 21 cm forest studies.",
  "abstract_shown": true,
  "affiliations": [
    "Max-Planck-Institut für Astronomie",
    "INAF-Osservatorio di Astrofisica e Scienza dello Spazio",
    "Gemini Observatory",
    "The Netherlands Institute for Radio Astronomy",
    "Leiden University",
    "National Radio Astronomy Observatory",
    "Universidad Diego Portales",
    "Indian Institute of Technology Indore"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 10,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Silvia Belladitta",
      "orcid": "0000-0003-4747-4484"
    },
    {
      "affs": [
        0
      ],
      "name": "Eduardo Bañados",
      "orcid": "0000-0002-2931-7824"
    },
    {
      "affs": [
        2
      ],
      "name": "Anniek J. Gloudemans",
      "orcid": "0009-0009-8274-441X"
    },
    {
      "affs": [
        3,
        4
      ],
      "name": "Timothy W. Shimwell",
      "orcid": "0000-0001-5648-9069"
    },
    {
      "affs": [
        5
      ],
      "name": "Emmanuel Momjian",
      "orcid": "0000-0003-3168-5922"
    },
    {
      "affs": [
        4
      ],
      "name": "Huib Intema",
      "orcid": "0000-0002-5880-2730"
    },
    {
      "affs": [
        6
      ],
      "name": "Chiara Mazzucchelli",
      "orcid": "0000-0002-5941-5214"
    },
    {
      "affs": [
        0
      ],
      "name": "Christian Fendt",
      "orcid": "0000-0002-3528-7625"
    },
    {
      "affs": [
        7
      ],
      "name": "Bhargav Vaidya",
      "orcid": "0000-0001-5424-0059"
    },
    {
      "affs": [
        0
      ],
      "name": "Fabian Walter",
      "orcid": "0000-0003-4793-7880"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2hacst4xzd65h.bib",
  "cited_by": 0,
  "doi": "10.3847/1538-4357/aea578",
  "fulltext_url": null,
  "id": "2hacst4xzd65h",
  "issns": [
    "0004-637X",
    "1538-4357"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea578",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3106|Astrophysics and Cosmic Phenomena"
    ]
  ],
  "openalex": "W7221648148",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea578/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-13",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/2hacst4xzd65h.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:11Z",
      "native_id": "10.3847/1538-4357/aea578",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea578"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "openalex:3106|Astrophysics and Cosmic Phenomena",
      "subfield": "high-energy"
    }
  ],
  "title": "Rapid Radio Variability in the z ∼ 7 Blazar VLASS J0410−0139: Indication of a Hidden Population of Weak Radio Jets at Cosmic Dawn",
  "title_text": "Rapid Radio Variability in the z ∼ 7 Blazar VLASS J0410−0139: Indication of a Hidden Population of Weak Radio Jets at Cosmic Dawn",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:11Z",
  "url": "https://research.draconitic.com/paper/2hacst4xzd65h",
  "venue": "The Astrophysical Journal",
  "versions": []
}