{
  "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$-$11 GHz observations of the most distant known blazar, VLASS J041009.05$-$013919.88 at z$\\sim$7, obtained with the upgraded Giant Metrewave Radio Telescope (uGMRT), the LOw Frequency ARray (LOFAR) and the Very Large Array (VLA). The first uGMRT epoch (300$-$820 MHz, April 2023) revealed an inverted radio spectrum which, combined with earlier (2021$-$2022) VLA data (1.5$-$11 GHz), unveiled a double-peaked spectrum potentially indicative of multi-epoch jet activity. A second uGMRT campaign (August 2023), simultaneous with new VLA observations (1.5$-$11 GHz), instead revealed a flat low-frequency and a peaked high-frequency spectrum, ruling out this interpretation. While limited by the two uGMRT epochs, variability analysis favors intrinsic jet processes, indicating a highly relativistic, closely aligned jet ($θ<3$ deg, $δ>19.3$, $Γ>9.7$). The equipartition magnetic field ($> 1$ mG) exceeds the equivalent Cosmic Microwave Background field at $z\\sim7$ (0.2 mG), indicating 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 $z\\sim7$, detectable only with deep ($\\leq μ$Jy) next-generation radio observations. LOFAR 144 MHz observations (April and July 2024) yielded $\\sim$2 mJy, well below the $\\sim$8 mJy predicted from uGMRT epochs, confirming strong variability at rest-frame $\\sim$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": [],
  "article_type": null,
  "arxiv": "2609.05628",
  "author_total": 10,
  "authors": [
    {
      "name": "Silvia Belladitta"
    },
    {
      "name": "Eduardo Bañados"
    },
    {
      "name": "Anniek J. Gloudemans"
    },
    {
      "name": "Timothy W. Shimwell"
    },
    {
      "name": "Emmanuel Momjian"
    },
    {
      "name": "Huib Intema"
    },
    {
      "name": "Chiara Mazzucchelli"
    },
    {
      "name": "Christian Fendt"
    },
    {
      "name": "Bhargav Vaidya"
    },
    {
      "name": "Fabian Walter"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/0ej08avmptb85.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.05628",
  "fulltext_url": null,
  "id": "0ej08avmptb85",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.05628",
  "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"
    ],
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "openalex",
      "3106|Astrophysics and Cosmic Phenomena"
    ]
  ],
  "openalex": "W7212034883",
  "pdf_url": "https://arxiv.org/pdf/2609.05628",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-04",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/0ej08avmptb85.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:19:48Z",
      "native_id": "2609.05628",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.05628"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "Rapid radio variability in the $z\\sim7$ 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": "preprint",
  "updated_at": "2026-10-30T12:19:48Z",
  "url": "https://research.draconitic.com/paper/0ej08avmptb85",
  "venue": "arXiv",
  "versions": []
}