Rapid radio variability in the z∼7 blazar VLASS J0410−0139: Indication of a hidden population of weak radio jets at Cosmic Dawn

@misc{belladitta2026rapid,
  title = {{Rapid radio variability in the $z\sim7$ blazar VLASS J0410$-$0139: Indication of a hidden population of weak radio jets at Cosmic Dawn}},
  author = {Silvia Belladitta and Eduardo Bañados and Anniek J. Gloudemans and Timothy W. Shimwell and Emmanuel Momjian and Huib Intema and Chiara Mazzucchelli and Christian Fendt and Bhargav Vaidya and Fabian Walter},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.05628},
  eprint = {2609.05628},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.05628},
}

TY  - UNPB
TI  - Rapid radio variability in the z∼7 blazar VLASS J0410−0139: Indication of a hidden population of weak radio jets at Cosmic Dawn
AU  - Silvia Belladitta
AU  - Eduardo Bañados
AU  - Anniek J. Gloudemans
AU  - Timothy W. Shimwell
AU  - Emmanuel Momjian
AU  - Huib Intema
AU  - Chiara Mazzucchelli
AU  - Christian Fendt
AU  - Bhargav Vaidya
AU  - Fabian Walter
PY  - 2026
DA  - 2026/09/04
PB  - arXiv
DO  - 10.48550/arxiv.2609.05628
UR  - https://arxiv.org/abs/2609.05628
AB  - 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, 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∼7 (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∼7, detectable only with deep (≤ μJy) next-generation radio observations. LOFAR 144 MHz observations (April and July 2024) 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.
ER  -