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Rapid Radio Variability in the z ∼ 7 Blazar VLASS J0410−0139: Indication of a Hidden Population of Weak Radio Jets at Cosmic Dawn

Year: 2026

Type: article

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.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/aea578