This work stands apart: the nearest neighbors study fast radio bursts, radio continuum, and pulsars, but not blazars at z~7. It continues the line of studying the most distant radio-loud active galactic nuclei, started with the discovery of J0410−0139 and its blazar sibling J0309+2717. Variability and spectral data for these objects exist, but low-frequency observations were limited to upper limits. This work fills the gap in the 0.144–0.82 GHz range, where emission should be boosted by the Doppler factor.
The source was observed with three instruments: uGMRT (300–820 MHz) in April and August 2023, VLA (1.5–11 GHz) simultaneously with the second uGMRT epoch, and LOFAR (144 MHz) in April and July 2024. Spectral energy distributions were constructed from fluxes at different frequencies. Variability was assessed using the VIP and MOSS indices, comparing the two uGMRT epochs. The size of the emitting region was constrained by light travel time, then brightness temperature and a lower limit on the Doppler factor δ were computed. The magnetic field was estimated from spectral curvature assuming equipartition between particles and field.
The first uGMRT epoch showed an inverted spectrum, which, together with earlier VLA data, suggested a double-humped shape typical of multi-episode jet activity. However, the second epoch, only 16 days later in the source rest frame, showed a flat spectrum, ruling out that interpretation. The variability is intrinsic to the jet. From it, a lower limit on the Doppler factor δ>19.3 was derived, corresponding to an angle between the jet and line of sight θ<3° and a Lorentz factor Γ>9.7. This means the source is highly boosted, and its intrinsic luminosity at 1.5 GHz does not exceed 0.4×$10^{30}$ erg/s/Hz, so it is radio-weak. The equipartition magnetic field is about 1 mG, higher than the equivalent CMB field of 0.2 mG at z~7, so synchrotron losses dominate. LOFAR observations give a flux of ~2.6 mJy, significantly lower than the ~8 mJy extrapolation from uGMRT data, confirming variability down to ~1 GHz in the rest frame.
Unlike neighboring works that study fast radio bursts or pulsars, this paper applies multi-wavelength radio diagnostics to the most distant blazar and for the first time constrains its low-frequency spectrum. It shows that even at z~7, jets behave like those in the local Universe, implying universal launching mechanisms. The main conclusion: the blazar nature of J0410−0139 implies a much more numerous population of radio-weak quasars that are currently unseen. This changes estimates of the number of active nuclei with jets in the early Universe.
The topic will advance with the advent of SKA-LOW, ngVLA, and DSA, which will reach sensitivities below the microjansky level and can find weak jets in quasars at z>6. Monitoring J0410−0139 at low frequencies will help catch high-flux states suitable for 21-cm forest searches. Further variability observations will separate intrinsic jet processes from interstellar scintillation and refine the parameters of the relativistic flow.
The results will impact cosmology of the reionization epoch, since powerful radio sources are needed as background lamps to study neutral hydrogen via 21-cm absorption. They are also important for understanding the growth of supermassive black holes and their feedback on galaxies.
Continue monitoring J0410−0139 with uGMRT and LOFAR to accumulate a light curve and determine characteristic variability timescales. Conduct deep surveys with sensitivity ≤1 μJy to search for radio-weak quasars at z>6.
This work directly addresses the unresolved problem of counting active nuclei with jets in the early Universe: how many quasars at z>6 have relativistic outflows, and what fraction of all quasars they represent. It also touches on the role of jets in ionizing the intergalactic medium and in the growth of supermassive black holes in the first billion years.
Doppler boosting δ>19.3 means that if the jet of J0410−0139 were not pointed at us but at a random angle, its radio flux would be thousands of times weaker, and we would simply not see it. In effect, we are looking down the barrel of a cosmic gun.
In Stanisław Lem's novel "Fiasco," the heroes try to make contact with a civilization hidden behind powerful natural radio sources resembling blazars. The variability of J0410−0139 makes it like a lighthouse that flashes and fades, as if someone were steering the beam.