A Lighthouse at the Edge of the Universe ✎

Original: "Rapid radio variability in the $$z\sim7$$ blazar VLASS J0410$$-$$0139: Indication of a hidden population of weak radio jets at Cosmic Dawn"

· Silvia Belladitta, Eduardo Bañados, Anniek J. Gloudemans, Timothy W. Shimwell, Emmanuel Momjian, Huib Intema, Chiara Mazzucchelli, Christian Fendt, Bhargav Vaidya, Fabian Walter

arXiv:2609.05628v1 · 2026-09-04 · arXiv non-exclusive · independent analysis · 2 min

High Energy Galaxies

The most distant blazar winks at us from an era when the universe was less than a billion years old, revealing a hidden fleet of radio-quiet quasars.

In the first few hundred million years after the Big Bang, the universe was a dark ocean in which the first stars and black holes ignited. Some of those black holes shot out narrow jets of plasma traveling at nearly the speed of light. If such a jet points straight at Earth, we see it as a bright spot: a blazar. The most distant known blazar, VLASS J0410−0139, shone when the universe was just 700 million years old, and now astronomers have caught it winking.

A blazar is not the black hole itself but its fiery exhaust. A jet aimed at us boosts the light, like a flashlight shining in your eyes rather than off to the side.

Twice in 2023, the uGMRT radio telescope in India picked up waves from the source at frequencies between 300 and 820 megahertz. Only 16 days passed between observations, as measured by the blazar's own clock, yet its spectrum changed: initially rising with frequency, it then flattened. This is not the signature of two separate flares but of a single living jet that breathes. From the rate of variability, astronomers calculated that the jet is boosted by at least a factor of 19.

Had the jet been pointed not at us but at some random angle, we would not see it. We have peered down the barrel of a cosmic gun.

Such a strong Doppler factor means the source is actually faint. Its intrinsic radio luminosity is hundreds of times lower than it appears. Consequently, there should be thousands of similar quasars with jets that we simply miss because their jets are pointed sideways. Current sky surveys see only the tip of the iceberg.

Finding this hidden fleet requires new instruments: SKA-LOW, ngVLA, and DSA. They will be sensitive to fluxes below a microjansky and will be able to catch the faint jets of quasars at z>6. Monitoring J0410−0139 will also help us pick the moment when the lighthouse flares up, using it as a background lamp to study neutral hydrogen in the early universe.

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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.

T_{B,var} = (1+z) \frac{\Delta S_\nu c^2}{2 k_B \nu_{obs}^2 \Omega}

Relates the variability amplitude ΔSν to the size of the emitting region Ω and redshift z, allowing estimation of temperature and thus the Doppler factor.

H_{CMB} = 3.26\times10^{-6} \times (1+z)^2 \ \text{Гс}

Shows the magnetic field that would produce the same energy density as the CMB at a given redshift. Used for comparison with the equipartition field of the jet.

Concepts

vla blazar blazar monitoring radio loud quasar radio pulsar

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A Lighthouse at the Edge of the Universe ✎