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arXiv · 2609.16384

NOCTURNE. II. Extreme radio variability in the heart of early-stage active galactic nuclei

Emilia Järvelä·Patrizia Romano·Anne Lähteenmäki·Marco Berton
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Abstract

We report the discovery of extreme 37 GHz radio variability in early-stage active galactic nuclei (AGN). Most of these sources belong to the class of narrow-line Seyfert 1 (NLS1) galaxies, which harbour fast-growing, low-mass supermassive black holes that accrete at high Eddington ratios. At 37 GHz, these extraordinary AGN exhibit amplitude variability of three to four orders of magnitude over timescales of a few days. Notably, despite several attempts, we have not detected relativistic jets in these sources, suggesting that this phenomenon may occur very close to the black hole and may represent a new form of AGN variability. One of these sources, which flared at 37 GHz, was followed up with the Karl G. Jansky Very Large Array and Swift XRT/UVOT. Based on these observations, we estimated an e-folding timescale of mere hours, leading to variability brightness temperatures and variability Doppler factors that are extremely difficult to explain by an incoherent emitter. This suggests a possible detection of coherent emission from an AGN, although more observations will be needed to distinguish between alternative scenarios.

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Emilia Järvelä, Patrizia Romano, Anne Lähteenmäki, Marco Berton. 2026-09-14. NOCTURNE. II. Extreme radio variability in the heart of early-stage active galactic nuclei. https://arxiv.org/abs/2609.16384

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