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We report on the discovery of one of the most extreme cases of high-frequencyradio variability ever measured in active galactic nuclei (AGN), observed ontimescales of days and exhibiting variability amplitudes of three to fourorders of magnitude. These sources, all radio-weak narrow-line Seyfert 1 (NLS1)galaxies, were discovered some years ago at Aalto University Mets\"ahovi RadioObservatory (MRO) based on recurring flaring at 37 GHz, strongly indicating thepresence of relativistic jets. In subsequent observations with the Karl G.Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0~GHz no signs of jets wereseen. To determine the cause of their extraordinary behaviour, we observed themwith the JVLA at 10, 15, 22, 33, and 45 GHz, and with the Very Long BaselineArray (VLBA) at 15 GHz. These observations were complemented with single-dishmonitoring at 37 GHz at MRO, and at 15 GHz at Owens Valley Radio Observatory(OVRO). Intriguingly, all but one source either have a steep radio spectrum upto 45 GHz, or were not detected at all. Based on the 37 GHz data the timescalesof the radio flares are a few days, and the derived variability brightnesstemperatures and variability Doppler factors comparable to those seen inblazars. We discuss alternative explanations for their extreme behaviour, butso far no definite conclusions can be made. These sources exhibit radiovariability at a level rarely, if ever, seen in AGN. They might represent a newtype of jetted AGN, or a new variability phenomenon, and thus deserve ourcontinued attention.

Unprecedented extreme high-frequency radio variability in early-stage active galactic nuclei