Extreme radio variability in early-stage active galactic nuclei

I report on the discovery of extreme 37 GHz radio variability originating from early-stage active galactic nuclei (AGN). Most of these AGN are narrow-line Seyfert 1 (NLS1) galaxies, harbouring fast-growing, low-mass supermassive black holes, accreting at high Eddington ratios. These AGN exhibit amplitude variability of three-to-four orders of magnitude over e-folding timescales of a few days. However, despite our relentless attempts, we have not detected relativistic jets in these sources, and thus, they might be displaying a new kind of variability phenomenon in AGN. Recently, one of these sources exhibited a series of 37 GHz flares and was followed up by several radio and X-ray facilities. Based on these observations, we estimated an e-folding timescale of some hours, leading to variability brightness temperatures and variability Doppler factors that are extremely hard to explain by an incoherent emitter, suggesting a possible detection of coherent emission from an AGN or brightness temperatures far exceeding the inverse Compton catastrophe limit. In this talk, I will present our multiwavelength investigation, including future plans, into this subset of AGN, and discuss the importance of the next-generation astronomical facilities for our efforts to unveil the nature of these extraordinary sources.