A few decades have passed since the identification of narrow-line Seyfert 1 (NLS1) galaxies as a subclass of active galactic nuclei (AGN). NLS1s show a Seyfert 1-like spectrum, but with emission line widths similar to those of Seyfert 2 spectra. These peculiar traits suggest a combination of factors: high accretion rates near the Eddington limit and low-mass black holes. Although few, jetted NLS1s have been discovered. Recently, seven sources with an inverted radio spectrum and extreme radio variability were identified, among a sample of radio-quiet and radio-silent jetted NLS1s. They show rapid high-frequency flares at 37 GHz that increase their flux density up to 9000-fold (Jy level) with an e-folding timescale of less than 10 hours. During quiescence and at lower frequencies, they achieve only mJy levels. Until now, their radio spectra are the only common feature. I will present the results of a multi-epoch analysis, devoted to search optical variability and periodicities using publicly available data. For this purpose I performed Fourier analyses, to find any similarity between such extreme flaring objects. Moreover, a long-term variability comparison between radio and optical light curves, and an optical spectra analysis have been carried out. Beside these the role of the Sardinia Radio Telescope in the timescale hunting will be presented. Such strong radio variability had never been observed before in any AGN, making this work an important key to understanding the phenomenon. Furthermore, understanding the physics behind these peculiar objects may ultimately help us reveal a new and unexplored population of jetted AGN.

Brief CV of Luca Crepaldi:

PhD student in Astronomia at the Università degli Studi di Padova.
Bachelor in 'Astronomy' and Master in 'Astrophysics and Cosmology' at the Università degli Studi di Padova, with a partial collaboration with the Metsähovi Radio Observatory (Finland).
Topic: Narrow-line Seyfert 1 galaxies multiwavelength studies, mainly in the optical and radio bands.
About 50 observing night with optical telescopes (UT4 at the Paranal observatory, NTT at the La Silla observatory, Tel. Galileo and Copernico at the Asiago observatory).