2025A&A...696A..74C

Over the last decade of active galactic nucleus (AGN) monitoring programs, the Metsähovi Radio Observatory has made multiple detections of seven powerful flaring narrow-line Seyfert 1 (NLS1) galaxies at 37 GHz. Several hypotheses have been proposed, but understanding this unique phenomenon is still far away. To look at the case from a different point of view, we performed an emission line analysis of the optical spectra, with the aim of identifying similarities among the sources, which in turn can possibly be tied with radio behavior. Our data were obtained with the Gran Telescopio Canarias. The results we obtained show that six out of the seven sources have typical properties for the NLS1 class, and one of them is an intermediate Seyfert galaxy. We found on average black hole masses above the median value for the class (> 10⁷ M_⊙), and a strong Fe II emission, which could be a proxy for an intense ongoing accretion activity. Although interesting, the characteristics we found are not unusual for this kind of AGN: the optical spectra of our sources do not relate with their unique radio properties. Therefore, further multi-wavelength studies will be necessary to narrow the field of hypotheses for this peculiar phenomenon.