Narrow-line Seyfert 1 (NLS1) galaxies are a subclass of active galactic nuclei (AGN) identified more than 30 years ago, but still not entirely understood. These objects are likely characterized by rapidly growing low-mass black holes. Interestingly enough, some of them have been detected in gamma-rays, a sign that they can harbor powerful relativistic jets. In my talk I will review their properties, showing how the true nature of NLS1s is that of early-stage AGN in a recently triggered activity phase, and how they are connected to other classes of kinematically young jetted AGN. I will also report on the discovery, in a handful of NLS1s, of relativistic jets remarkably faint at low radio frequency, but extremely bright and variable at high radio frequency. New observations of these sources are revealing an increasingly complicated picture, and I will outline the different scenarios we are developing to explain this new, unexpected phenomenon.
The colloquium will be held in presence and online in "Sala Rosino"
Stefano Ciroi
