Before the launch of the Fermi satellite only two classes of AGN were known to generate relativistic jets and thus emit up to the gamma-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The first two years of observations by the Large Area Telescope (LAT) on board Fermi confirmed that these two are the most numerous classes of identified sources in the extragalactic gamma-ray sky, but the discovery of variable gamma-ray emission from 5 radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s) revealed the presence of a possible emerging third class of AGN with relativistic jets.
Considering also that NLS1s are typically hosted in spiral galaxy, this finding poses intriguing questions about the nature of these objects, the onset of production of relativistic jets, and the cosmological evolution of radio-loud AGN. Here, we report on a first systematical investigation of the properties of a large sample of radio-loud NLS1 at MeV-GeV photon energies, utilizing the four-year accumulation of Fermi LAT data. In addition we discuss the radio-to-gamma-rays properties of the gamma-ray emitting Narrow-Line Seyfert 1 in the context of the blazar scenario, in particular of the two flaring NLS1s PMN J0948+0022 and SBS 0846+513.