Abstract
Before the launch of the Fermi Gamma-ray Space Telescope satellite only two classes of active galactic nuclei (AGN) were known to generate relativistic jets and thus to emit up to the γ -ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The discovery by the Large Area Telescope (LAT) on-board the Fermi satellite of variable γ -ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of an emerging third class of AGN with powerful relativistic jets. Considering that NLSy1 are usually hosted in late-type galaxies with relatively small black hole masses, this finding opened new challenging questions about the nature of these objects, the disc/jet connection, the emission mechanisms at high energies, and the formation of relativistic jets. In this review, I will discuss the broad-band properties of the γ -ray-emitting NLSy1 included in the Fourth Fermi LAT source catalog, highlighting major findings and open questions regarding jet physics, black hole mass estimation, host galaxy and accretion process of these sources in the Fermi era.
Highlights
A small percentage of active galactic nuclei (AGN) is radio-loud, and this characteristic is commonly ascribed to the presence of a relativistic jet, roughly perpendicular to the accretion disc
(Γ X < 2), which is similar to what is observed in FSRQ rather than radio-quiet narrow-line Seyfert 1 galaxies (NLSy1), suggesting that a significant contribution of inverse Compton (IC) radiation from a relativistic jet dominates that part of the X-ray spectrum
Most of the optical data in the spectral energy distribution (SED) are explained by synchrotron emission, the X-ray data by synchrotron self Compton emission, and the high-energy bump is modelled by an external Compton component with seed photons from a dust torus
Summary
A small percentage of active galactic nuclei (AGN) is radio-loud, and this characteristic is commonly ascribed to the presence of a relativistic jet, roughly perpendicular to the accretion disc. The discovery by Fermi-LAT of variable γ-ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of a possible third class of AGN with powerful relativistic jets (e.g., [6,7,8]). The detection of variable γ-ray emission from radio-loud NLSy1 poses intriguing questions about the nature of these sources, the production of relativistic jets, and the mechanisms of high-energy emission in the different class of AGN. In this context the study of NLSy1 has received increasing attention.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
