Abstract
The discovery in 2008 of high-energy gamma-rays from Narrow-Line Seyfert 1 Galaxies (NLS1s) made it clear that there were active galactic nuclei (AGN) other than blazars and radio galaxies that can eject powerful relativistic jets. In addition to NLS1s, the great performance of the Fermi Large Area Telescope made it possible to discover MeV-GeV photons emitted from more classes of AGN, like Seyferts, Compact Steep Spectrum Gigahertz Peaked Sources (CSS/GPS), and disk-hosted radio galaxies. Although observations indicate a variety of objects, their physical characteristics point to a central engine powered by a relatively small-mass black hole (but, obviously, there are interpretations against this view). This essay critically reviews the literature published on these topics during the last eight years and analyzes the perspectives for the forthcoming years.
Highlights
The Status before 2012High-energy gamma rays from jetted narrow-line Seyfert 1 galaxies (NLS1) were detected for the first time in 2008 by the Fermi Large Area Telescope (LAT), after the early months of operations [1,2,3,4].In 2012, I wrote an extensive review on this topic, dealing with the history before and after the discovery of the gamma-ray emission, and I refer the reader to that article [5]
A certain degree of similarity with flat-spectrum radio quasars (FSRQs) was apparent since the early studies [1,2,3], but other observational differences did not permit to establish if NLS1s and FSRQs are drawn from the same population or if there are intrinsic differences
Non-jetted NLS1s are known to be active galactic nuclei (AGN) with small black hole masses and high accretion rates, which in turn is confirmed by the morphology of their host galaxy, generally spirals with pseudobulges or bars [11,12]
Summary
High-energy gamma rays from jetted narrow-line Seyfert 1 galaxies (NLS1) were detected for the first time in 2008 by the Fermi Large Area Telescope (LAT), after the early months of operations [1,2,3,4]. A certain degree of similarity with flat-spectrum radio quasars (FSRQs) was apparent since the early studies [1,2,3], but other observational differences (relatively low observed luminosities, compact radio morphology, narrowness of permitted emission lines, different host galaxy...) did not permit to establish if NLS1s and FSRQs are drawn from the same population or if there are intrinsic differences. This review mostly deals with NLS1s, and includes other AGN displaying relativistic jets as they are all member of the set of jetted AGN powered by small-mass black holes.
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