Jetted Narrow-Line Seyfert 1 Galaxies & Co.: where do we stand?

TL;DR

This paper reviews the discovery and characteristics of gamma-ray emitting Narrow-Line Seyfert 1 Galaxies and related AGN, highlighting recent findings and future prospects in understanding their relativistic jets.

Contribution

It provides a comprehensive review of the literature on gamma-ray NLS1s and other AGN classes emitting jets, summarizing recent discoveries and discussing future research directions.

Findings

Gamma-ray NLS1s demonstrate relativistic jets from small-mass black holes.

Fermi LAT has expanded the known classes of gamma-ray emitting AGN.

The physical nature of these objects remains under active investigation.

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 analyses the perspectives for the forthcoming years.