Peaked sources and narrow-line Seyfert 1s: A love story

Abstract

The first similarities between peaked sources (PS) and narrow-line Seyfert 1 (NLS1) galaxies were noticed already 20 years ago. Nowadays, it is known that several sources can share both classifications, and that part of the parent population of gamma-ray emitting NLS1s could be hiding among PS. In this brief review, we describe how and why this orientation-based unification was developed. We also show how the recent discovery of absorbed radio jets in NLS1s basically invisible at frequencies below 10 GHz, could impact our knowledge of PS and, in particular, render the widely used radio-loudness parameter obsolete.