Narrow-line Seyfert 1 galaxies as an extreme of Seyfert activity

The recent progress in the study of narrow-line Seyfert 1 galaxies is summarized. A clear correlation between the strength of the soft X-ray excess and the optical line width of the Hβ line is detected. A similar relation is detected between the hard 2–10 keV continuum slope and the Hβ line width. The most extreme X-ray variability in radio-quiet active galaxies is seen to date in narrow-line Seyfert 1 galaxies. These newly discovered observational properties give rise to requirements on their physical interpretation. The extreme X-ray variability in some narrow-line Seyfert 1 galaxies is probably due to relativistic effects. The extreme soft X-ray excess emission detected in narrow-line Seyfert 1 galaxies may be due to a more highly ionized accretion disk, with respect to broad line Seyfert 1 galaxies. Narrow-line Seyfert 1 galaxies are therefore thought to form an extreme of Seyfert activity. Because of their extreme properties, the study of narrow-line Seyfert 1 galaxies should allow us to progress in solving the diverse problems more generally posed by the Seyfert phenomenon.