Introduction to Narrow Line Seyfert 1 Galaxies

ROSAT, ASCA and BeppoSAX observations have shown many NLS1s to have characteristic, unique and extreme X-ray properties. These include strong soft X-ray excess emission, extremely rapid and large-amplitude X-ray variability, steep 2-10 keV power-law continua, and unusual discrete spectral features. The discovered relations between the slopes of the soft and hard X-ray continua and the optical line widths in NLS1s may allow us to make statements on the physical conditions in the accretion disc and the accretion disc corona. X-ray observations of NLS1s are therefore a new and effective tool in the study of the innermost parts of active galactic nuclei. NLS1s do not appear to form a distinct class, but are rather connected to the 'standard' broad-line Seyfert 1s through a continuum of properties. More generally, soft X-ray excess strength is directly connected with the strong set of optical emission line correlations studied by [8]. These correlations emerged as the 'primary eigenvector' of the Principal Component Analysis (PCA) performed by Boroson & Green, and they relate optical line width, Fe II emission and forbidden-line emission. The primary eigenvector represents the strongest set of optical emission line correlations found among type 1 Seyferts/quasars, and ultrasoft NLS1s fall toward the narrow-H^/strong-Fe II/weak-[OIII] extreme of the primary eigenvector. A proper understanding of the Boroson & Green eigenvector would be a major step forward towards determination of the structure and kinematics of Seyfert/quasar emission line regions.

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