2.4.2. Narrow line Seyfert 1 galaxies
Osterbrock & Pogge (1985) identified a class of AGNs having all the properties of the Seyfert 1s with, however, very narrow Balmer lines and strong optical Fe II lines; they are called ``narrow-line Seyfert 1s'' or NLS1s. Quantitatively, a Seyfert 1 is called a NLS1 if the FWHM of the ``broad'' component of the Balmer lines is smaller than 2 000 km s-1 () and if the ratio of [O III] is < 3 (). However, there is a continuous distribution of optical line widths in Seyfert 1s and the separation between BLS1s (broad line Seyfert 1s) and NLS1s is arbitrary ().  have found a few objects with ``narrow'' broad Balmer lines which have both weak Fe II and strong [O III], as well as objects with ``broad'' lines and strong Fe II and weak [O III]. Three of the superstrong Fe II emitters have relatively broad Balmer components (FWHM > 3 000 km s-1, seet table 2).
NLS1s are generally found to be radio quiet; there are only two radio loud NLS1s known so far (PKS0558-504 and RGBJ0044+193) ().
The soft photon spectral index (0.1-2.4 keV) of Seyfert 1s, which measures the relative strength of their soft component, is well correlated with the Balmer line width; strong soft X-ray excess objects are NLS1s (; ; ; ). However,  have shown that this correlation holds for the high luminosity AGNs but does not appear for the low-luminosity objects.
There is a correlation between the strength of the high-ionization lines and the X-ray spectral index; strong high-ionization lines occur predominantly in objects with a soft X-ray excess (). In NLS1s, [Fe VII] 6087 and [Fe X]6375 are often present in emission ().
The soft excess component of NLS1s can be modelled by a black body with temperatures in the range 0.1 to 0.25 keV (; ), rather similar to the temperature range found for the BLS1 soft excesses. It is often so strong that it rules out models in which the soft excess is produced through reprocessing of the hard continuum ().
NLS1s very frequently exhibit rapid and/or high-amplitude X-ray variability (; ; ; ). They have larger amplitudes than the BLS1s (). The nuclear component of NGC4051 shows strong soft X-ray variability with X-ray intensity changing by a factor 2-3 on a time-scale of a few 100 sec (); it has also been observed once while its HX flux was about 20 times fainter than its historical average value; the observed flat spectrum and intense iron line (EW ~ 600 eV) are best explained assuming that the active nucleus has switched off, leaving only a residual reflection component visible (). Observed variabilities by a factor ~ 2 in a few hours show that a substantial fraction of the soft component comes from a compact region, smaller than a light-day; this radiation must therefore be seen directly and not be reprocessed by a larger region (; ). Giant-amplitude X-ray variability (up to about two orders of magnitude) has been observed in IRAS13224-3809 (), PHL1092 (; Brandt et al. 1999a), REJ1237+264 (; Grupe et al. 1995a), W7 () and RXJ0947.0+4721 ().
There is a very strong anticorrelation between the H FWHM and the amplitude of X-ray variability in the ~0.5-10 keV bandpass; this correlation is consistent with rapid variability and narrow lines being a result of a small central mass (; ).
Standard accretion disks are not able to account for the soft X-ray excess unless the Eddington ratio is close to unity (; ). The observed correlation between short-term X-ray variability and spectral steepness leads to the same conclusion (; ).
If the broad-line emitting region is gravitationally linked to the central BH, one can show that the FWHM of the lines depends on the mass of the BH, the ratio of the luminosity to the Eddington luminosity and the angle between the rotation axis of the gas disk and the line of sight. The NLS1s could be objects with a low mass BH radiating near the Eddington limit, while the AGNs with relatively narrow broad Balmer lines but with strong [O III] and weak Fe II could be AGNs seen perpendicularly to the disk (; ).
A serious concern for models based upon a fundamental difference in central mass is that  found broad components to UV emission lines of both NLS1s and BLS1s while  have observed variable broad (4900 km s-1 FWHM) line components in the spectrum of the NLS1 Mark110.
NGC5905 (; ), RXJ1242.6-1119 (Komossa & Greiner 1999) and RXJ1624.9+7554 () are interesting cases. All three are highly variable X-ray sources; in the high state, the spectra are extremely soft; optical spectra, taken several years after the X-ray outburst in each case, show no signs of Seyfert-like activity. Komossa & Bade (1999) suggested that these high amplitude X-ray outbursts could be explained by tidal disruption of a star by a central supermassive BH.