Wandel & Boller 1998). The line widths are attributed to virial motion ($M \approx V^2r/G$) in the vicinity of a supermassive black hole, but the orbital velocities are smaller than in more typical AGNs because of greater distance to the central source.

2.  NLS1s are low-inclination (i.e., nearly face-on) systems (Osterbrock & Pogge 1985). In this model, the line widths are again due to orbital motion around the central black hole, and the bulk of the broad-line region (BLR) gas orbits in a common plane that is almost perpendicular to the line of sight, leading to relatively small Doppler widths.

3.  NLS1s have relatively low black-hole masses, but high accretion rates. Again, the basic assumption is that the BLR motions are virial, but the central source has a lower mass. The luminosity can be kept relatively high by supposing that the accretion rate (relative to the Eddington rate) is correspondingly high in these sources.

The third of these explanations forms the current paradigm for NLS1s, as it explains not only the narrow emission lines, but possibly also the steepness of the soft X-ray spectrum: the temperature structure of an optically thick, geometrically thin accretion disk is given by $$T(r) = 