X-ray spectral complexity in narrow-line Seyfert 1 galaxies

We present a systematic analysis of the X-ray spectral properties of a samp le of 22 'narrow-line' Seyfert 1 galaxies for which data are available from the ASCA public archive. Many of these sources, which were selected on the basis of their relatively narrow H beta linewidth (FWHM less than or equal to 2000 km s(-1)), show significant spectral complexity in the X-ray band. Their measured hard power-law continua have photon indices spanning the ra nge 1.6-2.5 with a mean of 2.1, which is only slightly steeper than the nor m for 'broad-line' Seyfert 1s. All but four of the sources exhibit a soft e xcess, which can be modelled as blackbody emission (T-bb approximate to 100 -300 eV) superposed on the underlying power law. This soft component is oft en so strong that, even in the relatively hard bandpass of ASCA, it contain s a significant fraction, if not the bulk, of the X-ray luminosity, apparen tly ruling out models in which the soft excess is produced entirely through reprocessing of the hard continuum. Most notably, six of the 22 objects show evidence for a broad absorption fe ature centred in the energy range 1.1-1.4keV, which could be the signature of resonance absorption in highly ionized material. A further three sources exhibit 'warm absorption' edges in the 0.7-0.9keV bandpass. Remarkably, al l nine 'absorbed' sources have H beta linewidths below 1000 km s(-1), which is less than the median value for the sample taken as a whole. This tenden cy for very narrow linewidths to correlate with the presence of ionized abs orption features in the soft X-ray spectra of NLS1s, if confirmed in larger samples, may provide a further clue in the puzzle of active galactic nucle i.