An XMM-Newton study of six narrow-line Seyfert 1 galaxies at z=0.35-0.92

We report a detailed analysis of the XMM-Newton spectra of six narrow-line Seyfert 1 (NLS1) galaxies at redshift z = 0.35-0.92. Compared with the NLS1s at lower redshift in the previously most-studied sample, these NLS1s have larger black hole (BH) masses (log M-BH > 7.5) with similar or even lower Eddington ratios. Our extended XMM-Newton sample of NLS1s shows strong soft X-ray excess emission below 2 keV. The quantified soft excess strength does not show an obvious discrepancy from previous studies of the lower redshift NLS1s. The systematic effect in the measurement of the Eddington ratio mainly lies in the bolometric correction factor. We also tentatively fit the spectra assuming two more physical models for the soft excess: warm Comptonization and relativistic reflection from the inner accretion disc. In the first scenario, we confirm the ubiquity of a warm and optically thick corona. The behaviour of a single source can be better explained by relativistic reflection, although we cannot distinguish which model is a more favourable explanation for the soft excess based on the best-fitting statistics.