X-ray Properties of Narrow-line Seyfert 1 Galaxies with Very Small Broadline Widths

Narrow-line Seyfert 1 galaxies (NLS1s) with very small broadline widths (say, FWHM(Hβ) lsim1200 km s^-1) represent the extreme type of Seyfert 1 galaxies that have small black hole masses (M _BH) and/or high Eddington ratios (L/L _Edd). Here, we study the X-ray properties of a homogeneously and optically selected sample of 13 such objects, termed as very narrow line Seyfert 1 galaxies, using archival XMM-Newton data. It is found that the Fe Kα emission line is at most weak in these objects. A soft X-ray excess is ubiquitous, with the thermal temperatures falling within a strict range of 0.1-0.2 keV. Our result highlights the puzzling independence of the thermal temperature by extending the relations to even smaller FWHM(Hβ), i.e., smaller M _BH (~10⁶ M _sun) and/or higher L/L _Edd. The excess emission can be modeled by a range of viable models, though the disk reflection and Comptonization models generally give somewhat better fits over the smeared absorption and the p-free models. At the Eddington ratios around unity and above, the X-ray spectral slopes in the 2-10 keV band are systematically flatter than the predictions of the relationship with L/L _Edd suggested previously. Short timescale (1-2 hr) X-ray variability is common, which, together with the variability amplitude computed for some of the objects, is supportive of the scenario that NLS1s are indeed active galactic nuclei with relatively small M _BH.