X-Ray Properties of Narrow-Line Seyfert 1 Galaxies with Very Small Broad-Line Widths

Details X-Ray Properties of Narrow-Line Seyfert 1 Galaxies with Very Small Broad-Line Widths X-Ray Properties of Narrow-Line Seyfert 1 Galaxies with Very Small Broad-Line Widths

2010-11-15

arxiv.org/abs/1011.3283v3

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

11 figures and 4 table. Accepted for publication in the Astrophysical Journal

Scientific paper

Narrow-line Seyfert\,1 galaxies (NLS1s) with very small broad-line widths (say, FWHM(\hb) $\la $ 1200\,\kms) represent the extreme type of Seyfert\,1 galaxies that have small black hole masses (\mbh) and/or high Eddington ratios (\redd). 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 (VNLS1s), using archival \xmm\ data. It is found that the Fe K$\alpha$ 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(\hb), i.e., smaller \mbh\ ($\sim 10^6$ \msun) and/or higher \redd. 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 Risaliti et al.'s predictions of the relationship with \redd\ suggested previously. Short timescale (1--2 hours) X-ray variability is common, which, together with the variability amplitude computed for some of the objects, are supportive of the scenario that NLS1s are indeed AGN with relatively small \mbh.

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