Soft X-ray properties of "narrow-line" Seyfert 1 galaxies

TL;DR

This study systematically analyzes the soft X-ray spectra of 46 narrow-line Seyfert 1 galaxies, revealing steeper spectra and potential correlations with optical line widths, and discusses models explaining these properties.

Contribution

It provides the first comprehensive analysis of ROSAT observations of NLS1 galaxies, highlighting their unique X-ray spectral characteristics and evaluating various theoretical models.

Findings

NLS1 galaxies have steeper soft X-ray spectra than typical Seyfert 1s.

An anticorrelation exists between X-ray slope and H-beta line width.

Photon indices can reach values up to about 5 in NLS1s.

Abstract

We report on AGN with extremely soft X-ray spectra observed with ROSAT. From their optical emission lines these objects are classified as narrow-line Seyfert 1 galaxies (NLS1), almost all with extremely large Fe II/H-beta ratios and relatively narrow optical lines of hydrogen. Our results are based on a systematic study of 46 NLS1. We find that NLS1 have generally steeper soft X-ray continuum slopes than normal Seyfert 1s, and there may exist an anticorrelation between 0.1-2.4 keV continuum slope and the FWHM of the H-beta line. Objects with steep 0.1-2.4 keV continuum slopes and H-beta FWHM > 3000 km s^{-1} are clearly discriminated against by nature. When simple power-law models are fit to the data, photon indices reach values up to about 5, much higher than is usually seen in Seyfert 1s. We discuss steep ROSAT spectra in light of soft X-ray excess and hard X-ray tail models. We consider models for NLS1 where they are Seyfert 1s with extremal values of pole-on orientation, black hole mass and/or accretion rate, warm absorption and BLR thickness and confront these models with the known properties of NLS1. All simple models appear to have drawbacks, but models with smaller mass black holes and thicker BLRs show some promise. We suggest specific further tests of the models.