X-ray spectral properties of Seyfert galaxies and the unification scheme

Abstract

Context. The unification scheme of Seyfert galaxies predicts that the observed differences between types 1 and 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density owing to the edge-on view of the obscuring torus. Aims. We study the 0.5-10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim of testing the predictions of Seyfert unification scheme in the X-ray regime. Methods. We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in parameters that are independent of the orientation of AGN and the host galaxy. Results. The 0.5-10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model that consists of (i) an absorbed powerlaw; (ii) a narrow Gaussian fitted to the Fe Kα emission line; and (iii) an often seen soft-excess component characterized by either a thermal plasma model with temperature kT ∼ 0.1-1.0 keV or a steep powerlaw or both. The 2.0-10 keV hard X-ray continuum emission in several Seyfert type 2s is dominated by reflection and suggests the Compton-thick obscuration. Results of the statistical comparison between the distributions of (i) the observed X-ray luminosities in the soft (0.5-2.0 keV) and hard (2.0-10.0 keV) bands; (ii) the X-ray absorbing column densities; (iii) the equivalent widths of Fe Kα line; and (iv) the flux ratios of hard X-ray to [OIII] λ5007 Å, for the two Seyfert subtypes are consistent with the obscuration and orientation-based unification scheme.

Measuring the level of nuclear activity in Seyfert galaxies and the unification scheme

“…Using the same sample we have shown in our previous paper (Singh et al 2011, hereafter Paper I) that the comparisons of X-ray properties e.g., distributions of the observed X-ray luminosities, the absorbing column densities, the equivalent widths of Fe Kα line and the flux ratios of 2.0-10 keV hard X-ray to [OIII] line emission of the two Seyfert subtypes, are consistent with the Seyfert unification scheme.…”

“…It has been argued that the samples based on the properties that are independent to the orientation of the obscuring torus are more appropriate to test the unification scheme (Schmitt et al 2003;Lal et al 2011;Singh et al 2011). Such samples are less prone to the biases that are generally inherent in the samples derived from flux limited surveys.…”

X-ray spectral parameters for a sample of 95 active galactic nuclei

“…For Seyfert 2 with absorption ≥10 23 cm -2 the value of the relative reflection is bigger. Currently, the main the reason for this influence indicates the reflection from gas-dusty surrounding torus, namely that reflection is bigger for objects with higher inclination angle than for objects with lower one (Awaki et al 1991;Ricci et al 2011;Singh et al 2011;Ueda et al 2007). As a result, sometimes, on the average, in the 2-10 keV range only pure reflection spectrum from the torus walls can be observed, but not from the accretion disk (Awaki et al 1991;Singh et al 2011).…”

“…Currently, the main the reason for this influence indicates the reflection from gas-dusty surrounding torus, namely that reflection is bigger for objects with higher inclination angle than for objects with lower one (Awaki et al 1991;Ricci et al 2011;Singh et al 2011;Ueda et al 2007). As a result, sometimes, on the average, in the 2-10 keV range only pure reflection spectrum from the torus walls can be observed, but not from the accretion disk (Awaki et al 1991;Singh et al 2011). If the gas-dusty torus is not a homogeneous structure and consists of individual clouds, we can see a very large value of R Ricci et al 2011;Ueda et al 2007).…”

“…The main criterion that determines the Seyfert types in X-rays is the neutral absorption value (see e.g., Ricci et al 2011;Singh et al 2011). For our sample this criteria is true; H N values for type 2 h a s s y s t e m a t i c a l l y h i g h e r t h a n f o r t y p e 1 ( s e e F i g .…”