XMM-Newton observations of Seyfert galaxies from the Palomar spectroscopic survey: the X-ray absorption distribution

Institute of Astronomy & Astrophysics, National Observatory of Athens, I. Metaxa & B. Pavlou, Penteli, 15236 Athens, Greece

Received: 18 November 2008
Accepted: 16 March 2009

Abstract

We present XMM-Newton spectral analysis of all 38 Seyfert galaxies from the Palomar spectroscopic sample of galaxies. These are found at distances of up to 67 Mpc and cover the absorbed 2–10 keV luminosity range ~10³⁸–10⁴³ erg s^-1. Our aim is to determine the distribution of the X-ray absorption in the local Universe. Three of these are Compton-thick with column densities just above 10²⁴ cm^-2 and high equivalent width FeK_α lines (>700 eV). Five more sources have low values of the X-ray to [OIII] flux ratio suggesting that they could be associated with obscured nuclei. Their individual spectra show neither high absorbing columns nor flat spectral indices. However, their stacked spectrum reveals an absorbing column density of N_H ~ 10²³ cm^-2. Therefore the fraction of absorbed sources (>10²² cm^-2) could be as high as 55±12%. A number of Seyfert-2 appear to host unabsorbed nuclei. These are associated with low-luminosity sources L_X < 310⁴¹ erg s^-1. Their stacked spectrum again shows no absorption while inspection of the Chandra images, where available, shows that contamination from nearby sources does not affect the XMM-Newton spectra in most cases. Nevertheless, such low luminosity sources are not contributing significantly to the X-ray background flux. When we consider only the brighter, >10⁴¹ erg s^-1, 21 sources, we find that the fraction of absorbed nuclei rises to 75±19% while that of Compton-thick sources to 15–20%. The fraction of Compton-thick AGN is lower than that predicted by the X-ray background synthesis model in the same luminosity and redshift range.