Suzaku observations of hard X-ray selected Seyfert 2 galaxies

Subject headings: galaxies: active -X-rays: observations -Seyfert galaxies: individual (NGC 5728, NGC 4992, ESO263-G13, ESO 137-G34, ESO323-G32)

A fraction as high as 50% of Seyfert 2 galaxies in the nearby Universe are obscured in the X-ray band by column densities of the order of, or larger than the inverse of the Thomson cross-section (N H ≥ σ -1 T ≃ 1.5 × 10 24 cm -2 ), hence dubbed Compton thick (CT). If the optical depth (τ = N H σ T ) for Compton scattering does not exceed values of the order of “a few”, X-ray photons with energies higher than 10-15 keV are able to penetrate the obscuring material and reach the observer. For higher values of τ , the entire X-ray spectrum is depressed by Compton down scattering and the X-ray photons are effectively trapped by the obscuring material irrespective of their energy. The former class of sources (mildly CT) can be efficiently detected by X-ray instruments sensitive above 10 keV, while for the latter (heavily CT) their nature may be inferred through indirect arguments, such as the presence of a strong iron Kα line over a flat reflected continuum. The search for and the characterization of the physical properties of CT AGN is relevant to understand the evolution of accreting Supermassive Black Holes (SMBHs). In particular, mildly CT AGN are the most promising candidates to explain the so far largely unresolved spectrum of the X-ray background around its 30 keV peak (Worsley et al. 2005;Treister & Urry 2005;Gilli et al. 2007). According to the Gilli et al. (2007) XRB synthesis model, their integral contribution to the hard X-ray background is of the order of 25-30%. This fraction has been estimated under simplified hypotheses. In particular, the same luminosity function and cosmological evolution of unobscured and Compton thin AGN is assumed; moreover, the number density of “heavily” CT AGN is the same of “mildly” CT. While these assumptions are not inconsistent with the present observational framework (Risaliti et al. 1999;Guainazzi et al. 2005), it should be noted that absorption column densities in excess of 10 24 cm -2 were measured or inferred for about a few tens of nearby AGN (Comastri 2004;Della Ceca et al. 2008), and only a handful of them are known beyond the local Universe (Norman et al. 2002;Iwasawa et al. 2005). CT AGN may represent a significant fraction of the accretion power in the Universe and indeed a correction for the CT contribution to the estimates of the SMBH local mass density has to be included in the calculations (Marconi et al. 2004).

An unbiased census of extremely obscured AGN would require to survey the hard X-ray sky above 10 keV with good sensitivity. Such an argument is one of the key scientific drivers of the nustar (Harrison et al. 2010) and astro-h (Takahashi et al. 2010) missions, which will be launched in the next few years, and of the nhxm mission study (Pareschi et al. 2010).

For the time being, one has to rely on the observations obtained by the high-energy detectors on board bepposax, integral, swift and, more recently, Suzaku. All-sky surveys were performed using both the IBIS coded-mask telescope onboard integral and the BAT detector on board swift. Though limited to bright and thus low-redshift sources, they have proven to be quite successful. More than one hundred AGN are reported in both integral (Beckmann et al. 2009) and swift catalogues (Tueller et al. 2010;Cusumano et al 2010).

Hard X-ray selection is less biased against absorption and thus provides a useful benchmark to study the column density distribution of obscured AGN and the fraction of CT sources in the local Universe. The large majority of Integral and Swift sources were observed by XMM and Chandra and their broad band X-ray spectra discussed by Winter et al. (2008). While the already known, nearby CT AGN were recovered by Swift and Integral surveys, the fraction of “newly” discovered CT AGN is surprisingly low and apparently inconsistent, by about a factor of 2, with that predicted by Gilli et al. (2007) in the local Universe. On the basis of these findings, it has been proposed (Treister et al. 2009) that the contribution of CT AGN to the hard X-ray background may be significantly lower (by a factor of 2 to 3) than previously thought, with important implications for the evolution of the accretion power.

Since absorption column densities are often measured on relatively poor quality Xray spectra taken from non-simultaneous observations, above and below about 10 keV, and combining different instruments, the CT fraction and, more in general, t

…(Full text truncated)…