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We present the X-ray spectroscopic study of the Compton-thick (CT) active galactic nuclei (AGN) population within the Chandra Deep Field South (CDF-S) by using the deepest X-ray observation to date, the Chandra 7 Ms observation of the CDF-S. We combined an optimized version of our automated selection technique and a Bayesian Monte Carlo Markov chains (MCMC) spectral fitting procedure, to develop a method to pinpoint and then characterize candidate CT AGN as less model dependent and/or data-quality dependent as possible. To obtain reliable automated spectral fits, we only considered the sources detected in the hard (2−8 keV) band from the CDF-S 2 Ms catalog with either spectroscopic or photometric redshifts available for 259 sources. Instead of using our spectral analysis to decide if an AGN is CT, we derived the posterior probability for the column density, and then we used it to assign a probability of a source being CT. We also tested how the model-dependence of the spectral analysis, and the spectral data quality, could affect our results by using simulations. We finally derived the number density of CT AGN by taking into account the probabilities of our sources being CT and the results from the simulations. Our results are in agreement with X-ray background synthesis models, which postulate a moderate fraction (25%) of CT objects among the obscured AGN population. [ABSTRACT FROM AUTHOR]

The hard X-ray sky at energies above 10 keV has been extensively explored by the Swift/Gehrels and the NuSTAR missions in the 14−195 keV and the 3−24 keV bands. respectively. The main population of the hard X-ray detected sources are active galactic nuclei (AGN). A discrepancy has been reported between the number count distributions of the two missions in the sense that the extrapolation of the Swift/Gehrels number counts in the flux regime sampled by NuSTAR lies significantly below the NuSTAR counts. We explore this aspect anew by comparing the number count distributions derived from the Swift/Gehrels 105-month catalogue with those from the serendipitous NuSTAR source catalogue. We use a novel number count distribution estimator that makes use of the C-statistic on the unbinned data. In the 14−195 keV band, the Swift/BAT counts follow a Euclidean slope with α = 1.51 ± 0.10 (90% confidence level). The NuSTAR counts in the 8−24 keV band present a steeper slope with α = 1.71 ± 0.20 because of an upturn at fluxes below ∼2 × 10−13 erg cm−2 s−1. The same upturn is observed in the soft (3−8 keV) NuSTAR number counts, which generally also present a steep slope with α = 1.82 ± 0.15. Only the bright part of the NuSTAR 3−8 keV counts agrees with the Chandra number counts in the 2−10 keV band, while the fainter part (below ∼7 × 10−13 erg cm−2 s−1) of the soft NuSTAR counts is in marked disagreement with the Chandra counts. Next, we compare the derived number counts in the different bands using our X-ray AGN population synthesis models. The comparison between the Chandra and the Swift/BAT number counts shows a very good agreement for the standard AGN spectrum with a power-law slope Γ = 1.9, a high-energy cut-off at ∼130 keV and a 2−10 keV reflection component of 3%. On the other hand, using this standard AGN spectral model, only the bright part of the NuSTAR 8−24 keV and 3−8 keV number counts agrees with the model predictions. It is therefore most likely that the disagreement between the X-ray number counts in the different bands is caused by the faint NuSTAR number counts. We discuss various possibilities for the origin of this disagreement. [ABSTRACT FROM AUTHOR]

The evolution of the accretion history of the Universe has been studied in unprecedented detail owing to recent X-ray surveys performed by Chandra and XMM-Newton. A focus on the most heavily obscured or Compton-thick active galactic nuclei (AGNs) is missing in these studies. These AGNs evade detection even in X-ray surveys owing to their extreme hydrogen column densities, which exceed 1024 cm−2. Recently, the all-sky hard X-ray survey performed by Swift/BAT brought a breakthrough, allowing the detection of many of these AGNs. This is because of the very high energy bandpass (14–195 keV) of this instrument, which helps to minimise attenuation effects. In our previous work, we identified more than 50 candidate Compton-thick AGNs in the local Universe, corresponding to an observed fraction of about 7% of the total AGNs population. This number can only be converted to the intrinsic Compton-thick AGNs number density if we know their exact selection function. This function sensitively depends on the form of the Compton-thick AGN spectrum, that is the energy of their absorption turnover, photon-index and its cut-off energy at high energies, and the strength of the reflection component on the matter surrounding the nucleus. For example, the reflection component at hard energies 20–40 keV antagonises the number density of missing Compton-thick AGNs in the sense that the stronger the reflection the easier these sources are detected in the BAT band. In order to constrain their number density, we analysed the spectra of 19 Compton-thick AGNs that have been detected with Swift/BAT and have been subsequently observed with NuSTAR in the 3–80 keV band. We analysed their X-ray spectra using the MYTORUS models which properly take into account the Compton scattering effects. These were combined with physically motivated Comptonisation models, which accurately describe the primary coronal X-ray emission. We derived absorbing column densities that are consistent with those derived by the previous Swift/BAT analyses. We estimate the coronal temperatures to be roughly between 25 and 80 keV corresponding to high energy cut-offs roughly between 75 and 250 keV. Furthermore, we find that the majority of our AGNs lack a strong reflection component in the 20–40 keV band placing tighter constraints on the intrinsic fraction of Compton-thick AGNs. Combining these results with our X-ray background synthesis models, we estimate a percentage of Compton-thick AGNs in the local Universe of ≈20 ± 3 % relative to the type-II AGNs population. [ABSTRACT FROM AUTHOR]

We present an X-ray spectroscopic study of optically selected (SDSS) Seyfert 2 (Sy2) galaxies. The goal is to study the obscuration of Sy2 galaxies beyond the local universe, using good quality X-ray spectra in combination with high S/N optical spectra for their robust classification. We analyzed all available XMM-Newton archival observations of narrow emission line galaxies that meet the above criteria in the redshift range 0.05 < z < 0.35. We initially selected narrow line AGN using the SDSS optical spectra and the BPT classification diagram. We further modeled and removed the stellar continuum, and we analyzed the residual emission line spectrum to exclude any possible intermediate-type Seyferts. Our final catalog comprises 31 Sy2 galaxies with median redshift z ~0.1. X-ray spectroscopy is performed using the available X-ray spectra from the 3XMM and the XMMFITCAT catalogs. Implementing various indicators of obscuration, we find seven (~23%) Compton-thick AGN. The X-ray spectroscopic Compton-thick classification agrees with other commonly used diagnostics, such as the X-ray to mid-IR luminosity ratio and the X-ray to [OIII]5007 luminosity ratio. Most importantly, we find four (~13%) unobscured Sy2 galaxies, at odds with the simplest unification model. Their accretion rates are significantly lower than the rest of our Sy2 sample, in agreement with previous studies that predict the absence of the broad line region below a certain Eddington ratio threshold. [ABSTRACT FROM AUTHOR]

Wide-area X-ray and far-infrared surveys are a fundamental tool to investigate the link between AGN growth and star formation, especially in the low-redshift universe (z ≲ 1). The Herschel Terahertz Large Area survey (H-ATLAS) has covered 550 deg2 in five far-infrared and sub-mm bands, 16 deg2 of which have been presented in the Science Demonstration Phase (SDP) catalogue. Here we introduce the XMM-Newton observations in the H-ATLAS SDP area, covering 7.1 deg2 with flux limits of 2 × 10-15, 6 × 10-15, and 9×10-15 erg s-1 cm-2 in the 0.5-2, 0.5-8, and 2-8 keV bands, respectively. We present the source detection and the catalogue, which includes 1700, 1582, and 814 sources detected by EMLDetect in the 0.5-8, 0.5-2, and 2-8 keV bands, respectively; the number of unique sources is 1816. We extract spectra and derive fluxes from power-law fits for 398 sources with more than 40 counts in the 0.5-8 keV band. We compare the best-fit fluxes with those in the catalogue, which are obtained assuming a common photon index of Γ = 1.7; we find no bulk difference between the fluxes and a moderate dispersion of s = 0.33 dex. Using the fluxes from the spectral fits wherever possible, we derive the 2-10 keV Log N-Log S , which is consistent with a Euclidean distribution. Finally, we release the computer code for the tools developed for this project. [ABSTRACT FROM AUTHOR]

Heavily obscured, Compton thick (CT, NH > 1024 cm-2) active galactic nuclei (AGN) may represent an important phase in AGN/galaxy co-evolution and are expected to provide a significant contribution to the cosmic X-ray background at its peak. However, unambiguously identifying CT AGN beyond the local Universe is a challenging task even in the deepest X-ray surveys, and given the expected low spatial density of these sources in the 2-10 keV band, large area surveys are needed to collect sizable samples. Through direct X-ray spectra analysis, we selected 39 heavily obscured AGN (NH > 3 × 1023 cm-2) at bright X-ray fluxes (F2-10 ≳ 10-14 erg s-1 cm-2) in the 2 deg² XMM-COSMOS survey. After selecting CT AGN based on the fit of a simple absorbed two power law model to the shallow XMM-Newton data, the presence of bona fide CT AGN was confirmed in 80% of the sources using deeper Chandra data and more complex models. The final sample comprises ten CT AGN (six of them also have a detected Fe Ka line with EW ~ 1 keV), spanning a wide range of redshifts (z ~ 0.1-2.5) and luminosity (L2-10 ~ 1043.5-1045 erg s-1) and is complemented by 29 heavily obscured AGN spanning the same redshift and luminosity range. We collected the rich multi-wavelength information available for all these sources, in order to study the distribution of super massive black hole and host properties, such as black hole mass (MBH), Eddington ratio (λEdd), stellar mass (M*), specific star formation rate (sSFR) in comparison with a sample of unobscured AGN. We find that highly obscured sources tend to have significantly smaller MBH and higher λEdd with respect to unobscured sources, while a weaker evolution in M* is observed. The sSFR of highly obscured sources is consistent with the one observed in the main sequence of star forming galaxies, at all redshifts. We also present and briefly discuss optical spectra, broadband spectral energy distribution (SED) and morphology for the sample of ten CT AGN. Both the optical spectra and SED agree with the classification as highly obscured sources: all the available optical spectra are dominated by the stellar component of the host galaxy, and to reproduce the broadband SED, a highly obscured torus component is needed for all the CT sources. Exploiting the high resolution Hubble-ACS images available, we are able to show that these highly obscured sources have a significantly larger merger fraction with respect to other X-ray selected samples of AGN. Finally we discuss the implications of our findings in the context of AGN/galaxy co-evolutionary models, and compare our results with the predictions of X-ray background synthesis models. [ABSTRACT FROM AUTHOR]

The majority of active galactic nuclei (AGNs) are obscured by large amounts of absorbing material that makes them invisible at many wavelengths. X-rays, given their penetrating power, provide the most secure way for finding these AGNs. The XMM-Newton serendipitous source catalog, of which 3XMM-DR4 is the latest version, is the largest catalog of X-ray sources ever produced; it contains about half a million detections. These sources are mostly AGNs.We have derived X-ray spectral fits for very many 3XMM-DR4 sources (≳114 000 observations, corresponding to ∼77 000 unique sources), which contain more than 50 source photons per detector. Here, we use a subsample of ≃1000 AGNs in the footprint of the SDSS area (covering 120 deg2) with available spectroscopic redshifts. We searched for highly obscured AGNs by applying an automated selection technique based on X-ray spectral analysis that is capable of efficiently selecting AGNs. The selection is based on the presence of either a) flat rest-frame spectra from a simple power-law fit; b) flat observed spectra from an absorbed power-law fit; c) an absorption turnover, indicative of a high rest-frame column density; or d) the presence of an Fe Kα line with a large equivalent width (>500 eV). We found 81 highly obscured candidate sources. Subsequent detailed manual spectral fits revealed that 28 of them are heavily absorbed by column densities higher than 1023 cm-2. Of these 28 AGNs, 15 are candidate Compton-thick AGNs on the basis of either a high column density, consistent within the 90% confidence level with NH > 1024 cm-2, or a large equivalent width (>500 eV) of the Fe Kα line. Another six are associated with near-Compton-thick AGNs with column densities of ∼5 x 1023 cm-2. A combination of selection criteria a) and c) for low-quality spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly absorbed AGNs with an efficiency of 80%. [ABSTRACT FROM AUTHOR]