A. Akylas

Constraining the fraction of Compton-thick AGN in the universe by modelling the diffuse X-ray background spectrum

This paper investigates which constraints can be placed on the fraction of Compton-thick active galactic nuclei (AGN) in the Universe from modelling the spectrum of the diffuse X-ray background (XRB). We present a model for the synthesis of the XRB that uses as input a library of AGN X-ray spectra generated by Monte Carlo simulations. This is essential to account for the Compton scattering of X-ray photons in a dense medium and the impact of that process on the spectra of heavily obscured AGN. We identify a small number of input parameters to the XRB synthesis code that encapsulate the minimum level of uncertainty in reconstructing the XRB spectrum. These are the power-law index and high-energy cutoff of the intrinsic X-ray spectra of AGN, the level of the reflection component in AGN spectra, and the fraction of Compton-thick AGN in the Universe. We then map the volume of the space allowed to these parameters by current observational determinations of the XRB spectrum in the range 3−100 keV. One of the least-constrained parameters is the fraction of Compton-thick AGN. Statistically acceptable fits to the XRB spectrum at the 68% confidence level can be obtained for Compton-thick AGN fractions in the range 5−50%. This is because of degeneracies among input parameters to the XRB synthesis code and uncertainties in the modelling of AGN spectra (e.g. level of reflection fraction). The most promising route for constraining the fraction of Compton-thick AGN in the Universe is via the direct detection of those sources in high-energy (>10 keV) surveys. We show that the observed fraction of Compton-thick sources identified in the Swift/BAT serendipitous survey limits the intrinsic fraction of Compton-thick AGN, at least at low redshift, to 10−20% (68% confidence level). We also make predictions on the number density of Compton-thick sources that current and future X-ray missions are expected to discover. Testing those predictions with data will place tight constraints on the intrinsic fraction of Compton-thick AGN as a function of redshift.

The Compton-thick AGN in the Chandra Deep Field North

We present X-ray spectral analysis of the brightest sources (f2−10 keV > 10 −15 erg cm −2 s −1 ) in the Chandra Deep Field North. Our sample consists of 222 sources; for the vast majority (171) either a spectroscopic or a photometric redshift is available. Our goal is to discover the Compton-thick AGN in a direct way i.e. through their X-ray spectra. Compton-thick AGN give away their presence in X-rays either directly through the absorption turnover redshifted in the Chandra passband, or through a flat, reflection-dominated, spectrum. The above selection criteria yield 10 Compton-thick AGN candidates of which the nine are reflection dominated. The IR or sub-mm data where available, corroborate the presence of a heavily obscured nucleus in most cases. All the five candidate Comptonthick sources with available 24 μm data present very high values of the f24/fR flux ratio suggesting that they are dust obscured galaxies. The low fx/fIR ratio also suggest the presence of obscured nuclei in many cases. Four of the candidate Compton-thick sources are associated with sub-mm galaxies at high redshifts z ∼ 2. The number count vs. flux distribution of the candidate Compton-thick AGN as well as their distribution with redshift agree reasonably well with the predictions of the X-ray background synthesis models of Gilli et al.

A wide search for obscured active galactic nuclei using XMM–Newton and WISE

Heavily obscured and Compton-thick active galactic nuclei (AGNs) are missing even in the deepest X-ray surveys, and indirect methods are required to detect them. Here we use a combination of the XMM–Newton serendipitous X-ray survey with the optical Sloan Digital Sky Survey (SDSS), and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray-to-infrared luminosity selection method in finding heavily obscured AGNs. We select the sources which are detected in the hard X-ray band (2–8 keV), and also have a redshift determination (photometric or spectroscopic) in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the spectral energy distributions of the 2844 sources which have three, or more, photometric data points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 μm luminosity to the observed 2–10 keV X-ray luminosity and the intrinsic relation between the X-ray and the mid-infrared luminosities. With this approach, we find 20 candidate heavily obscured AGNs and we then examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64 per cent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78 per cent; out of the nine detected spectroscopically in the SDSS) based on the [O III] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12–19, and the number of Compton-thick AGNs to be 2–5, showing that the method is reliable in finding obscured AGNs, but not Compton thick. However, those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical–infrared selection and the scatter of the Lx-LMIR relation limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. The (observed) X-ray luminosities of heavily obscured AGNs are relatively low (L2−10keV<1044ergs−1), even though most of them are located in the ‘quasi stellar object (QSO) locus’. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ∼25 per cent of the heavily obscured AGNs of our sample.

On the Lx – L6 μm ratio as a diagnostic for Compton-thick AGN

As the mid-IR luminosity represents a good isotropic proxy of the AGN power, a low X-ray to mid-IR luminosity ratio is often claimed to be a reliable indicator of Compton-thick AGN. We assess the efficiency of the X-ray to mid-IR luminosity ratio diagnostic by examining the 12 μm IRAS AGN sample (intrinsic L2−10 keV > 10 42 erg s −1 ) for which high signal-to-noise ratio XMM-Newton observations are now available. We find that the vast majority (ten out of eleven) of the AGN that were classified as Compton-thick on the basis of X-ray spectroscopy by Brightman & Nandra have a low LX/L6 μm luminosity ratio, i.e. lower than a few percent of the average AGN ratio, as is typical of reflection-dominated Compton-thick sources. At low LX/L6 μm ratios, we also find a comparable number of AGN, most of which are heavily absorbed but not Compton-thick. This implies that although most Compton-thick AGN have low LX/L6 μm ratios, at least in the local, Universe, the converse is not necessarily true. We then extend our analysis to higher redshifts. We perform the same analysis in the Chandra Deep Field South, for which excellent quality Chandra (4 Ms) and XMM-Newton (3 Ms) X-ray spectra are available. We derive accurate X-ray luminosities for Chandra sources using X-ray spectral fits, as well as 6 μm luminosities from spectral energy distribution fits. We find in total eight AGN (intrinsic L2−10 keV > 10 42 erg s −1 ) with low LX/L6 μm ratios, after excluding one source where the 6 μm emission primarily comes from star-formation. One of these sources has been already found to host a Compton-thick nucleus, while for another one at a redshift of z = 1.22 we argue it is most likely Compton-thick on the basis of its combined Chandra and XMM-Newton spectrum. In agreement with the low redshift sample, we find a large number of non Compton-thick “contaminants” with low X-ray to mid-IR luminosity ratios. Our results suggest that al owLX/L6 μm ratio alone cannot help us to ascertain whether a Compton-thick AGN is present, albeit the majority of low LX/L6 μm AGN are heavily obscured. More interestingly, the two most reliable Compton-thick AGN in the high redshift Universe have high LX/L6 μm ratios, showing that this method cannot provide complete Compton-thick AGN samples.

Chandra and Spitzer observations of CDFS X-ray obscured QSOs

We present Chandra and Spitzer data for the 186, extragalactic, hard 2-10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%), there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, strictly according to X-ray criteria, i.e. defined as having large hydrogen column densities (N H > 10 22 cm -2 ) and luminosities (L x > 10 44 erg s -1 ). The surface density of X-ray obscured QSOs is ∼210 deg -2 . We find 18 candidate Compton-thick N H > 10 24 cm -2 sources, of which three have QSO luminosities (L x > 10 44 erg s -1 ). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range z = 1.3-4.3. Eight of these show narrow-line optical spectra, two show no obscuration in their optical spectra that present broad lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, X/O > 1, and red colours, I - 3.6 μm > 4. Combining the X-ray with the mid-IR 8 pm or 24 μm flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for detecting obscured QSOs.

The XMM–Newton/2dF survey – IV. The X-ray spectral properties of the hard sources

We present an analysis of the X-ray spectral properties of 61 hard X-ray-selected (2-8 keV) sources from the bright (f 2-8keV > 10 -14 erg cm -2 s -1 ) XMM-Newton/2dF survey. This comprises nine XMM-Newton pointings in the North Galactic Pole region (∼1.6 deg 2 ) and overlaps with the SDSS, 2QZ and 2dFGRS surveys. Our sources contribute about 50 per cent of the 2-10 keV X-ray background down to the flux limit of 10 -14 erg cm -2 s -1 . The hardness ratio distribution of the sample suggests a deficit of heavily absorbed sources. A spectral fit to the coadded total source spectrum yields a steep photon index, r = 1.83 +0.04 -0.05. All but eight sources have optical counterparts down to the SDSS photometric limit of r 22.5. Spectroscopic identifications exist for 34 sources. The vast majority (24 sources) are associated with broad-line (BL) active galactic nuclei (AGN), while only seven present narrow or no emission lines. Five sources are probably associated with Galactic stars. Finally, we present photometric redshifts for another 17 probable AGN. The combined spectrum of the 24 spectroscopically identified BL AGN is steep (r = 2.02 +0.04 -0.05), while that of the seven AGN that do not present broad lines is flatter, with r = 1 64 +0.11 -0.11. The spectrum of the eight optically unidentified sources is flat with r 1.1. Spectral fits to the individual BL AGN reveal large absorption (rest-frame column density >10 22 cm -2 ) in only two cases. The individual spectra of the narrow-line AGN present significant evidence for even a moderate absorption (3 × 10 21 cm -2 ) in only one case.

X-ray detected infrared excess AGN in the Chandra deep fields: a moderate fraction of Compton-thick sources

We examine the properties of the X-ray-detected infrared excess AGN or dust obscured galaxies (DOGs) in the Chandra deep fields (CDFs). We find 26 X-ray-detected sources that obey the 24 μ mt oR-band flux ratio criterion f24/fR > 1000. These are at a median redshift of 2.3, while their IR luminosities are above 10 12 L� . Their X-ray luminosities are all above a few times 10 42 erg s −1 in the 2–10 keV band, which unambiguously argues that they host AGN. Nevertheless, their IR spectral energy distributions are split between AGN (Mrk231) and star-forming templates (Arp220). Our primary goal is to examine their individual X-ray spectra in order to assess whether this X-ray-detected DOG population contains heavily obscured or even Compton-thick sources. The X-ray spectroscopy reveals a mixed bag of objects. We find that four out of the 12 sources with adequate photon statistics and hence reliable X-ray spectra show evidence for a hard X-ray spectral index with Γ ∼ 1 or harder, which is consistent with a Compton-thick spectrum. In total 12 out of the 26 DOGs show evidence for flat spectral indices. However, owing to the limited photon statistics we cannot distinguish whether they are flat because they are reflection-dominated or because they show moderate amounts of absorption. Seven DOGs show relatively steep spectra Γ > 1.4, indicative of low column densities. All the above suggests a fraction of Compton-thick sources that does not exceed 50%. The average X-ray spectrum of all 26 DOGs is hard (Γ ∼ 1.1) or even harder (Γ ∼ 0.6) when we exclude the brightest sources. These spectral indices agree with the stacked spectrum of X-ray-undetected sources (Γ ≈ 0. 8i n the CDFN). This could suggest (but not necessarily prove) that X-ray undetected DOGs, in a similar way as the X-ray-detected ones presented here, are hosting a moderate fraction of Compton-thick sources.

Compton-thick AGN in the 70-month Swift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

The 70-month Swift/BAT catalogue provides a sensitive view of the extragalactic X-ray sky at hard energies (>10 keV) containing about 800 Active Galactic Nuclei. We explore its content in heavily obscured, Compton-thick AGN by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data. We apply a Bayesian methodology using Markov chains to estimate the exact probability distribution of the column density for each source. We find 53 possible Compton-thick sources (with probability 3 to 100%) translating to a ~7% fraction of the AGN in our sample. We derive the first parametric luminosity function of Compton-thick AGN. The unabsorbed luminosity function can be represented by a double power-law with a break at

The XMM-Newton survey in the H-ATLAS field

L_{\star} 2 \times 10^{42}