X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South

Abstract

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Γ with an intrinsic redshifted absorption N H , a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Γ and N H free. The weighted mean value for the slope of the power law is Γ 1.75 ± 0.02, and the distribution of best fit values shows an intrinsic dispersion of σ int 0.30. We do not find hints of a correlation between the spectral index Γ and the intrinsic absorption column density N H . We then investigate the absorption distribution for the whole sample, deriving the N H values in faint sources by fixing Γ = 1.8. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates. By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N H distribution representative of the whole AGN population, f (N H )dN H , from the observed one. f (N H ) shows a lognormal shape, peaking around log(N H ) 23.1 and with σ 1.1. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN. We find that the fraction of absorbed sources (with N H > 10 22 cm −2 ) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

XMM-Newtonobservations of the Lockman Hole: X-ray source catalogue and number counts

“…The faintest among theses sources has a 2−10 keV flux of ∼9 × 10 −16 erg cm −2 s −1 . In the Chandra deep field Tozzi et al (2006) spectroscopically detected 14/280 candidates at 2−10 keV fluxes greater that ∼1.5 × 10 −15 erg cm −2 s −1 . Polletta et al (2006), detected 5/567 X-ray selected candidates in the Chandra SWIRE survey at 2−10 keV fluxes greater than ∼4.6 × 10 −15 erg cm −2 s −1 .…”

“…In order to obtain the first measurement of the surface density of Compton thick sources, we multiplied the source fraction estimated above in the 2−10 keV band by the cumulative number density predicted by Gilli et al (2007). The resulting log N − log S is shown in the upper panel of (Tozzi et al 2006), the XMM-COSMOS ) and the Chandra-SWIRE survey (Polletta et al 2006). Lower panel: the fraction of Compton thick source candidates in the Lockmann Hole, the CDFS, the XMM-COSMOS and Chandra-SWIRE, respectively from left to right.…”

“…Lower panel: the fraction of Compton thick source candidates in the Lockmann Hole, the CDFS, the XMM-COSMOS and Chandra-SWIRE, respectively from left to right. The solid lines represents the prediction of the X-ray background model by Gilli et al (2006).…”

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

“…The spectra and the best-fit models for these sources are reported in Figure 4. This disagreement is not unusual, several works using low-energy data have thus far attempted the spectral identification of distant CT sources in the medium/deep survey fields (e.g., Tozzi et al 2006;Georgantopoulos et al 2007Georgantopoulos et al , 2009Georgantopoulos et al , 2013Comastri et al 2011;Feruglio et al 2011;Brightman & Ueda 2012;Brightman et al 2014;Buchner et al 2014;Lanzuisi et al 2015), and given the range of possible spectral shapes for CT sources and the limited counting statistics (from few tens to ∼100 counts) for these faint sources, especially at the highest energies 5 6 keV  -where the effective area starts to drop, researchers have always struggled to consistently identify CT sources (see, e.g., Castelló-Mor et al 2013). Moreover, their analysis, at least for sources with redshifts 2  (i.e., the great majority of the potential contributors to the CXB, according to populationsynthesis models), have been limited by sampling of the restricted lower-energy part of the reflection component and of the heavily obscured primary emission.…”

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

“…A detailed investigation of X-ray obscuration can only be obtained by fitting high-quality X-ray spectra with hundreds to thousands of counts (e.g., Akylas et al 2006;Tozzi et al 2006;Mainieri et al 2007;Lanzuisi et al 2013;LaMassa et al 2014). As the requisite signal-to-noiseratio for such an analysis is not available for the majority of sources discovered in X-ray surveys, a simpler diagnostic, the hardness ratio (HR), is often used as a proxy of X-ray extinction (e.g., Kim et al 2007;Fiore et al 2008Fiore et al , 2009Wilkes et al 2009;Civano et al 2012).…”