Searching for highly obscured AGNs in the XMM-Newton serendipitous source catalog

The majority of active galactic nuclei (AGNs) are obscured by large amounts of absorbingmaterial that makes them invisible at many wavelengths. X-rays, given their penetratingpower, provide the most secure way for finding these AGNs. The XMM-Newtonserendipitous source catalog, of which 3XMM-DR4 is the latest version, is thelargest catalog of X-ray sources ever produced; it contains about half a milliondetections. These sources are mostly AGNs. We have derived X-ray spectral fits for verymany 3XMM-DR4 sources (≳114000 observations, corresponding to ~77000 unique sources), which contain morethan 50 source photons per detector. Here, we use a subsample of ≃1000 AGNs in the footprint of the SDSS area(covering 120 deg^2)with available spectroscopic redshifts. We searched for highly obscured AGNs by applyingan automated selection technique based on X-ray spectral analysis that is capable ofefficiently selecting AGNs. The selection is based on the presence of either a) flatrest-frame spectra from a simple power-law fit; b) flat observed spectra from an absorbedpower-law fit; c) an absorption turnover, indicative of a high rest-frame column density;or d) the presence of an Fe Kalpha line with a large equivalent width (>500 eV). We found81 highly obscured candidate sources. Subsequent detailed manual spectral fits revealedthat 28 of them are heavily absorbed by column densities higher than 10^23 cm^-2. Of these 28 AGNs, 15 arecandidate Compton-thick AGNs on the basis of either a high column density, consistentwithin the 90% confidence level with N_H>10^24 cm^-2, or a large equivalent width (>500 eV) of the FeKalpha line.Another six are associated with near-Compton-thick AGNs with column densities of~ 5 x 10^23cm^-2. Acombination of selection criteria a) and c) for low-quality spectra, and a) and d) formedium- to high-quality spectra, pinpoint highly absorbed AGNs with an efficiency of80%.

Note:

18 pages, 10 figures, accepted for publication in A&A

X-rays: general
X-rays: diffuse background
surveys
galaxies: active

References(44)

Figures(30)

[2008]

(6)

LaMassa

[2008]

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Burlon

[2008]

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Guainazzi

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Chartas

[2008]

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Lewis

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(2011). a highly absorbed candidate by our selection criteria. Of the sources in out parent sample that are classified as CT AGN within the sample of type 2 Seyfert galaxies extracted from the SDSS presented in LaMassa et al our classification agrees in all but two cases: SDSS J080359.20+234520.4, and

Goulding

SDSS J112301.31+470308.6. In both cases, the automated fits return very flat photon indices in the hard band, but because of the very low number of counts in that band, consistent with being larger than 1.4 at the 90% confidence level. As a result, these two sources are not flagged as candidates by our selection criteria. It is important to note that these two sources are classified as CT

AGN in based on their X-ray luminosity to optical, and mid-IR luminosity ratios, and not from their X-ray spectral analysis. Finally, we cross-correlated our parent sample with the CT AGN within the two type 2 QSO samples that were also extracted from the SDSS and reported in Vignali et al

LaMassa

[2010]

and Jia et al. . Our classification agrees with that in those works except in one case: SDSS J091345.48+405628.2

Nevertheless, as pointed out in its X-ray spectrum is complex and dominated by soft emission, and in addition, different X-ray spectral analysis of this source, both using

Jia

XMM-Newton and Chandra data, have been published reporting a non-CT classification in some cases

6.3. Hardness ratios versus automated spectral-fit selection

Rest-frame hardness ratios (or X-ray colors) have been proposed by several other studies as an alternative to manual spectral fitting for the selection of highly absorbed sources. The downside of these methods is that to obtain rest-frame colors (or fluxes) from X-ray count rates, a spectral model has to be assumed. This could strongly decrease the accuracy of the selection technique, especially if the assumed spectral model is a poor representation of the actual spectral shape. To compare our proposed technique with color-selection techniques, X-ray colors were computed for our sample following the two different X-ray color selection techniques presented in Brightman & Nandra and

Iwasawa

Brightman & Nandra presented an X-ray color selection calibrated by using rest-frame fluxes derived from best-fit models and manual spectral fits. To this end, they used XMMNewton spectra for a sample of 126 local AGN, extracted from a parent sample selected in the mid-IR, for which they carried out a detailed X-ray spectral analysis. Brightman & Nandra defined two X-ray colors, HR1 and HR2, based on rest-frame

fluxes computed in the rest frame bands: 1-2 keV (band 1), 24 keV (band 2), and 4-16 keV (band 3), as follows:

HR1 = F(band2)-F(band1)

F(band2)+F(band1) HR1 = F(band3)-F(band2)

F(band3)+F(band2). (1)

First, we applied this method to our sample of 81 highly absorbed candidates by using the best-fit model obtained from the manual fits. The results are plotted in Fig. 9 in the top-left panel. The dashed line corresponds to the proposed dividing line between highly absorbed sources (NH > 1023 cm-2

and mildly absorbed or unabsorbed sources in Brightman & Nandra

[2012]

The solid line corresponds to the wedge defined in that work to contain all their CT AGN. We found similar results, that is, all highly absorbed sources in our sample but one lie above the dashed line. Nevertheless, we found a higher number of contaminants with lower NH of ∼ 20%, while a value of only

7% was reported in Brightman & Nandra . Moreover, not all our CT candidates fall within the solid wedge

Brightman & Nandra also proposed that their selection technique could be applied to X-ray colors derived by using observed count rates, assuming a simple power-law model (Γ =

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