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

Abstract

The majority of active galactic nuclei (AGN) 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 AGN. The XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources ever produced; it contains about half a million detections. These sources are mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources ($\gtrsim$ 114 000 observations, corresponding to $\sim$ 77 000 unique sources), which contain more than 50 source photons per detector. Here, we use a subsample of $\simeq$ 1000 AGN in the footprint of the SDSS area (covering 120 deg${}^2$) with available spectroscopic redshifts. We searched for highly obscured AGN by applying an automated selection technique based on X-ray spectral analysis that is capable of efficiently selecting AGN. The selection is based on the presence of either a) flat rest-frame spectra; b) flat observed spectra; c) an absorption turnover, indicative of a high rest-frame column density; or d) an Fe K$\alpha$ line with an 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 10${}^{23}$ cm${}^{-2}$. Of these 28 AGN, 15 are candidate Compton-thick AGN on the basis of either a high column density, consistent within the 90% confidence level with N${}_{\mathrm{H}}$ $>$10${}^{24}$ cm${}^{-2}$, or a large equivalent width (>500 eV) of the Fe K$\alpha$ line. Another six are associated with near-Compton-thick AGN with column densities of $\sim$ 5$\times$10${}^{23}$ 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 AGN with an efficiency of 80%.

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

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• Where are Compton-thick radio galaxies? A hard X-ray view of three candidates
• The XMM-Newton survey in the H-ATLAS field
• XMMFITCAT: The XMM-Newton spectral-fit database
• Radio observations of four active galactic nuclei hosting intermediate-mass black hole candidates: studying the outflow activity and evolution
• Multi-wavelength observations of the triple-peaked AGN Mrk 622
• A possible binary AGN in Mrk\,622?
• Discovery of H2O Megamasers in Obscured Active Galactic Nuclei
• Harnessing the XMM-Newton data: X-ray spectral modelling of 4XMM-DR11 detections and 4XMM-DR11s sources
• Selection of Compton-thick AGN from a hard photometric sample using XMM-Newton observations