2014A&A...569A..71C

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 (>114000 observations, corresponding to ∼77000 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 120deg²) 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 (>500eV). 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 10²³cm^–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 N_H>10²⁴cm^–2, or a large equivalent width (>500eV) of the Fe Kα line. Another six are associated with near-Compton-thick AGNs with column densities of ∼5x10²³cm^–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%.