Conference proceedings to appear in "The Central Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP) 2007
astro-ph/0701109

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Abstract: We review some of the main physical and statistical properties of the X-ray absorber in AGNs. In particular, we review the distribution of the absorbing column density inferred from X-ray observations of various AGN samples. We discuss the location of the X-ray absorber and the relation with the dust absorption at optical and infrared wavelengths. Finally, we shortly review the recent findings on X-ray absorption at high luminosities and at high redshift.

Table of Contents

• INTRODUCTION
• THE WARM ABSORBER
• COLD ABSORPTION: GENERAL PROPERTIES AND WARNINGS
• THE N_H DISTRIBUTION AMONG AGNs
• THE LOCATION OF THE COLD X-RAY ABSORBER
• X-RAY ABSORPTION VERSUS OPTICAL AND INFRARED ABSORPTION
• X-RAY ABSORPTION AT HIGH LUMINOSITIES AND AT HIGH REDSHIFT
• OPEN ISSUES
• REFERENCES

¹ INAF - Astronomical Observatory of Rome, Italy
² INAF - Arcetri Astrophysical Observatory, Firenze, Italy