The Obscured AGN Population Probed by X-ray Observations

Abstract

Abstract. X-ray properties of obscured AGN are reviewd and recent results obtained from ASCA, BeppoSAX, and Chandra are presented.There is a population of AGN that can only be revealed by hard X-ray observations. Studies of the X-ray background suggest that obscured AGN should significantly outnumber their unobscured counterparts. X-ray Spectra of Obscured AGN Effects of X-ray AbsorptionThe energy distribution of AGN can be approximated by a power-law and is roughly fiat over the X-ray band. However, we now know at least in wellstudied bright Seyfert galaxies that there are a significant excess of soft X-ray (below half keV) emission and a roll-over of the hard X-ray continuum at a few hundred keV. The soft X-ray excess is believed to be related to the accretion disk while the high energy roll-over is probably due to a cut-off in energy of thermal electrons in a corona which Comptonizes the disk emission into the power-law continuum. There are other subtle spectral distortions due to reflection off the accretion disk surface. These phenomena are likely happening in a region within 100 gravitational radii of a central black hole.When cold matter lies in the line of sight at radii much further out, a series of photoelectric edges of various elements as well as Hydrogen are imposed on the X-ray continuum and make a low energy cut-off. The energy of the absorption cut-off increases as the column density of the absorbing matter increases. Therefore, going up across the X-ray energy band has the same effect as increasing wavelength in the optical/near-infrared for obscured objects. Assuming the Galactic dust to gas ratio, the optical depth which a near-infrared observation (say, in the K band) can probe is equivalent to that at 2-3 keV. For example, ROSAT, whose bandpass was up to 2 keV, was sensitive to relatively unobscured AGN but no longer sensitive when an absorption column density exceeds 10 22 cm -2 (of course, this restriction is relaxed for high redshift objects). X-ray observatories such as ASCA, BeppoSAX, Chandra and XMMNewton have sensitivity above 2 keV, which enables these satellites to search for highly obscured AGN. It should, however, be noted that simply shifting the energy window to higher energies does not necessarily guarantee detection of more heavily obscured objects, because of a) increasing importance of Comp- 225terms of use, available at https://www.cambridge.org/core/terms. https://doi