X-ray observations have probably provided the most direct test of the unified model. Indeed, the X-ray spectrum of many Sy2s is characterized by a powerlaw similar to that observed in Sy1s and a photoelectric cutoff due to absorbing, cold gas along our line of sight. Since the absorbing column in Sy2s is generally larger than 1022 cm-2, these studies were mostly obtained with satellites sensitive in the high energy band above ~ 1 keV such as Ginga, ASCA and BeppoSAX.
If the absorbing column is larger than ~ 1024 cm-2, i.e. the medium is thick to Compton scattering, then the transmitted component is completely suppressed below 10 keV and the spectrum observed in the 2-10 keV band is dominated by reprocessed components. More specifically, in this case the hard X-ray spectrum is characterized by a flat Compton reflection component, ascribed to the inner surface of the torus, and/or a steeper component due to a ionized, warm scattering medium (eg. Matt et al. 1997). If the absorbing medium has a column in the range 1024-1025 cm-2, then the transmitted component can be still observed in the 10-300 keV band as an emission excess (eg. Done et al. 1996); for larger absorbing columns even the 10-300 keV band is dominated by the reflection component. The cold reflector also produces a prominent fluorescence iron line at 6.4 keV (EW ~ 1-2 keV with respect to the reflected continuum). Yet, Netzer et al. (1998) pointed out that also the Narrow Line Clouds can contribute significantly to the observed 6.4 keV line, provided that their column density is large enough. Instead, the warm scattering medium emits lines of He- and H-like iron at 6.7 and 7 keV, which can reach large equivalent widths as well (Matt et al. 1996). Until a few years ago only a couple of Compton thick sources were known. This is because the reflection efficiency of both the cold and warm mirrors is low (~ 10-2-10-3) and, therefore, this class of objects is more difficult to detect. This highlights one of the major problems in past hard X-ray surveys, as discussed in the following section.