The X-ray emission from active galactic nuclei is believed to originate close to the central engine (for a recent review see [1]). In the various versions of unified models the blocking torus is further from the center than the X-ray emission site. Thus in sources where we have a line-of-sight to the central engine we expect to see non-thermal X-ray emission while along other line-of-sights the emission will be blocked at lower energies by photo-electric absorption.
So, one of the tests for Cygnus-A being a buried quasar, that is an active galactic nucleus for which our line of sight to the central engine is blocked by an obscuring torus, is whether we see non-thermal X-ray emission with a low energy cut-off due to photo-electric absorption. It is possible that the torus could be Compton-thick in which case no direct X-ray emission would be visible and we would have to hope to observe scattered flux as seen in some AGN (eg NGC1068 : [2]).
In order to understand the history of X-ray observations and the recent new results it helps to understand some features of X-ray instrumentation. There have been two basic types of instrument. Firstly, the collimated medium energy X-ray spectrometer, operating in the energy range 2 - 40 keV, which provides spectral information but whose spatial resolution is around a degree at best (although source positions can be determined more accurately by scanning the satellite or using a modulation collimator). Secondly, there is the X-ray imager, operating in the energy range 0.1 - 3.5 keV, which provides spatial resolutions down to a few arcseconds but very little spectral information. ASCA is the first satellite to carry telescopes and detectors capable of spatial resolution down to 1 - 2 arcminutes for X-rays over the wide energy range of 0.5 - 10 keV, and thus unify the two classes of detector.