Iron lines from active galactic nuclei

The cleanest examples of observed relativistic iron lines have been found in AGN. There are three principal reasons why AGN tend to present cleaner disk reflection signatures than GBHCs. Firstly, the primary X-ray continuum is often found to be very featureless, being well modeled by a simple power-law form. This makes it straightforward to subtract the continuum in order to study the underlying disk reflection signatures. The physical reason underlying the simplicity of the continuum form in AGN is the wide separation of temperatures between the accretion disk surface (with $kT\sim 10{\rm\thinspace eV}$) and the disk corona (with $kT\sim 100{\rm\thinspace keV}$) -- thermal Comptonization of the optical/UV disk emission by the corona produces a featureless power-law spectrum across much of the X-ray band. The accretion disks in GBHCs, on the other hand, can get so hot ($kT\sim 1{\rm\thinspace keV}$) that the Wien tail of the thermal disk emission can overlap with the red-wing of any iron line, making continuum subtraction much more challenging. Secondly, we expect there to be a wide range of parameter space over which AGN accretion disks are not strongly ionized, thereby simplifying the study of the X-ray reflection signatures. On the other hand, GBHCs may well be generically ionized due to the high temperature of the disk. Finally, since GBHCs typically lie in the plane of the Galaxy, they are usually much more heavily absorbed than type-1 AGN, further complicating the modeling of the continuum radiation.