2.1.3. Ionization of the Disk Surface
X-ray irradiation can photoionize the surface layers of a disk (Ross & Fabian 1993; Ross, Fabian & Young 1999). As discussed above, the fluorescent line that the illuminated matter produces depends upon its ionization state. A useful quantity in this discussion is the ionization parameter
where Fx(r) is the X-ray flux received per unit area of the disc at a radius r, and n(r) is the comoving electron number density: it measures the ratio of the photoionization rate (which is proportional to n) to the recombination rate (proportional to n2). The iron line emission for various ionization parameters has been investigated by Matt et al (1993, 1996). They found that the behaviour split into four regimes depending on the value of (also see Fig. 2).
Figure 2. Reflection spectra from ionized matter for various values of the ionization parameter . The dotted lines show the level of the illuminating power-law continuum for each value of .
Note that ionization of the reflector paradoxically causes the observed iron edge to strengthen at moderate values of . This is because the edge is saturated in reflection from a cold absorber, as is absorption at lower energies where oxygen and iron-L are the main absorbers. Ionization of oxygen and iron leads to the iron-K edge being revealed, and so apparently becoming stronger, as the reflected flux below the edge increases.
The Matt et al. (1993, 1996) calculations assume a fixed density structure in the atmosphere of the accretion disk. Nayakshin, Kallman & Kazanas (1999) have relaxed this assumption and included the effect of thermal instability in the irradiated disk atmosphere. In their solutions, the cold dense disk that produces the X-ray reflection features is blanketed with an overlying low-density, highly ionized, region. For weak irradiation, the ionized blanket is thin and does not affect the observed spectrum. However, for strong irradiation, the ionized blanket scatters and smears the ionized reflection features. In their models, it can be difficult to produce highly ionized iron lines in reflection - the effect of increasing ionization is to dilute the `cold' reflection signature. The extent of this effect will depend on the Compton temperature of the radiation field.