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 F_x(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).

1. < 100 ergs cm s^-1: The material is weakly ionized. X-ray reflection from the accretion disk produces a cold iron line at 6.4 keV. Since the total photoelectric opacity of the material is large even below the iron edge, the Compton backscattered continuum only weakly contributes to the observed spectrum at 6 keV, and the observed iron K-shell absorption edge is small. This regime is termed `cold' reflection, since the reflection spectrum around the energy of the iron-K features resembles that from cold, neutral gas.

   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 .

2. 100 ergs cm s^-1 < < 500 ergs cm s^-1: In this intermediate regime, the iron is in the form of FeXVII-FeXXIII and there is a vacancy is the L-shell (n = 2) of the ion. Thus, these ions can resonantly absorb the corresponding K line photons. Successive fluorescent emission followed by resonant absorption effectively traps the photon in the surface layers of the disk until it is terminated by the Auger effect. Only a few line photons can escape the disk leading to a very weak iron line. The reduced opacity below the iron edge due to ionization of the lower-Z elements leads to a moderate iron absorption edge.

3. 500 ergs cm s^-1 < < 5000 ergs cm s^-1: In this regime, the ions are too highly ionized to permit the Auger effect. While the line photons are still subject to resonant scattering, the lack of a destruction mechanism ensurs that they can escape the disk and produces a `hot' iron line at ~ 6.8 keV. There is a large absorption edge.

4. > 5000 ergs cms^-1: When the disk is highly ionized, it does not produce an iron line because the iron is completely ionized. There is no absorption edge.

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.