X-ray reflection by photoionized accretion discs

Calculations of X-ray reflection spectra from ionized, optically-thick material are an important tool in investigations of accretion flows surrounding compact objects. We present the results of reflection calculations that treat the relevant physics with a minimum of assumptions. The temperature and ionization structure of the top five Thomson depths of an illuminated disc are calculated while also demanding that the atmosphere is in hydrostatic equilibrium. In agreement with Nayakshin, Kazanas & Kallman, we find that there is a rapid transition from hot to cold material in the illuminated layer. However, the transition is usually not sharp so that we find a small but finite region in Thomson depth where there is often a stable temperature zone at T ∼ 2 × 10 K due to photoelectric heating from recombining ions. As a result, the reflection spectra often exhibit strong features from partially-ionized material, including helium-like Fe K lines and edges. The reflection spectra, when added to the illuminating spectra, cannot be adequately fit by the reflection models pexriv and pexrav in the ASCA energy ranges. More accurate fits, both statistically and parametrically, are often obtained with use of the constant density models of Ross & Fabian. Fitting our results with these group of models yield a spurious R − Γ correlation in the 0.5–10 keV energy range, where R is the reflection fraction for a power-law continuum of index Γ, confirming the suggestion of Done & Nayakshin that at least part of the R − Γ correlation reported by Zdziarski, Lubiński & Smith for Seyfert galaxies and X-ray binaries might be due to ionization effects. However, large uncertainties in the fit parameters prevent confirmation of the correlation in the 3– 20 keV energy range. Although many of the reflection spectra show strong ionized features, these are not typically observed in most Seyfert and quasar X-ray spectra. However, the data are not yet good enough to place constraints on the illumination properties of discs, as instrumental and/or relativistic effects could mask the ionized features predicted by the models.