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 often we find a small but finite region in Thomson depth where there is a stable temperature zone at T similar to 2 X 10(6) 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, were fitted by the publicly available constant-density models (i.e. PEXRIV, PEXRAV and the models of Ross & Fabian). We find that owing to the highly ionized features in the spectra these models have difficulty correctly parametrizing the new reflection spectra. There is, evidence for a spurious R-Gamma correlation in the ASCA energy range, where R is the reflection fraction for a power-law continuum of index Gamma, confirming the suggestion of Done & Nayakshin that at least part of the R-Gamma correlation reported by Zdziarski, Lubinski & 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.