The effects of high density on the X-ray spectrum reflected from accretion discs around black holes

Current models of the spectrum of X-rays reflected from accretion discs around black holes and other compact objects are commonly calculated assuming that the density of the disc atmosphere is constant within several Thomson depths from the irradiated surface. An important simplifying assumption of these models is that the ionization structure of the gas is completely specified by a single, fixed value of the ionization parameter xi, which is the ratio of the incident flux to the gas density. The density is typically fixed at n(e) = 10(15) cm(-3). Motivated by observations, we consider higher densities in the calculation of the reflected spectrum. We show by computing model spectra for n(e) greater than or similar to 10(17) cm(-3) that high-density effects significantly modify reflection spectra. The main effect is to boost the thermal continuum at energies less than or similar to 2 keV. We discuss the implications of these results for interpreting observations of both active galactic nuclei and black hole binaries. We also discuss the limitations of our models imposed by the quality of the atomic data currently available.