X-ray reflection from cold matter in Active Galactic Nuclei and X-ray binaries
Abstract
Weak, broad emission lines due to low-ionization stages of iron, and other spectral features, have recently been observed in the X-ray spectra of Active Galactic Nuclei and in some Galactic X-ray binaries. These features are due to X-ray irradiation of relatively cold, dense gas very close to the central compact object. The observed spectra consist of both direct radiation from the primary X-ray source and ‘reflection’ from the surrounding accretion flow. The reflected spectrum has features imprinted in it by photoabsorption, iron fluorescence and Compton scattering. The strength, shape and broadening of this reflected spectrum is a diagnostic of the geometry, ionization state and iron abundance of the accretion flow, and through timing studies may yield the mass of the central object. We present here results of detailed Monte Carlo calculations of the interaction between X-rays and dense, neutral material for use in the detailed interpretation of the X-ray spectra of AGN and X-ray binaries. The angular dependence, intensity and spectrum of the ‘reflected’ radiation are computed in a semi-infinite, plane-parallel configuration for power-law input spectra. We find, as expected, that the angular distribution of continuum photons able to escape from the slab is highly anisotropic and a strong function of the initial energy and the angle of the incident X-rays. Furthermore, the competition within the slab between multiple electron (down-) scattering of high-energy photons and…
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