X-ray reflection from cold matter in Active Galactic Nuclei and X-ray binaries

• Published in: Monthly notices of the Royal Astronomical Society 1991-03, Vol.249 (2), p.352-367
• Main Authors: George, I. M., Fabian, A. C.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Stellar systems. Galactic and extragalactic objects and systems. The universe; Unidentified sources and radiation outside the solar system; X-ray sources; x-ray bursts
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/249.2.352

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 photoelectric absorption of low-energy photons leads to a ‘Compton reflection hump’ in the albedo in the 20–100 keV band. Due to the high abundance and fluorescence yield of iron, a significant fraction of the incident photons with energies above the iron K-shell edge at 7.1 keV give rise to iron K-shell fluorescent photons with an energy of 6.4 keV. For illustration, our results are applied to two simple geometries: a semi-isotropically illuminated slab, and a disc illuminated from above by a central point source. In both cases we find our results to be consistent with recent observations of unobscured Active Galactic Nuclei. The significance of our results for these and other objects is briefly discussed.