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

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.